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Microfluidic and nanofluidic phase behaviour

characterization for industrial CO2, oil and gas

Bo Bao, Jason Riordon, Farshid Mostowfi and David Sinton

Version Post-print/accepted manuscript

Citation

(published version)

Bao, Bo, Jason Riordon, Farshid Mostowfi, and David Sinton.

"Microfluidic and nanofluidic phase behaviour characterization for industrial CO 2, oil and gas." Lab on a Chip 17, no. 16 (2017): 2740-

2759. DOI: 10.1039/C7LC00301C

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Received 00th January 20xx,

Accepted 00th January 20xx

Microfluidic and nanofluidic phase behaviour characterization for energy applications

Bo Bao,a Jason Riordon,b Farshid Mostowfic and David Sinton*b

Microfluidic systems that leverage unique micro-scale phenomena have been developed to provide rapid, accurate and

robust analysis, predominantly for biomedical applications. These attributes, in addition to the ability to access high

temperatures and pressures, have motivated recent expanded applications in phase measurements relevant to energy

applications. We here present a comprehensive review of this exciting new field, separating microfluidic and nanofluidic

approaches. Microfluidics is practical, and provides similar phase properties analysis to established bulk methods with

advantages in speed, control and sample size. Nanofluidic phase behaviour can deviate from bulk measurements, which is

of particular relevance to emerging unconventional oil and gas production from nanoporous shale. In short, microfluidics

offers a practical, compelling replacement of current bulk phase measurement systems, whereas nanofluidics is not

practical, but uniquely provides insight into phase change phenomena at nanoscales. Challenges, trends and opportunities

for phase measurements at both scales are highlighted.

1. Introduction

Fluid phase characterization is crucial to a wide range of

processes, including protein crystallization, polymer analysis,

surfactant screening, chemical reactor engineering, the

transport and storage of natural gas and carbon dioxide,

supercritical fluid processing and extraction, and of particular

interest to this review, oil and gas operations. Fluid phase

behaviour is obtained through either theory/simulation or

experimental methods. Theory/simulation approaches

calculate the thermodynamic properties of fluid mixtures using

the equation of state. Performing direct measurement of fluid

phase, however, remains an expensive and time-consuming

proposition. Energy-relevant fluid processes are governed by

microscale fundamentals in terms of viscosity, wetting

behaviour, surface tension and capillary effects. The inherent

technical challenges associated with informing these processes

using conventional reservoir fluid analysis include complex

small-scale transport phenomenon, complex microscale phase

behaviours, intensive capital investment, high labour costs and

long run times.

Microfluidic approaches are particularly well suited to

perform phase behaviour analysis: fluid properties are generally

similar to bulk systems, with benefits in rapid heat and mass

transfer, lab-on-a-chip processing, optical access for

visualization, potential for multiplexing, and precise control of

temperatures, pressures and compositions. Microfluidic

technologies have been widely investigated and developed for

sample preparation, separation, sensing, detection and fluid

manipulation. Successful applications include, but are not

limited to, point-of-care diagnostics, cell analysis, drug

metabolism, omics and environmental analysis.1 In recent

years, microfluidic techniques have been increasingly applied to

energy applications, with applications in enhanced oil recovery,

heavy oil/bitumen production, shale and tight gas/oil

production, carbon transport and storage, reservoir fluid

analysis and other petrochemical processes.

The advancement of fabrication techniques in the last two

decades, paired with the recent boom in unconventional and

gas recovery from nanoporous shale and tight oil reservoirs, has

led to increased interest in nanofluidic phase characterization

devices. These systems are designed to visualize fluid behaviour

in nanoconfinement, which is typically distinct from micro- and

macroscale behaviour. Nanofluidic behaviour is a deviation

from microfluidic approaches where the data is generally

readily applicable to the larger scales and the small operating

scale offers speed and simplification. Although not as

established as microfluidics-based phase measurements,

nanofluidic phase measurements are rapidly emerging.

1.1 Complementary reviews and our focus

Microscale phase characterization. Several review papers

include various aspects of microscale phase characterization.

Thome reviewed boiling phenomena in microchannels,

addressing topics such as flow regimes (e.g. bubble flow, slug

flow, annular flow and mist flow), heat transfer coefficients and

evaporation models.2 Tibirica et al. conducted a comprehensive

literature review on how boiling in microchannels has

progressed over the last two decades. The authors nicely review

macro-to-micro transition, flow regimes, pressure drop, heat

transfer coefficients, critical heat flux, superficial void fraction

and liquid entrainment.3 Later, microscale phase behaviours

were reviewed in the context of chemical and material

synthesis. Guenther et al. reviewed transport characteristics of

multiphase flows, and systematically addressed the

characterization techniques, multiphase flow instability, flow

regimes and interfacial shapes of gas-liquid flows, liquid-liquid

flows, phase separation and coalescence, mixing and

dispersion, and applications to chemical and material

synthesis.4

As compared to foundational work in biomedicine, energy

systems represent a much more recent application of

microfluidic phase behaviour characterization and have

required new innovations. In an early article, Sinton et al.

provided a brief overview of the early successes and potential

opportunities of microfluidics in energy, including solar energy

(photocatalysis and photosynthesis), electrochemical energy

a. Interface Fluidics, 11421 Saskatchewan Dr. NW, Edmonton, Alberta, Canada b. Department of Mechanical and Industrial Engineering, University of Toronto, 5

King's College Road, Toronto, Canada. Tel: +1 (416) 978 1623; E-mail: sinton@mie.utoronto.ca

c. Schlumberger-Doll Research, Cambridge, USA Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x

(fuel cell and battery), oil and gas (reservoir fluid analysis and

micromodels informing sub-surface operations), and carbon

management.5 Marre et al. summarized high-temperature and

high-pressure supercritical microfluidic applications in terms of

reactor material, packaging techniques, flow characteristics for

flow-through chemistry and material synthesis applications.6

Lifton later provided a comprehensive review of microfluidics

for enhanced oil recovery, covering visualization technologies,

and modelling of a suite of hydrocarbon recovery processes,

e.g. water/surfactant flooding, polymer flooding, nanoparticle

flooding, foam flooding, microbial enhanced oil recovery, heavy

oil extraction and carbon sequestration.7

Nanoscale phase characterization. Fluid phase behaviour in

nanometer-confined geometries (< 100nm) is a topic of

increasing interest, with applications spanning biology,

chemistry, physics and engineering, as highlighted originally in

an elegant review by Eijkel et al.8 The nanometer length scale

leads to unique fluid behaviour which is very different from the

bulk scale. Bocquet et al. conducted an in-depth review in

continuum validity, characteristic length scales (e.g. molecular

length, Debye length of electrostatic diffuse layer and slip

length), associated transport phenomena and practical

applications.9 A few years later, Bocquet et al. highlighted

current challenges and recent progress of emerging nanofluidic

approaches, and applications to membranes (used for

separation, desalination and energy harvesting), gene

sequencing, super hydrophobic surfaces, and unconventional

oil and gas.10

Nanofluidic devices have been fabricated by (i)

nanolithography such as electron beam lithography and

focused ion beam techniques; (ii) standard micro-electro-

mechanical-systems (MEMS) techniques such as etching and

bonding; and (iii), nanomaterial-based methods such as

nanoporous substances and nanowires, as presented in a

comprehensive review by Duan et al.11 Napoli et al. summarized

the nanofluidic studies in biomolecule applications, including

biomolecule separation, interaction kinetics, concentration and

detection.12 Segerink et al. presented state-of-the-art

nanofluidic techniques in point of care applications using single

nanopores, nanoporous membrane, nano-confinement and

concentration polarization.13 Sparreboom et al. focused on the

transport aspect of nanofluidics and systematically discussed

the applications, including flow detection, pumping,

electrokinetic control of molecules, energy conversion, and

molecule separation.14 Cole et al. conducted a review on

adsorption-desorption mechanisms of hydrocarbons within

nanopores of mineral surfaces.15 Barsotti et al. performed a

systematic review of capillary condensation in nanoporous

media and pointed to potential applications in hydrocarbon

recovery from tight reservoirs.16

Here we critically review both microfluidic and emerging

nanofluidic phase behaviour characterization methods for

energy applications. The focus is on applications that have been

most pursued and those rapidly emerging, specifically:

microfluidic pressure-volume-temperature (PVT) systems that

probe fluid phase transitions, dew/bubble/critical points and

phase diagrams (P-T, P-x, T-x); microfluidic solubility, diffusivity,

miscibility (one phase is mixed into the other or dispersed

phases are merged into a homogeneous phase), microfluidic

precipitation (generation of a solid phase from a liquid phase,

e.g. precipitation of asphaltene, wax and salt); and, recent

advancements in nanofluidic phase visualization methods.

