Super stable foams stabilized by colloidal ethyl cellulose particles

Dynamic Article LinksC<Soft Matter

Cite this: DOI: 10.1039/c1sm06518a

www.rsc.org/softmatter PAPER

Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8AView Online / Journal Homepage

Super stable foams stabilized by colloidal ethyl cellulose particles

Huajin Jin,a Weizheng Zhou,a Jian Cao,*a Simeon D. Stoyanov,*b Theodorus B. J. Blijdenstein,b

Peter W. N. de Groot,b Luben N. Arnaudovb and Edward G. Pelanb

Received 8th August 2011, Accepted 8th November 2011

DOI: 10.1039/c1sm06518a

Here we report the preparation of super stable liquid foams with various bubble sizes stabilized by

colloidal ethyl cellulose (EC) particles. What is novel and different in this particle stabilized foam is that

both the initial material (EC) and processes used are in principle food grade, thus it may offer scope in

food applications. The particles were prepared using a conventional anti-solvent precipitation method,

involving the dissolution of EC polymer into acetone, followed by fast mixing with water (anti-solvent),

leading to the precipitation of EC particles, then followed by the rotary evaporation of acetone. The

interfacial tension of the resulting dispersion is 36 mNm�1, indicating that particles co-exist with surface

active and water soluble components, which is most likely a low molecular weight EC fraction. The

average particle diameter is 0.13 mmand their zeta potential is�50mV at pH¼ 6, increasing to�5mV at

pH¼ 3. This negative surface potential is attributed to adsorption of hydroxyl ions, known to occur on

many hydrophobic surfaces, including oil–water, air–water and hydrophobic particle–water. As a result,

there is strong electrostatic repulsion between EC particles at neutral and low ionic strength, which

stabilizes EC dispersion and also significantly increases the adsorption barrier of EC particle at the air–

water interface. Due to their similar origin, both inter-particle repulsion and adsorption barrier can be

controlled by pH and/or ionic strength, which leads to dispersion destabilization and at the same time

good foamability and extreme foam stability at acidic conditions (pH< 4) and/ormoderate or high ionic

strengths (I > 20 mM). Foam coarsening shows an initial stage with coarsening time of approximately 1

week, followedby aplateau,where the coarsening has been arrested for a period ofmonths. Byusing cryo

scanning electronmicroscopy, we reveal that these EC foams are Pickering stabilized, where ECparticles

are closely packed at the air–water interface forming a single or multi-layers. We also show that super

stable EC foams can be prepared using various aeration techniques, allowing us to vary the bubble

diameter from a few microns to hundreds of microns.

1 Introduction

Liquid foams are foams where the continuous phase is not

solidified, gelled or cross linked. When the liquid is water, they

are also called aqueous foams. Aqueous foams occur as inter-

mediate and/or end products in a wide range of areas and

industries like food, home and personal care, medical and

pharmaceutical, construction, automotive, petrochemical and

mining.1 In addition, liquid foams have been also used as

a template to synthesize novel materials etc.2–4 Since wet foams

are thermodynamically unstable dispersed systems, which drain,

coarsen and coalesce,5 for their successful application it is

important to prolong and control their lifetime. The standard

strategy of controlling foam stability is to use conventional

surface active ingredients like synthetic or natural surfactants

aUnilever R&D Shanghai, 5/F, 66 Linxin Road, Shanghai, 200335, P. R.China. E-mail: [email protected] R&D Vlaardingen, Olivier van Noortlaan 120, 3133 ATVlaardingen, the Netherlands. E-mail: [email protected]

This journal is ª The Royal Society of Chemistry 2011

(emulsifiers), lipids, block co-polymers or proteins, which are

able to cover the bubble surfaces and thus to slow down the foam

collapse. Nevertheless, such foams typically survive only for few

hours due to processes of coalescence and disproportionation.6

In addition to foam stability issues, bubble size plays an

important role in many industrial foam applications. For

example, it is well known that small bubbles provide creamier

and smoother textures in ice and whipped cream.7 In applications

like ultrasonic imaging and targeted drug delivery, for both

safety and application reasons it is critical to be able to produce

small mono disperse bubbles, with diameter less than 10 mm.8–10

In this context, the challenge is even higher since smaller bubbles

have higher internal pressure, Pb¼ Pl + 2g/r (here g is the surface

tension and, Pl and Pb are the pressures in the foam liquid and in

the bubble with radius r respectively), which by using the ideal

gas law, translates into higher concentration of the gas inside

them. This coupled with the fact that the gas has finite solubility

in the continuous phase, and that in real foams small bubbles do

co-exist with bigger bubbles and with atmosphere, leads to faster

foam coarsening, where small bubbles shrink and large ones

Soft Matter

http://dx.doi.org/10.1039/c1sm06518a






http://pubs.rsc.org/en/journals/journal/SM

Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

grow.11 One strategy to stop or slow down this process is to use

low water soluble (or insoluble) surfactants, block copolymers

and/or proteins that are irreversibly attached to the air–water

interface for the time scales of the disproportionation process,

which could range from hours to days, months or years.