2. Current phase characterization methods

2.1 Traditional pressure-volume-temperature (PVT) systems

Traditionally, fluid phase measurements are conducted in a PVT

cell with a transparent window for visualization, and the

majority of such instruments are dedicated to oil and gas. When

crude oil travels from a reservoir to the surface, the oil

experiences significant changes in pressure and temperature

which result in significant changes in thermophysical properties.

The fluid may also cross one or more phase boundaries. Fig. 1

depicts the typical phase behaviour of crude oil. The fluid is

initially in liquid form at reservoir conditions but as the fluid

travels along the production path, its pressure and temperature

drop. Furthermore, other phases – gaseous and solid phases in

this case - may appear as the fluid travels to the surface. A main

purpose of a PVT apparatus is to recreate the desired conditions

which the fluid experiences while traveling to the surface. A

conventional PVT cell is an apparatus designed for measuring

thermophysical properties of reservoir fluids such as saturation

pressure (bubble or dew point), and gas/liquid volume ratio at

different pressures. PVT measurements provide experimental

data points for tuning and validating the equation of state of the

crude oil.

A typical PVT cell has a volume of 100 ml to 1L, sustains

pressure up to 60 MPa and temperatures up to 175 ºC. Routine

PVT experiments include constant-composition expansion,

differential liberation, constant-volume depletion, and multi-

stage separator tests. All of these experiments are aimed at

emulating the production process in a reservoir through natural

depletion. In addition, other properties such as chemical

composition can be obtained by adding analysis units such as

chromatography tools to the PVT system. Consequently,

current PVT systems are the mainstream technology for fluid

phase behaviour studies in the energy and environmental

industries.17 A typical PVT system, however, requires several

hours to establish a single data point due to large heat and mass

transfer times within the large cell volume. Moreover,

purchasing and operating a commercial PVT system is

expensive, with capital costs in the hundreds of thousands of

dollars. Long wait time and high expenses have motivated

research in alternate phase behaviour characterization

technologies.

2.2 Optical methods

Optical methods offer unique advantages of high sensitivity, in-

situ measurement, automatic detection and potential for

distributed sensing over long distances. Optical methods

employ a variety of principles, namely, chilled mirror,

absorption spectroscopy, Fabry-Perot hygrometry and fiber-

optic reflectometry. A chilled mirror measures the dew point

temperature by using a cooled mirror surface to induce

condensation, and allow monitoring of light adsorption and

scattering effects.18 Absorption spectroscopy is a very sensitive

non-contact technique that acquires a broadband spectrum

covering the vibration frequency unique to the chemical

species.19 The Fabry-Perot hygrometer consists of a multi-

layered material with high and low refractive indices. The

condensed liquid inside the surface material layer changes the

refractive index and shifts the signal wavelength. This technique

has the disadvantage of slow response and coating degradation

after long-term use. 18 The fiber-optic reflectometer detects the

refractive index change associated with phase change via the

reflected signal. The fiber-optic probe installed in an equilibrium

cell can function at high temperatures (300 ºC) and pressures

(30 MPa).20,21 Contamination and fouling of the fiber-optic

requires additional care for long term use.

2.3 Electrical and acoustic methods

Electrical and acoustic methods utilize electrical and acoustic

signals to detect phase change, and are based upon

capacitance, quartz crystal microbalance, surface acoustic

waves and sound speed. A capacitance probe uses moisture-

sensitive dielectric material sandwiched between two

electrodes. The condensed liquid changes the dielectric

constant of the layer material, which can be detected by an

electrical circuit. This approach has the advantage of low

installation costs but is vulnerable to contamination or fouling

from system impurities. A quartz crystal microbalance employs

mechanical oscillation to detect the fluid phase transition. The

condensed phase increases the mass of the oscillator which

decreases the resonance frequency. This technique is widely

used in natural gas moisture measurement because of its high

accuracy (down to 10 ppb). The surface acoustic wave sensor

also tracks the mass change of fluid in contact with the sensor.

Early work shows a sensitivity such that a phase shift of 30º is

equivalent to 3µg/cm2.22 The speed of sound in a fluid is a direct

indicator of fluid density and state of phase. The phase

behaviour of rich gas mixtures was successfully measured by the

decompression wave speed.23

3. Micro/nano-fluidic phase characterization

3.1 Chief benefits of miniaturization: rapid heat and mass transfer

and accessible temperatures and pressures

Compared to bulk-scale conventional methods, microfluidic

approaches leverage unique advantages, including significant

reduction in heat and mass transfer time. In a simple scenario

where fluid is heated by heat convection while flowing through

a section of tube, the temperature of fluid at the outlet (To) can

be predicted by a simple 1-D heat transfer model:

𝑇𝑤 − 𝑇𝑜

𝑇𝑤 − 𝑇𝑖= 𝑒

−4ℎ𝐿

𝜌𝑣𝑐𝑝𝑑

In this equation, Tw represents the wall temperature, h is the

heat transfer coefficient, ρ is the density, and cp is the heat

capacity of a fluid. If all other parameters are fixed, a reduction

in channel dimension d will generate an outlet temperature To

closer to Tw. This relation demonstrates why a smaller channel

enables rapid heat transfer. Miniaturized features in

microfluidic systems also improve mass transfer. Consider a

simple 1-D diffusion model in a tube. At the boundary, the

solute concentration changes from 0 to Co at t = 0+, initiating a

1-D diffusion process through the solvent in the tube. The solute

concentration C at the right interface can be predicted by the

solution to Fick’s second law:

𝐶 = 𝐶0 𝑒𝑟𝑓𝑐(𝐿

2√𝐷𝑡)

The diffusion time is proportional to the square of the length

(t ∝ L2). The relation here clearly indicates that small

dimensions can substantially reduce the time required for

diffusion and equilibrium. In summary, the microscale features

inherent to microfluidic systems provide efficiency in both

energy and mass transport, as compared to bulk-scale

conventional systems.

The inherently small fluid-exposed surface areas in

microfluidic systems, allow for very high fluid pressures with

moderate bonding force. The small fluid volumes also

inherently reduce the impact of a failure. Likewise with proper

material selection, high temperatures are readily accessible.

The result is that, with proper engineering and safety

considerations, microfluidic systems can provide ready access

to high temperatures and pressures that are difficult to achieve

with bulk systems.

3.2 Foundational microfluidic phase characterization methods in

other fields

Protein crystallization characterization. Microfluidic phase

change characterization was first used to investigate the phase

behaviour of biological and biomedical samples, specifically

protein crystallization.24 Experimentally determining the phase

behaviour of bio-macromolecules is of high importance, given

that their two thermodynamic phases – in-solution or

precipitated - inherently govern their biophysical behaviours

and biochemical functions. Conventional techniques are

Figure 1. Typical P-T phase envelope of a reservoir fluid. Dashed lines indicate quality lines. Dash-dot line indicates the production path of a reservoir fluid. The black dot indicates the critical point.

incapable of providing a systematic full-factorial investigation of

these natural complex macromolecules.

Multiplexed microfluidic approaches have been introduced

to reduce sample consumption and measurement time in these

applications. Hansen et al. demonstrated rapid screening of

protein crystallization conditions using a multilayer silicone

elastomer-glass made microfluidic chip with 144 parallel

reactions controlled by on-chip micromechanical valves.24 In a

later study, a microfluidic approach was combined with positive

displacement injection metering, which allowed sequential

injection of precise sample aliquots from a single channel to an

array of reaction chambers. For the first time, large-scale

systematic combinatorial screening enabled rapid mapping of

the crystallization phase diagram, with a 72-fold improvement

in crystallization compared to conventional sparse matrix

screens.25 A phase diagram of twelve protein crystallizations

was generated by massive combinatorial screening.26 In

addition to screening the phase diagram of proteins, kinetic

optimization and protein structures were also experimentally

investigated using micro-reactors exploiting free interface

diffusion.27 Lau et al. upgraded their previous design25 to

include three modules, including a formulation module, a

droplet injector and a two-phase storage module. The

automated chip was used to generate a four-dimensional

solubility phase diagram which included multiple protein

concentrations, precipitant molecular weights, precipitant

concentrations and pH.28

Zheng et al. developed a PDMS microfluidic device to

observe the micrographs of the protein crystals (e.g. lysozyme,

thaumatin, bovine liver catalase and glucose isomerase), where

sample concentration was controlled by varying the flow rates

of dispersed fluids.29 Later, this device was upgraded by

coupling to a glass capillary and performing on-chip X-ray

diffraction to study protein crystallization.30 Gerdts et al. used a

similar principle that combined plug flow induced by flow

focusing and in situ X-ray diffraction to study protein

crystallization.31 Khvostichenko et al. developed a polymer

microfluidic platform using small-angle X-ray scattering to

screen the phase behaviour of lipid mesophases.32

Polymer characterization. An understanding of the phase

diagram of a polymer-solvent mixture is crucial to polymer

preparation, purification and characterization processes.33 Shim

et al. developed a two-layer-structured PDMS phase chip and

leveraged water permeation through PDMS to vary solute

concentration in microdroplets stored in wells, which

successfully measured the concentration-concentration phase

diagram of a mixture of a salt (ammonium sulfate) and a

polymer (poly-ethylene glycol).34 Selimovic developed another

phase chip to screen the temperature-composition phase

diagram of a polymer (ammonium sulfate) system and protein

crystallization (e.g. bovine protein and lysozyme).35 Mao et al.

detected the lower critical solution temperature and

temperature-composition phase diagram of mixed polymer

solutions by introducing temperature gradients in micro-

capillary tubes.36,37 Further, Zhou et al. developed a single-layer

PDMS microchamber to construct the temperature-

composition phase diagram of PNIPAM solutions point by

point.38 Shi et al. continued this work by using the design of the

PDMS microchip with glass capillary 30, and mapping the phase

diagram of PNIPAM solutions under a dark-field microscope.39

Later, Shangguan et al. upgraded the previous device by

replacing PDMS with chemically resistant Teflon and using a

new configuration with low-angle laser scattering to study the

critical temperature and critical concentrations of poly(vinyl

acetate) in solvents of isobutanol and benzene.40

Salt characterization. The precipitation of salts is central to a

wide range of applications. Leng et al. designed a PDMS

microfluidic chip to quantify the crystallization of potassium

chloride (KCl) in water and the temperature-composition phase

diagram of binary systems of docusate sodium salt and

water.41,42 Laval et al. successfully measured the temperature-

concentration phase diagram of adipic acid in water in a PDMS

microfluidic device with temperature gradient set by Peltier

modules and concentration gradients set by varying the flow

rate of solute in dispersed droplets.43 The principle has been

further utilized to study the nucleation kinetics and polymorphs

of an aqueous solution of potassium nitrate.44

Surfactant characterization. Lee et al. successfully generated

the ternary composition phase diagram of a surfactant system

by detecting the fluorescence signal intensity of a mixed fluid

sample in a glass microfluidic chip.45 Blumenschein et al. utilized

magnetic particles as liquid carriers in a microfluidic tube to

detect the phase change of water and surfactant.46

4. Microfluidic phase change characterization for energy

4.1 Microfluidic PVT

PVT systems generally investigate fluids at reservoir conditions

(elevated pressures, high temperatures and harsh chemical

environments) and thus typical microfluidic device materials

(PDMS or PMMA) used for biological applications are no longer

applicable. Marre et al. developed silicon-glass based micro-

reactors and associated packaging hardware for high-pressure

(up to 30 MPa) and high-temperature (up to 400 ºC)

petrochemical applications.47 The flat glass cover allowed clear

visualization of phase and transport behaviour in microchannels

etched in the silicon wafer. Compared to wet-etching (HF) glass-

glass devices, the silicon-glass device demonstrated advantages

in terms of feature resolutions and high depth-to-width ratios

owing to deep reactive ion etching (DRIE). The maximum

pressure depends largely on the total area of the channels,

bonding quality and access wells. The use of overburden cells

has recently enabled pressures up to 86 MPa (Table 1).48,49

While expensive and labour intensive to fabricate, silicon-glass

microfluidics are the norm for microfluidic PVT systems.

Mostowfi et al. developed the first microfluidic PVT system

capable of detecting the bubble point of multi-component

hydrocarbons (Table 1).50 The device was fabricated by

standard silicon lithography, reactive ion etching and anodic

bonding. As shown in Fig. 2a, hydrocarbon mixtures in the liquid

phase flowed into a long serpentine channel which exerted a

pressure drop along the channel. Geometrical restrictions along

the microchannel assisted bubble nucleation at locations where

pressure dropped below the bubble point. Importantly, local

pressure along the microchannel was monitored by integrated

membrane-based pressure sensors. This microfluidic PVT

system significantly improved measurement time, reducing

sampling time to ~15 mins from the hours required for

conventional PVT. The long serpentine microchannel design

was further applied in the measurement of equilibrium gas-oil

ratios of crude oil (Table 1).51 Adequate mixing of gas and oil

phases is necessary to guarantee an accurate measurement at

equilibrium. The conventional direct flash method relies on

operator experience, which is unreliable. By using a microfluidic

approach, live crude oil flows through the long microchannel

and experiences different phase patterns from single-phase

liquid to two-phase slug flow and annular flow, as pressure

decreases linearly with travel distance. An image of the

microfluidic chip is shown in Fig. 2b. The same chip design was

applied to conduct constant-composition expansion tests on

black oil samples.

Pinho et al. developed a microfluidic approach to investigate

the thermodynamics of multicomponent fluid samples (CO2 +

cyclohexane) at high pressures and temperatures (Table 1).52 To

accommodate harsh pressure and temperature conditions, a

long serpentine microchannel design was implemented, which

contained a hollow insulating zone developed earlier by Marre

et al.,47 as shown in Fig. 2c. Dew point and bubble point were

detected by variation of temperatures at isobaric conditions.

The P-T phase diagram was then constructed by connecting a

series of bubble and dew points. The intersection of the bubble

point line and dew point line corresponds to the critical point.

This technique also allowed for a “stop-flow” mode of operation

by adding a low-resistance bypass flow path in parallel which

substantially reduced the flow speed in microchannels. Using

the stop-flow mode enabled the capture of quality images at a

lower camera sampling rate (4 fps) compared to that required

for continuous flow (300 fps). In addition, P-x and T-x phase

diagrams were generated by varying the mixing ratio of the

binary component sample.

Recently, Bao et al. developed a microfluidic device to

measure the P-T phase diagram in a single run by multiplexing

multiple pressure and temperature conditions on a single

device (Table 1).53 As shown in Fig. 2d, linear pressure gradients

were established by pressure drop in the resistor channel, while

linear temperature gradients were obtained by positioning a

heater and a chiller on the microfluidic chip. The orthogonal

pressure and temperature gradients generated 10,000

individual P-T conditions, which enabled the measurement of a

phase diagram in a single run. The multiplexing strategy

demonstrated a hundred-fold improvement in measurement

speed compared to conventional PVT methods.

Song et al. developed a microfluidic chip to ascertain the

role of impurities within an industrial supercritical CO2 stream

(ultralow content < 0.5%) on dew formation (Table 1),54 as

shown in Fig. 2e. Water dew point was detected by sweeping

pressures (15 min per step for equilibrium) at constant

temperatures. This approach led to a 3-fold reduction in error

compared to other methods.

Togo et al. developed a Teflon capillary tube-based

microfluidic apparatus to observe the liquid-liquid slug flow of

mixtures of aromatic hydrocarbons and water (Table 1),55 as

shown in Fig. 2f. The phase equilibrium was determined by a

PVT equilibrium cell with a transparent sapphire window and a

stainless steel capillary tube used as a pre-heating coil. The

microscale capillary tube improved the mixing of two phases. P-

x phase diagrams of the binary systems were generated and

showed strong agreement with literature data and predictions

by the equation of state.