However, in this case the challenge shifts, since one needs to find

ways to deliver such types of surfactants at the interface, to

counterbalance their natural anti-foaming action. In some cases

this is possible, for example, Dressaire et al. have shown that it is

possible to prepare sub-micron micro-bubbles by covering them

with a layer of insoluble self assembled surfactant and demon-

strated that such bubbles can survive more than one year.11

An alternative approach that overcomes some of the above

mentioned difficulties is to use colloidal particle dispersions,

composed of particles with the right surface activity, size and

shape. This is also known as the Pickering stabilization mech-

anism and recently has drawn a lot of scientific and industrial

interest.3,4,12–27 The source of the Pickering stability is related to

the very high energy of colloidal particle attachment at the

air–water interface, which for spherical particles is E ¼ p r2 g

(1 � |cosq|)2, where q is the contact angle between the air–water

interface and the particle surface and r is the particle radii.

Even for nanosized particles, this energy could be of the order

of hundreds or thousands thermal energy units, thus exceeding

typical surfactant adsorption energies by orders of magnitude.13

However, what is seldom discussed in the literature is that in

order to adsorb such particles onto the air–water interface,

often it is necessary to overcome a large adsorption barrier of

the particle to the interface which could be of the order of the

particle attachment energy. The origin of such a barrier can be

of electrostatic or steric nature and sometimes the conventional

aeration processes are not able to provide sufficient energy to

overcome it, unless measures have been taken to lower it.

Therefore in order to obtain bubbles and liquid foams stable

for months, it is vital to control both the wettability of particles

and particle-interface interactions.

As discussed earlier, bubble size is another important aspect

for foam applications. This is especially true for the case of

spherical particle stabilized foams, where reported average

bubble size is of the order of tens and hundreds particle diameters

and normally lies in between hundreds of microns and few mil-

limetres, which typically is too large for many applications. To

our knowledge, there are limited number of papers in the liter-

ature, that describe particle stabilized foams with bubbles in the

range below 50–100mm.4,9,20,25

For the industrial application of Pickering stabilized foams in

foods there are additional challenges, since all the materials,

processes of making particles and the way they are applied

should be food grade and prepared using scalable and cost

effective industrial processes.27 In particular, most of the

colloidal particles that are capable of producing Pickering

stabilized foams are either made from non food grade materials

and/or processes or their surfaces have been modified using non

edible materials or a process that involves a chemical modifica-

tion.12,14–21,23,25 Currently, only a few kinds of food grade parti-

cles and rods were developed to stabilize air bubbles. For

example, Campbell et al. prepared anisotropic food-grade ethyl

cellulose (EC) micro-rods via a liquid–liquid dispersion method,

and the resultant rods showed potential application for enhanced

Soft Matter

stability of food foams.22,24 Zhou et al. modified micro-scale

CaCO3 rods and particles with fatty acids and obtained CaCO3

rods with suitable wettability. Stable CaCO3 foamwith a life time

of more than several months can be obtained by simply shaking

a dispersion of modified CaCO3.26 However, the bubble size of

these foams can only be tuned at a limited range, and micro-

bubbles in the order of 10 mm diameter cannot be achieved

because of the size and surface properties of the particles.

In this paper, we have decided to further investigate the

possibility of preparation of colloidal particles made from ethyl

cellulose (EC) and the usage of them as efficient stabilizers of

foams made with small bubbles. The reason we choose EC is that

it is accepted as a food additive in several markets (FDA GRAS

status28 and an E-number in the EU29). Ethyl cellulose is

a hydrophobic, non water soluble cellulose ether polymer,

comprising of an anhydroglucose (cellulose) repeating backbone,

where a high degree of hydroxy-groups on the anhydroglucose

are etherified with ethyl-groups.30 The main sources of cellulose

used in EC are cotton and wood and thus could be considered as

stainable sources. However the processes of cellulose extraction

and modification used to produce EC on industrial scale are

relatively harsh and solvents used are difficult to fully recover. As

a result, EC is one of the most expensive cellulose derivatives

available on the market today.