Luther et al. determined gas-liquid phase equilibriums of

binary and ternary mixtures by using Raman spectroscopy to

investigate segmented flows in a glass capillary (Table 1),56 as

shown in Fig. 2g. The compositions of mixtures (CO2, water and

acetone) at elevated pressures were computed by the

measured Raman spectrum and a P-x phase diagram was

constructed. Spectroscopy has the advantages of being non-

invasive, highly-automated and commercial analytical tool that

can be applied at remote sites. However, spectroscopy

techniques are subject to interpretation errors caused by the

optical effects of thin films of liquid on the surface wall.

Sullivan et al. developed a glass-silicon-glass sandwich-

structured microfluidic chamber to detect the bubble point of

binary mixture using thermal pulses to stimulate bubble

nucleation (Table 1).57 As shown in Fig. 2h, bubble nucleation

was triggered by local pulse heating using an integrated

platinum electrode and detected by either microscope

observation or an embedded thermal conductivity sensor.

Bubble point was determined by observing the subsequent

behaviour of bubble (grow or shrink).

4.2 Microfluidic solubility measurement

Microfluidic phase behaviour characterization can be applied

not only to traditional PVT studies, but also to the study of

related fluid phenomena such as dissolution. In such cases

microfluidics offers the additional advantage of relatively rapid,

and observable, mass transfer owing to small diffusion lengths.

Energy applications requiring solubility data include: (i) CO2

capture and sequestration in reservoirs; (ii) CO2 enhanced oil

recovery; and (iii) interaction between oil (pure hydrocarbon,

crude oil, bitumen) and solvent (toluene, propane, butane)

during oil production.

Abolhasani et al. developed a silicon-glass-based

microfluidic platform to measure CO2 solubility in solvents using

co-injection (Table 1).58 CO2 and solvent were co-injected into a

long serpentine microchannel, as shown in Fig. 3a. The solubility

of CO2 was determined by visualizing and tracking the shrinkage

of the gas plug volume at multiple locations along the

microchannel. This method achieved both speed and accuracy

(2-5% deviation from literature data).

Lefortier et al. developed another visualization- and co-

injection-based microfluidic tool to screen CO2 solubility in pure

and mixed solvents (Table 1).59 Bubble shrinkage and expansion

Table 1. Summary of microfluidic phase characterization methods for energy

(c) (d)

(e)

(f) (g)

(i)

(a) (b)

(h)

Figure 2. Microfluidic PVT: (a) measurement of the bubble point and phase diagram; 50 (b) gas-oil ratio measurement; 51 (c) investigation of multicomponent system thermodynamics at high pressures and temperatures; 52 (d) direct measurement of the fluid phase diagram; 53 (e) dew point measurement of gas with impurities; 54 (f) measurement of P-x phase diagram using microfluidic mixing; 55 (g) determination of P-x phase diagram in microfluidic segmented flows using Raman spectroscopy; 56 (h) measurement of bubble point using on-chip heating. 57

Reproduced from ref. 50, 51 and 52 with permission from the Royal Society of Chemistry, from ref. 53, 54, 56 and 57 with permission from American Chemical Society, from ref. 55 with permission from Elsevier.

Measured property Principle Sample Device T (ºC) P (MPa) Speed Accuracy Ref.

bubble point, Vl/Vt - P diagram

single-phase injection / pressure gradient

methane + decane + propane + others

silicon-glass chip

22 4 < 15 min ± 2.5 % 50

gas-oil ratio + composition

single-phase injection / gas chromatography

Synthetic + real oil silicon-glass chip

15 < 37.6 a few min s.d.: 0.54 % 51

saturation pressure + volume ratio

single-phase injection / expansion

black oil silicon-glass chip

< 150

< 86

2-3 hrs < 2 % error 49

Bubble/dew point, critical point, P-T, T-x and P-x diagram

multiphase co-injection / pressure gradient

alkanes + CO2 + H2

silicon-glass chip

27-227 6 to 15 5 × faster than HPOC methods

2 % 52

P-T diagram, critical point

static with pressure + temperature gradient

CO2 / CO2 + N2 silicon-glass chip

12-32 5.5-8.5 3 hrs / 10,000 data

1.2 % error 53

dew point stepwise pressure variation

CO2 / CO2 + N2 + O2

glass chip 31-50 6.6-14.4 15 min per pressure

s.d: 4.84 % 54

P-x diagram composition

multiphase co-injection

p-xylene + water / decalin + water

Teflon / stainless steel tube

300-350 < 25 residence time: 1.6-3.2 min

±1 K, ±0.1MPa, ±0.004

55

bubble/dew point, P-x diagram

multiphase co-injection

acetone with CO2 + N2 combinations

glass capillary 30-60 6-10 NA 2-5 % 56

bubble point stepwise pressure variation

CO2 + hexadecane

silicon-glass chip

21 0.76, 0.53

NA 0.5 kPa 57

CO2 solubility multiphase co-injection

CO2 + DMC / He + DMC

silicon-glass chip

27, 30, 35, 39

0.10-0.13 5 min 2 to 5 % 58

CO2 solubility multiphase co-injection / bubble evolution

CO2 + solvents glass chip room temp.

0.1 NA Indicated by Figure

59

CO2 solubility multiphase co-injection / confocal Raman + optical

CO2 + salt water silicon-glass chip

22-100 1.1-10 <45 min NA 60

mass transfer rate multiphase co-injection / Raman

ethyl acetate + water + CO2

glass capillary 25, 37 8.5 0.8 to 8 s for equilibrium

NA 61

diffusion coefficients

multiphase co-injection / Raman

toluene + cyclohexane

silicon-glass chip

25, 40 and 55

NA 5 min 8.5 % deviation

62

CO2 diffusivity immobilized sample / diffusion model

CO2 + Athabasca bitumen

glass chip 21 1.0-5.6 ~ 10 min 8 to 18 % 63

toluene-bitumen mutual diff. coef.

pre-filling + contact toluene + Athabasca bitumen

Teflon chip 21 0.1 minutes ± 15 % deviation

64

CO2 diffusivity pre-filling + contact CO2 + water / CO2 + brine

PMMA chip 26 0.5-5 ~ 90 s 5 to 7% error 6 to 14% uncertainty

65

minimum miscibility pressure

pre-filling + new phase contact / fluorescence microscopy

CO2 + oils silicon-glass chip

25-60 5-12 < 30 min variations of 1-2 MPa

66

asphaltene / carboxylic acid content

multiphase co-injection / UV-VIS adsorption

biodegraded heavy oil

glass chip NA NA residence time < 3 s

± 5 % error 67

asphaltene content multiphase co-injection / optical absorption / weight

crude oil glass chip NA 0.1 < 30 min ± 10 to 50 % relative error

68

asphaltene yield curve

multiphase co-injection / optical absorption spectroscopy

crude oil glass chip 22-24, 65 0.1 1 day < 20 % error

69

asphaltene solubility multiphase co-injection

crude oil glass chip 30, 50, 70

0.1 35 min 0.6 % error 70

asphaltene deposition

multiphase co-injection / UV-VIS spectroscopy

asphaltene from Wyoming crude oil

silicon-glass microreactor

70 0.1 residence time < 5 min

NA 71

wax appearance temperature

single-phase injection / pressure

hydrocarbons and crude oil

glass chip -5-70 0-3.4 1 to 2 hrs < 1 oC error 72

salt precipitation pre-filling + flooding / microscopy

CO2 + salt water PMMA / glass chip

21 NA NA -6 to 14 % 73

in the glass microfluidic chip was observed using an optical

microscope, as shown in Fig. 3b. Solubility data of pure, binary

and ternary solvent mixtures were rapidly calculated by a

solubility model based on measured bubble volumes. In

addition to visualization, other probing tools such as Raman

spectroscopy were also utilized to measure solubility in

microfluidics.

Liu et al. developed a silicon-glass microfluidic chip to study

CO2 solubility in water and brine by using confocal Raman

spectroscopy, as shown in Fig. 3c (Table 1).60 The relationship

between CO2 solubility and Raman band intensity ratios

(ICO2/IH2O) was established by fitting a third order polynomial

function. This microfluidic method took 45 min to reach

equilibrium, which is long in comparison to other microfluidic

approaches but still short as compared to the 24 hrs of

relaxation time generally required in macroscale methods.

Luther et al. developed another optical microscopy and

Raman spectroscopy-based microfluidic tool (a glass capillary)

to investigate mass transfer in multiphase mixtures composed

of oil, water and CO2 (Table 1).61 The co-injection method

allowed variation of composition by changing the flowrate of

each constituent. Fig. 3d shows the observed reduction of

bubble volume and appearance of the emulsion system with

increasing residence time in the micro-capillary tube. Chemical

composition was also characterized by Raman spectroscopy as

previously.56

4.3 Microfluidic diffusivity measurement

Microfluidic approaches to the measurement of diffusivity have

also had a strong impact on the field. In both carbon

sequestration and enhanced oil recovery, it is important to

understand the mass transport behaviour of injected gas as it

flows into the reservoir. In addition to benefits in terms of fast

heat and mass transfer, the reduced volume within microfluidic

devices reduces or removes bulk influences, such as convection

and gravity, and allows simple diffusion measurements not

possible in bulk systems.

Solvents & oil. Lin et al. measured the diffusion coefficients of

a toluene-cyclohexane mixture in a microfluidic device using

confocal Raman microscopy (Table 1).62 As shown in Fig. 4a, this

work used a Y-shaped co-flow design to mix two constituents

and Raman microscopy to quantify the fraction of each

constituent. The diffusivity was calculated based on the

extracted diffusion data and a 1-D diffusion model.

Diffusion of viscosity-reducing solvent into an oil phase is of

particular interest for the recovery of heavy and extra heavy oil.

Bitumen typically refers to extra heavy oils with a viscosity

higher than 10 Pa s or American Petroleum Institute (API)

gravity less than 10º. In recent years, a few microfluidic

methods have been developed to quantify diffusion within

bitumen, which leverage high efficiency microfluidics heat and

mass transfer. Fadaei et al. measured CO2 diffusivity in bitumen

at elevated pressures within a 1-D glass straight microchannel

(Table 1).63 As shown in Fig. 4b, the diffusivity was calculated

based on the measured oil swelling data over time and a 1-D

diffusion model. The measurement required ~10 min.

Importantly, the obtained result showed strong agreement with

literature data of the conventional method (pressure decay and

volume change). The effective ‘trigger’ that started the diffusion

process was a rapid increase in the CO2 pressure. However, this

method did not readily translate to application with a liquid-

phase solvent, as significant diffusion occurred from first

contact of the liquid solvent and oil. For this purpose, Fadaei et

al. employed a T-shaped microfluidic chip to guarantee a sharp

initial condition for toluene-bitumen diffusivity measurement

(Table 1).64 As shown in Fig. 4c, toluene was introduced into the

bitumen within a dead-end channel with the diffusion

measurement starting at first contact. Toluene mass fraction

was correlated to the transmitted light intensity using bright

field microscopy. Similarly, diffusivity data was obtained using

the measured concentration map and a 1-D diffusion model.

CO2 & water. Also of relevance to probing phase-relevant fluid

phenomena, Sell et al. measured CO2 diffusivity in water and

brine within a PMMA microfluidic chip (Table 1).65 As shown in

Fig. 4d, a T-junction design enabled a fresh CO2-liquid interface.

The concentration of CO2 in water and brine was correlated to

local pH level indicated by a pH-dependent dye by using

fluorescence microscopy. The diffusivity of CO2 was determined

using the derived CO2 concentration distribution and a 1-D

diffusion model. This study provided insightful data for carbon

sequestration in saline aquifers. Sequestration, however, is not

as common a practice as CO2-enhanced oil recovery, largely due

to economics.

4.4 Microfluidic miscibility measurement

Microfluidics enabled the rapid measurement of minimum

miscibility pressure (MMP) between injected CO2 and the oil

phase - a key parameter in CO2 enhanced oil recovery. At MMP,

the interfacial tension between the CO2 and the oil phase

vanishes forming a homogeneous miscible phase. Conventional

MMP measurement methods suffer from long experiment time

(days to weeks) and operator dependency. Nguyen et al.

developed a fluorescence-based microfluidic method to

Figure 3. Microfluidic solubility: (a) measurement of CO2 solubility in solvents by evaluating gas plug sizes; 58 (b) rapid screening of CO2 solubility in pure and mixed solvents; 59 (c) studying CO2 solubility in water and brine using confocal Raman spectroscopy; 60 (d) investigation of multiphase CO2, oil and water. 61 Reproduced from ref. 58, 59 with permission from the Royal Society of Chemistry, from ref. 60, 61 with permission from Elsevier.

visualize and measure the MMP of CO2 in synthetic and crude

oil samples (Table 1).66 As presented in Fig. 4e, the sharp oil-CO2

interface vanishes at MMP. Crude oil is naturally fluorescent

due to the aromatic components, allowing easy differentiation

of the oil phase from CO2 using fluorescence microscopy. This

microfluidic approach achieved an accuracy (0.5 MPa)

competitive with the gold standard rising bubble apparatus

method within a much shorter time (30 mins).

4.5 Microfluidic precipitation measurement

Asphaltenes. Liquid-solid phase change is relevant to a range of

energy applications. The precipitation and deposition of solids

within oil and gas reservoirs is of particularly interest.

Asphaltenes are a group of heavy and semi-solid sub-

components found in crude oils, along with lighter sub-

components of saturates (e.g. alkanes), aromatics and resins.

Asphaltenes are defined by their solubility. Current techniques

to quantify asphaltenes in crude oil depend on solvent-based

precipitation and weighing of the asphaltene aggregates as

found in the ASTM protocol.74 The drawbacks of the standard

approach include large sample requirements (250 mL), long

waiting times (~2 days) and associated laboratory constraints

and costs.

The measurement of asphaltene content within oil is an

industrial challenge that microfluidic technologies are well-

positioned to address. Bowden et al. determined the

asphaltene and carboxylic acid content of heavy oil with an H-

shaped glass microfluidic chip applying optical spectroscopy

and compositional analysis (Table 1).67 As shown in Fig. 5a,

asphaltenes were precipitated by co-injection of extracting

phase and oil samples in the microchannel. Deasphalted oil

(maltene) phase from the collection vial was characterized using

UV-VIS adsorption spectra and GC-MS. Later, Schneider et al.

integrated the co-injection, filtration and optical spectroscopy

onto a glass microfluidic device to quantify the asphaltene

weight content of crude oil samples (Table 1).68 As shown in Fig.

5b, a microfluidic channel was used for co-flow and mixing

between oil phase and solvent (toluene) or precipitant

(heptane). The filter membrane installed downstream collected

any precipitated asphaltene. Absorbance spectroscopy was

applied on the toluene-diluted oil and heptane-deasphalted oil.

It was found that the asphaltene content was linearly

(a) (c)

(b) (d)

Figure 4. Microfluidic diffusivity and miscibility: (a) measurement of toluene-cyclohexane diffusion coefficients using confocal Raman microscopy; 62 (b) rapid measurement of CO2-bitumen diffusivity; 63 (c) measurement of bitumen-toluene mutual diffusion coefficients; 64 (d) measurement of CO2-water/brine diffusivity; 65 (e) fast measurement of minimum miscibility pressure of CO2 in crude oils. 66 Reproduced from ref. 62 with permission from Elsevier, from ref. 63, 64, 65 and 66 with permission from American Chemical Society.

proportional to the difference in absorbance spectra between

the diluted oil and deasphalted oil. Notably, this microfluidic-

spectroscopy approach reduced the asphaltene measurement

time from days to minutes, reduced oil sample requirements by

orders of magnitude, and demonstrated high repeatability.

Later, Sieben et al. measured the asphaltene yield curve and

evaluated the asphaltene solubility by varying the titrant-oil

ratios (Table 1).69,70 These works have been developed and

commercialized to form the world’s first petroleum analysis

instrument using microfluidics – an early and important

commercial offering in this area. Hu et al. investigated

asphaltene deposition in a glass packed-bed microreactor to

investigate the role of geometry on the precipitation process

(Table 1).71 Microscale quartz particles were packed in the

microchamber (~40% porosity) to mimic the porous reservoirs.

Fig. 5c shows the packed-bed microreactor capable of

withstanding high pressures and the visualized asphaltene

deposition process as a function of time. Precipitated

asphaltene percentage was quantitatively determined by UV-

VIS spectra.