We report a simple and versatile approach to prepare EC

particles and describe their functionality to stabilize foams.

There are four main points we want to highlight as being novel

and interesting: (1) colloidal particles are derived from ethyl

cellulose using simple physical post-processing procedure; (2) EC

particle–particle and particle–air–water interface interactions

can be fine-tuned by using electrolytes or acids; (3) stable foams

with a life time of several months are formed by aerating EC

dispersion at the appropriate conditions; and (4) by using range

of different aeration methods, we demonstrate that bubbles with

different sizes (1–1000mm) are obtained.

The paper is organized as follows: firstly, the EC dispersion is

characterized towards particle size and z-potential, foamability

and foam stability. Secondly, morphology of the resulting

bubbles is investigated using optical and electron microscopy.

Subsequently, the application of different aeration methods

towards bubble size variation is discussed. Finally, the specific

disproportionation stability of EC particle stabilized foams is

investigated using turbidimetry.

2 Materials and methods

2.1 Materials

Ethyl cellulose, with an ethoxyl content of 48%, was purchased

from Sigma-Aldrich (247499-100G, batch: 08521KH). Rather

than giving the exact molecular weight, the supplier gives the

viscosity of 5.0 wt% ethyl cellulose in 80/20(v/v) toluene/ethanol

to be 100cps. Xanthan (Keltrol RD) was purchased from CP

Kelco (Beringen, Belgium). HCl, H2SO4, HAc, NaOH, NaCl,

and KCl were bought from Sinopharm Chemical Reagent Co.,

Ltd (China), and CaCl2 and MgCl2 were purchased from Sigma-

Aldrich. All the chemicals are used as received, unless otherwise

noted.



Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

2.2 Preparation of EC dispersion

The desired amount of ethyl cellulose powder was dissolved in

acetone, using a magnetic stirrer (IKA ETS-D5) at 30 �C for 30

min to obtain a clear 1.0 wt% EC solution. After complete

dissolution, the same volume of deionised water (anti-solvent)

was quickly poured into this solution under stirring. This abrupt

change of solvent quality resulted in a system that was super-

saturated with respect of EC, leading to the formation of EC

colloidal particles via a nucleation and growth mechanism. The

resulting turbid dispersion was then put in a rotary evaporator

(Buchi R-200, Heidolph), where all the acetone and part of the

water were removed to get 1.0 wt% or 2.0 wt% EC dispersion in

water. By using head space gas chromatography with a flame

ionized detector, we had measured that the residual level of

acetone in 1, 2 and 4 wt% EC dispersions was around 0.4 mg ml�1

irrespective of EC concentration. We also made EC particle

stabilized foams at low pH, left it to drain and collected the froth,

which was dissolved in ethanol and then the concentration of

acetone in the EC ethanol solution was measured. The residual

amount of acetone turned out to be below 0.01 mg ml�1 inde-

pendent of the EC concentration, indicating that there is hardly

any residual acetone trapped in the EC particles. The obtained

EC dispersions were stored in a fridge at a temperature of 4 �Cand brought to ambient temperature shortly before used.

2.3 The morphology of EC particles

The morphology of EC particles was investigated by trans-

mission electron microscopy (TEM) (JEM-1200EX). A drop of

EC dispersion (0.1 wt%) was dropped onto Cu grid, followed by

drying it at room temperature for TEM measurement.

2.4 Measurement of size and z-potential of EC particles

Diluted EC particle dispersion (ca. 0.1 wt%) was used to measure

the particle size by using a Malvern Zetasizer Nano ZEN 3600,

which combined dynamic light scattering (DLS) and electro-

phoretic mobility. The viscosity of water was assumed in all cases

and a refractive index of 1.59 was used in the analysis. In order

to obtain the pH dependence of z-potential and particle size,

0.1 wt% EC particle dispersion was titrated by adding 0.1 MHCl

aqueous solution or 0.1 M NaOH aqueous solution to tune the

pH from 9 to 2.

2.5 Surface tension

Surface tension of 1.0 and 2.0 wt% EC dispersion were measured

by a surface tensiometer (Kr€uss GmbH, Hamburg Germany)

based on the Wilhelmy plate method. Surface tension has been

measured for period of 1 h, which is sufficient to achieve the

equilibrium value for the system (surface tension flats off after

first 200–400 s).