Wax appearance. Another challenge in the oil and gas industry

where microfluidics is having an impact is the precipitation of

paraffin wax from reservoir fluids. Waxes appear as fluid

temperature decreases during production. The undesired solid

phase can easily cause agglomeration and therefore impact the

production and transportation. Wax appearance temperature,

the highest temperature at which wax begins to crystallize is a

crucial flow parameter. A common approach to quantify the

‘wax appearance temperature’ is through cross-polar

microscopy. However, this method depends on the skill and

experience of the operator. Molla et al. developed an operator-

independent microfluidic technique to measure the wax

appearance temperature (Table 1).72 As shown in Fig. 5d, the

glass microfluidic chip houses a long serpentine channel.

External pressure drop along the channel is required to

generate a continuous flow of oil sample in the microchannel.

Wax deposition on the channel wall reduces the channel cross-

section and thus elevates the pressure required to maintain a

constant flowrate. Therefore, wax formation can be simply

measured by a pressure increase. Fig. 5e shows the visualization

of wax formation using a microscope.

CO2 sequestration. An analogous challenge arises in carbon

sequestration. During the injection of CO2 into saline aquifers,

(a) (b)

(c)

(d)

(e)

(b)(a)

(c) (d) (e)

Figure 5. Microfluidic precipitation: (a) determination of asphaltene content using UV-VIS spectroscopy and GC-MS; 67 (b) measurement of asphaltene content, asphaltene yield curves and asphaltene solubility using optical spectroscopy; 68, 69, 70 (c) investigation of asphaltene deposition in porous media using UV-VIS spectroscopy; 71 (d) measurement of wax appearance temperature of hydrocarbons; 72 (e) investigation of salt precipitation during CO2 injection into brine. 73 Reproduced from ref. 67, 69, 71, 72, 73 with permission from the Royal Society of Chemistry, from ref. 68, 70 with permission from American Chemical Society.

the drying effect of the CO2 results in salt precipitation that

blocks pores within the media and thus slows or even stops the

process. Considering the complexity of porous media, it is

challenging to characterize the salt precipitation using existing

laboratory analysis. Kim et al. investigated pore-scale salt

precipitation dynamics using a transparent plastic microfluidic

chip (Table 1).73 As shown in Fig. 5e, a random 2D channel

network was patterned on the PMMA-made chip where CO2

was injected to the pre-filled brine solution. Salt crystal growth

was observed by using bright-field and fluorescence

microscopy. It was found that porosity decreased ~15% to ~25%

after CO2 flooding.

5. Nanofluidic phase change visualization

In contrast to microfluidic systems, research in phase behaviour

at the nanoscale has not been motivated by ease of use, speed

or relevance-to-bulk but rather applicability to nanoporous

systems, and buoyed by the boom in unconventional oil and gas

resources.75,76,77 The vast amount of hydrocarbons reside in

pores with sizes ranging from one to hundreds of

nanometers.78,79 Nanofluidic phenomena also play a central role

in, for instance, the transpiration and ascent of sap in trees,

drying stresses in soil and concrete, smart drug delivery

strategies, hydrodynamic cavitation in hydraulic valves and

around propeller blades and transport in synthetic trees.10

Models based on the equation of state can accommodate

factors such as capillary pressure, critical point shift, and solid-

fluid interactions.80,81,82,83,84,85 The Peng-Robinson equation of

state can be combined with engineering density functional

theory to study the phase behaviour of pure substances and

mixtures.86 Molecular dynamics simulations are also a powerful

tool to study adsorption and the transport of fluids near

surfaces under nano-confinement. Density at a position of few

molecular layers next to a wall has been found to be typically

higher than in the centre of nanopores, which is indicative of

the role of adsorption phenomena in nanopores.87,88,89,90 These

studies have predicted that fluid phase behaviour under nano-

sized confinement deviates from that in the bulk state. These

models, each using different factors, inevitably have led to

disagreeing predictions.80,81,82,83,84,85 Molecular dynamic

simulations compute molecular interactions and predict

molecular distributions in nanoconfined environments,

however, they are incapable of modelling complicated solid-

fluid interactions, surface roughness and surface wettability. A

gap exists between these models and practical operation. There

is an urgent need for measurements to verify these models and

inform onsite production. Recent studies on fluid phase

behaviour at the nanoscale are primarily motivated by

understanding the fundamentals of specific thermodynamic

processes, such as evaporation, cavitation, boiling,

condensation, adsorption and desorption.

5.1 Current nanofluidic phase change visualization approaches

Nanopores. Nagashima et al. achieved bubble nucleation in an

electrolyte solution within a nanopore with extreme

superheating by Joule heating using an electrical pulse.91

Witharana et al. measured the bubble nucleation temperature

of water in nano- and micro-sized cavities and posts and found

measurements agreed with predictions by the Young-Laplace

equation. The overall trend indicated that boiling in confined

geometries requires a higher superheating temperature.92

Nanoporous networks. Vincent et al. investigated the drying-

induced cavitation in porous media of nanometer-sized pores

and found that fluid dynamics were governed by the interaction

of deterministic mass transport and a stochastic nucleation

process.93 He et al. validated surfactant performance in oil

displacement experiment using a nanofluidic device with both

homogeneous and heterogeneous pore structures. Weakly

emulsifying surfactant was found to yield higher oil recovery

than a non-surfactant-bearing control fluid.94 Recently, Kelly et

al. developed a micro/nanofluidic 2D network that resembles

the granular and fractured nanoporous media of shale rock.95

An imbibition experiment was conducted on IPA-water and air

and visualized by differential interference contrast microscopy.

Gruener et al. studied the imbibition dynamics of alkanes in

nanometer-sized cylindrical pores.96

Packed beds and consolidated porous media. Ally et. al

visualized the condensation of propane and CO2 in packed beds

(Table 2).97 As shown in Fig. 6a, a silicon-glass microfluidic chip

was filled with silica particles (150 nm diameter) which formed

a packed bed of pore throats with an average diameter of 15

nm. Direct observation of the colour and appearance of the

packed beds was enabled by optical microscopy. The bottom

part of Fig. 6a shows the colour change in the packed bed during

propane condensation under isothermal pressurization. The

results indicate that capillary condensation depends on pore

geometry and wettability, and that complete pore filling occurs

at a meniscus radius much below the interstitial pore radii of

the packed bed.

There has also been recent work in nano-scale capillary-

driven flow and imbibition triggered by capillary condensation

using a 3D porous silicon layer.98,99 3D porous silicon layer

represents an important 3D structure for future nanofluidic

phase studies.

Nanochannels. Nanofluidic visualization experiments have

enabled direct observation of flow behaviours in nanochannels,

including capillary filling, imbibition, pressure-driven flow and

multiphase displacement measurements. Duan et al. conducted

an evaporation-induced cavitation experiment in nanochannels

and found a unique phenomenon, where water menisci

remained pinned at the entrance while vapour columns expand

inside.100 Eijkel et al. found that corner flow effects can strongly

accelerate the drying process in nanochannels.101 Tas et al.

studied the capillary filling of water in 100-nm deep channels. It

was found that fluid filling in nanochannels behaved as

predicted by the Washburn model. However, apparent viscosity

was elevated due to electro-viscous effects at the boundary

surface.102 Phan et al. investigated the capillary filling in dead-

end nanochannels and observed two distinct stages.103 Wu et

al. studied pressure-driven flow and observed stratified flow

and annular flow in the liquid and gas displacement experiment

in 100 nm deep channels.104 Liu et al. studied oil/water

displacement in 500 nm deep channels using epi-fluorescence

and confocal microscopy. A residual water phase was found in

the oil phase that can increase superficial velocity and

potentially affect the permeability.105 Chauvet et al. examined

the roles of trapped gas bubbles in capillary filling of

nanoslits.106

Wang et al. conducted preliminary experiments to study the

evaporation of alkane in nanochannels by controlled heating at

atmospheric pressure (Table 2).107 The nanofluidic chip had two

parallel microchannels bridging a series of parallel

nanochannels (5 μm wide and 100 nm deep) orthogonal to

microchannels, which is the same design used in the transport

study by Wu et al.104 The evaporation process of alkanes was

observed by optical microscopy. As shown in the left side of Fig.

6b, nominally pure alkane (n-pentane) first evaporated in the

microchannel and then propagated into the nanochannels.

However, for a ternary alkane mixture, the evaporation

meniscus slowed down and stopped in the microchannel

without advancing into the nanochannels. This problem

occurred because of the fractionation process, where lighter

components exiting the mixture increase the boiling point of the

remaining heavier component in the liquid phase. The issue

demonstrated here is a challenge broadly for phase evaporation

experiments using both micro- and nanoscale channels. For

instance, if care is not taken, evaporation of light components

from the microchannels can change the composition of the fluid

in the nanochannels, by increasing the concentration of heavier

components. Using a similar design, Parsa et al. studied and

observed the reverse phase change process: condensation of

propane under nanoconfinement (Table 2).108 Condensation

was induced by isothermal pressurization in nanofluidic devices

with different depths and designs. As shown in Fig. 6c, one

nanofluidic device used the same design as presented in Fig. 6b,

while the other was patterned by a 2D random network

between two microchannels. It was found that the

condensation pressure in the 50 nm and 30 nm channels was

lower than the saturation pressure, while there was no

detectable difference for a 500 nm channel.

Alfi et. al investigated the nanoconfinement effect on

hydrocarbon phase behaviours in nanofluidic channels using

optical microscopy (Table 2).109 As shown in Fig. 6d, the glass

nanofluidic chip (width 5 μm, depth 50 nm) has the same design

as that used in the works by Wang and Parsa (See Fig. 6b and

6c). Isobaric heating induced bubble formation inside the

channels. Interestingly, the experiment result showed that the

confinement effect on the bubble point temperature was

negligible in the 50 nm channel. This study also indicated that

capillary-pressure-based models fail to predict the phase

behaviour under confinement and thus cannot be used for

precise reservoir simulations. Continuing this work, Alfi et al.

Figure 6. Nanofluidic phase characterization for energy applications: (a) condensation in nanoporous packed beds;97 (b) evaporation of alkanes in a nanochannel;107 (c) condensation of petroleum retrograde gas in nanochannels;108 (d) role of nanoconfinement on phase behavior and contact angle;109,110 (e) condensation in nanochannels;111 (f) bubble nucleation and growth in nanochannels.112 Reproduced from ref. 107, 108 with permission from Society of Petroleum Engineer, ref. 109 with permission from Elsevier, from ref. 97, 110 and 111 with permission from American Chemical Society, from ref. 112 with permission from Royal Society of Chemistry.

studied the nanoconfinement effect on contact angles using

the same device but with a shallower depth (10 nm) (Table 2).110

The authors attempted to measure the contact angle, and made

observations. However, we would question the validity of the

contact angle measurement here. This work highlights another

challenge of nanofluidic phase-based measurements, namely

that the contact angle observed in-plane (i.e. that imaged using

a conventional microscope) is not the relevant contact angle.

The relevant contact angle is that exhibited in the depthwise,

nanoscale direction and that is not possible to image with

Table 2. Summary of nanofluidic phase characterization methods for energy

Subject Principle Samples Device T (oC) P (MPa) Key findings Ref.

evaporation

of alkane in

nanochannel

1D nanochannel;

isobaric heating;

visualization;

n-pentane,

n-butane

+i-butane +

n-octane

silicon-

glass chip

72 0.1 1) Pure alkane vaporates first in microchannel and

then nanochannel;

2) Ternary alkane mixture vaporizes in microchannel

but meniscus cannot propagate into nanochannels

because of liberation of lighter component increases

the bubble point of remaining liquid.

107

condensation

/ vaporization

in

nanochannel

1D nanochannel

(straight channel /

random network);

isothermal

pressure

variation;

visualization

propane silicon-

glass chip

18-21 0.7 -0.8 1) 500 nm: condensation and vaporization pressures

are close to Psat

2) 50 and 30 nm: condensation and vaporization

pressure are lower than Psat. Comparison with Kelvin

Equation is good in some cases but not in others.

108

condensation

in nanopores

porous packed

beds; isothermal

pressurization;

visualization;

propane

CO2

packed

beds of

silica

particles

21 1 - 6 1) Capillary condensation depends on pore geometry

and wettability;

2) Complete pore filling occurs at a meniscus radius

well below the interstitial pore radii of the packed

bed.

97

confinement

effect of

phase

behaviour in

nanochannels

1D nanochannel;

isobaric heating;

visualization

hexane

heptane

octane

glass-

glass chip

69 -125 0.1 1) 50 nm: confinement effect in the form of wall-

molecule interactions is almost negligible;

2) capillary-pressure-based models fail to describe

the phase behaviour of hydrocarbons inside confined

media such as 50 nm pore.

109

nanoconfine

ment effect

on contact

angle of

hydrocarbon

phase

1D nanochannel;

visualization (epi-

fluorescence)

confocal

microscopy

hexane

heptane

octane

glass-

glass chip

23 -127 0.1 1) There is a significant deviation in contact angle

within 10-nm pores compared to bulk state –

nanoconfinement reduces contact angle;

2) Contact angle in nanoconfinement is also

temperature-dependent – higher temperature yields

to a smaller contact angle.

110

condensation

in

nanochannels

1D nanochannel;

isothermal +

isobaric;

visualization;

propane silicon-

glass chip

41 - 70 2 - 3.4 1) The rate of liquid condensate growth in

nanochannels is limited mainly by vapour flow

resistance;

2) There is no capillary condensation detected for

propane at 70 nm.

111

bubble

nucleation

and growth in

nanochannels

1D nanochannel;

isothermal

pressure

variation;

visualization;

brightfield

microscopy

propane silicon-

glass chip

70 -103 1.1 - 4.2 1) Hydrocarbons remain in the liquid phase in

nanochannels under pressures much lower than the

saturation pressure (closer to spinodal limit than

classical nucleation theory);

112

2) Depending on the initial nucleation location –

along the channel or at the end – two types of

bubble growth dynamics were observed.

existing methods. While there may be ways to deduce wetting

aspects from the in-plane measurement, it is critical that

researchers differentiate that observation from that of the

dominant contact angle in the nanoscale dimension.

To avoid the fractionation problem encountered by Wang

(see Fig. 6b), new strategies have been developed to ensure

phase change occurs in nanochannel (and not neighbouring

microchannels) by introducing local temperature controls.

Zhong et al. conducted a propane condensation experiment in

a dead-end nanochannel (70 nm deep, 5.5 μm wide) by cooling

the nanochannel while heating the service microchannel (Table

2).111 shown in Fig. 6e, an interesting condensation

phenomenon was observed, where a liquid bridge formed in the

vapour phase. Importantly, it was found that the condensate

growth rate in nanochannels was limited mainly by vapour flow

resistance. This study also claimed that capillary condensation

was absent in 70 nm confinement.

Bao et al. studied bubble nucleation and growth, in a series

of dead-end nanochannels (88 nm deep, 7 μm wide) (Table

2).112 Similarly, nanochannel regions were locally heated at a

controlled temperature while the service microchannel was

cooled. This local heating ensured that bubble nucleation

initially occurred only in the nanochannels. Fig. 6f presents the

observed bubble nucleation and growth in channels under the

isothermal pressure drawdown process, which mimics the

production path with decreased pressures. This work claimed

that propane remains in the liquid phase in nanochannels under

pressures much lower than the saturation pressure. The

nucleation conditions in terms of temperature and pressure

were found to be closer to the spinodal limit than classical

nucleation theory. Additionally, two types of bubble growth

dynamics were observed depending on the initial nucleation

location, either along the channel or at the channel end.

6. Challenges and future directions

6.1 Achieving promised multiplexing

To date, micro/nano-fluidic phase visualization approaches

have not demonstrated the extent of multiplexing possible

within the planar area of chips, let alone the third dimension.

Instead single channels are employed in the majority of cases.

Recent work with 10,000 phase measurement wells address this

issue partially, in the context of temperature and pressure, but

the challenge of precisely multiplexing concentration in such a

manner remains.53 Here the community can look to low

temperature, low pressure gradient generator methods for

some inspiration. Other such methods, such as hydrogel

gradients, are not likely to be applicable here.

6.2 Simplifying supporting hardware

Microfluidic phase measurements suffer from the same

criticisms of many biomedical microfluidics in that large

supporting infrastructure is generally required. The resulting

systems are not as portable as one would hope given the scale

of the device features. This criticism is even more applicable to

microfluidics for energy applications to date as the hardware

required is significantly larger, and often an order of magnitude

more expensive (i.e. traditional microfluidic syringe pumps

~1,000 USD, vs high-pressure, high-temperature pumps

~10,000 USD).

6.3 Developing products for an energy market

It is important to recognize that the market for phase

measurements is very different from that of most biomedical

diagnostics. The market is necessarily more niche, the cost-per-

test ceiling is higher, and the volume of tests is lower. Unlike

many microfluidic tests which ultimately aim for widespread

application by unexperienced users, microfluidic phase

measurements will generally be performed by specialists for

specialists. There are additional opportunities, however, for

distributed testing in energy applications, for instance regular

testing of hydrocarbons on pipelines or distribution networks.

6.4 Exploring new chip materials

To achieve commercial success, the development of new

microfluidic phase characterization devices will require faster,

low cost prototyping methods. Whereas other microfluidic

fields have leveraged low-cost materials such as plastics,

silicone polymers (PDMS), glass or paper, energy studies have

largely focused on strong materials capable of withstanding the

harsh physical and chemical conditions required for oil and gas

samples. Whereas silicon-glass-based microfluidic devices have

emerged as the standard for energy microfluidics, work is

needed to improve the efficiency of the microfabrication

process, which requires delicate cleanroom fabrication over

several days using photolithography and etching processes. For

certain applications, micromachining and laser ablation may

represent acceptable alternatives: micro-milling has been used

to manufacture 50 µm features on metal substrates and laser

cutting (vector and raster) has been demonstrated on silicon

wafers (Oxford lasers). While the quality and feature size are

inferior, the resolution may be sufficient for some applications.

Likewise, the community would benefit from exploring

alternative materials that – while not as robust as silicon-glass –

may provide sufficient resilience for a given application.

6.5 Further expanding pressures and temperature ranges

The harsh environments associated with energy applications

have led to the development of highly customized microfluidic

systems, capable of not-only withstanding high temperatures

and pressures, but providing an optical window on phase

behaviour. Silicon-glass and custom enclosures have enabled

studies at temperatures and pressures up to 400 ºC and 86 MPa,

respectively. Further improvements in experimental design,

however, will be necessary to achieve even more extreme

pressures and temperatures. Natural gas transportation in

northern environments, for example, will require phase studies

at low temperatures. Achieving either very high or very low

pressures and temperatures for micro/nano-fluidic phase

studies will require more robust materials as well as better

manifolds and infrastructure (O-rings, transducers, long

working-distance objectives). Likewise the deep off-shore oil

wells and deep hydraulic fracturing operations can exceed ~ 100

MPa in pressure. Studying such systems is not possible with

current micro and nanofluidic approaches.

6.6 Shifting from 2D to 3D

Constrained by fabrication processes and standard microscopy

methods, current microfluidic devices for phase

characterization are limited to 2D planar patterns. Typical

microfluidic fabrication methods including soft lithography and

hot-embossing (e.g. plastics), wet etching (e.g. glass), ion-

reactive etching (e.g. silicon), micromachining (e.g. steel) and

imprinting (e.g. paper), cannot produce complex 3D structures.

Current microfluidic studies that use 3D geometries are mostly

in natural porous media, packed particle beds, or composites

with regular shapes and cyclical patterns. Hydrocarbons,

however, reside in a highly heterogeneous 3D network. There is

a need to characterize phase properties inside 3D porous

structures with representative reservoir features.113 Additive

manufacturing, or 3D printing, is an emerging technique that

may contribute in this regard – particularly the now

commercially available and increasingly capable two-photon

printing capabilities. Alternatively, incorporating porous

substrates into microfluidic systems for study is a promising

approach, demonstrated for instance with nanoporous

silicon.98,99

6.7 Characterizing natural gas

In the context of energy applications, microfluidic phase

characterization techniques have been studied intensively in

CO2, alkanes, and other hydrocarbons, but not in natural gas or

methane. Such a gap in knowledge is ever more striking given

the rich phase change phenomena involved in natural gas

processes and their importance to recovery, transport, storage

and utilization. The absence of natural gas phase studies in

literature is in part due to the technical difficulties in reaching

the very low temperatures at which phase change occurs. With

the development of low temperature control systems, we see

potential opportunities in using microfluidic methods to

characterize natural gas phase behaviours to measure

hydrocarbon dew point or water dew point. Demand for such

technologies could stem from chemical plants and refineries

and associated thermophysical models they employ.

6.8 Increasing and improving nanoscale phase data

Given the often conflicting predictions from molecular

dynamics simulations and inherent challenges in properly

modelling surface roughness, wettability and broader solid-fluid

interactions, there is an urgent need to develop controlled

experiments to study nanofluidic phase change. The few

experiments that have been completed at the nanoscale have

shown discrepancies. For example, Parsa claimed that 50 nm

confinement led to a lower condensation pressure as compared

to bulk phase saturation pressure.108 However, Alfi stated that

50 nm confinement was negligible.109 Additionally, Luo declared

that 38 nm confinement played an insignificant role on bubble

point.114 More work in these areas is required to shed light on

important nanofluidic phase change phenomena, elucidate

what differences exist between bulk-scale and nanoscale phase

change behaviour, forge a better link with molecular dynamics

simulations, and explore the critical role of surface roughness,

adsorbed water, multicomponent systems, contact angle and

wettability.

6.9 Exploring sub-10 nm

Nanoscale phase change becomes increasingly interesting as

features are reduced to size of a few molecules. At such a small

scale, surface effects caused by interactions between the

surface and fluid samples are no longer negligible and begin to

change the fluid phase behaviour from what is expected at the

bulk scale. This change in behaviour has been modelled by a

variety of theories with different assumptions and observed by

a few visualization experiments as described previously. Though

they show discrepancy in quantitative result, there is no dispute

on the trend that the confinement effect becomes more

apparent as the scale decreases, especially in the sub-10 nm

regime. The molecular size of common substances of interest

range from sub-nanometer to a few nanometers (ex: water ~

0.3 nm, methane ~ 0.4 nm, decane ~ 1 nm 115 and hexadecane

~ 2.3 nm 116), further highlighting the interest in exploring

devices near that size. So far there has been no experimental

study on sub-10 nanometer phase behaviour using nanofluidic

devices (the closest work explored ion transport in 2 nm

channels).117 There is a need to explore the phase behaviour of

fluids in the sub-10 nm regime, and a great opportunity for tight

partnership between experimentalists and the molecular

modelling community, as the scales on which they work begin

to overlap.

7. Conclusion

This review paper comprehensively covers and assesses the

emerging field of microfluidics- and emerging nanofluidics-

based phase characterization studies for energy applications.

Microfluidic methods are generally motivated by improving the

time efficiency of fluid phase measurements, with a focus on

practical applications and displacing or informing traditional

methods. In contrast, nanofluidic approaches are motivated to

quantify and explain the fundamental physics of fluid behaviour

in nanoconfinement.

Recent developments in microfluidic PVT systems probing

liquid/vapour/supercritical phase transitions as well as

dew/bubble/critical points and phase diagrams (P-T, P-x, T-x)

were discussed. Related phase research areas, namely

microfluidic solubility, diffusivity, miscibility and precipitation

studies, were also explored. Lastly, developments in nanofluidic

phase measurements were discussed, where experiments are

pushing further and further into the nano-realm. Challenges

and trends within both the microfluidic and nanofluidic fields

were highlighted and discussed. There are tremendous

opportunities in the near future, particularly where researchers

can take advantage of the full microfluidic and nanofluidic tool

kits as well as expertise from adjacent fields such as

thermodynamics and molecular simulations. This space also

offers excellent opportunities for engagement with industry

and the development of spin-off companies.

Acknowledgements

The authors gratefully acknowledge support from the Natural

Sciences and Engineering Council of Canada through the

Collaborative Research and Development Grant Program, and

on-going funding through the Discovery Grants program,

Discovery Accelerator Supplement, the Canada Research Chairs

program, and the E.W.R. Steacie Memorial Fellowship (DS). The

authors also acknowledge support from Schlumberger Canada

Ltd and Alberta Innovates-Energy and Environment Solutions,

as well as infrastructure support through the Canada

Foundation for Innovation and Ontario Research Fund.

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