2.6 Foam generation

EC foams were produced by various foaming techniques such as

hand-shaking or using various mechanical mixers. EC dispersion

(10 ml, 2.0 wt%) was put into a 25 ml measuring cylinder, and

some electrolytes and/or acids were added, followed by sealing it

with Para Film. The resultant EC dispersion was shaken by hand


for 2 min. Unless told otherwise, EC foams were produced by

shearing 2.0 wt% EC dispersion for 3 min by four types of low

and high shear mixers. The low shear mixers included Aerolatte

(Shanghai Dixi Electronic Company) and a semi-professional

kitchen mixer (Kenwood KM 800). High shear mixers included

a Silverson mixer (L5T, Silverson machine LTD) with an

Emulsor Screens head andUltra-Turrax homogenizer (IKA T25)

with an 18 mm diameter dispersion head. For the latter two

cases, foams were generated by having the mixing head centered

and placed well below the surface of the liquid and at least 1 cm

above the bottom of the vessel. The mixing was started at the

lowest rotation speed of the mixer, which had been gradually

increased to the desired level in order to avoid mechanical

entrapment of large bubbles via the hydrodynamic perturbation

of the upper surface in the stirring vessel. Following this process

we form hardly any foam at low rotational speeds and foams

with around 20–40% vol fraction of air at high rotational speeds.

Depending on the mixer and mixing head it seems that the

threshold value of foam formation seems to be around the point

where high velocity fluid exiting the mixing head reaches cavi-

tating conditions. The latter is registered by occurrence of

a typical cavitation sound and increased turbidity of the solution

due to the formation of cavitating bubbles (when solution is

sufficiently transparent) and by volume increase of the solution

in the beaker. This hypothesis will be further explored in

a follow-up work, where similar type of micro-bubbles foams is

made by using cavitating nozzles.31

2.7 Foamability and foam stability of EC dispersion

The foamability was estimated by measuring the ratio of the

foam volume immediately after preparation to the initial liquid

volume. The foam stability was assessed by monitoring foam

volume (total volume minus the volume of drained liquid) and

bubble size over time at ambient temperature.

2.8 Optical observation of EC bubbles

A drop of deionised water was placed on the top of a slide glass,

and a small amount of EC foam was gently put on top of the

water droplet, without covering with a cover glass. Then the

microscopy images of foam were obtained by using an optical

microscope fitted with a digital camera. (DM LB 2, Leica

Microsystems Ltd, Germany).

2.9 Cryo SEM of bubble surfaces

The microstructure of wet EC bubbles was investigated by cryo-

SEM. Namely a tiny piece of the foam was dropped on top of

a rivet, plunge-frozen in melting ethane. This cryo-fixed sample

was stored in liquid nitrogen until further processing. Then the

sample was freeze-fractured in an Oxford CT 1500 HF cryo-

system, to obtain a freshly prepared cross-section through the

foam. The sample was freeze-etched at �90 �C for a short time

(at max 3 min) to reveal the ultrastructure of the liquid phase.

After freeze-etching the sample was sputter coated with gold–

palladium for better SEM contrast. Then the sample was trans-

ferred into the SEM (Jeol 6340 Semi-inlens or gatan 2500, SEM

JEOL 630) and investigated at �125 �C with an accelerating

Soft Matter


Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

voltage of 3 kV and a working distance of about 6 mm. Imaging

was performed using the in-lens secondary electron detector.

2.10 Disproportionation rate

A detailed description of the method, which has been developed

to decouple disproportionation from coalescence, is given else-

where.32 Briefly, EC dispersion was aerated to relatively dry foam

(air phase volume app. 0.75). Separately, 0.5 wt% xanthan

solution was prepared in a similar way, which was freed from air

bubbles formed during the dissolution process, by using a bench

top desiccator. Directly after preparation, the relatively dry foam

was gently mixed with xanthan gum solution to adjust the air

phase volume to 0.5. At this phase volume fraction, the bubble

volume is below close packing, minimizing the chance of coa-

lescence. Finally, the bulk yield stress invoked by xanthan

prevents creaming of air bubbles over time. By inhibiting the

creaming and coalescence, the rate of disproportionation can be

measured using turbidity measurements. To this end, sample

volumes of 20 ml of the thickened foams were studied in time

using a Turbiscan Lab Expert (Formulaction, Toulouse,

France). We extract the bubble size evolution d(t)/d(0) from the

average backscattering along the height of the foam sample. A

coarsening time, tcoarse, is determined using the model

d2ðtÞd2ð0Þ ¼ 1þ t

tcoarse(1)

3 Results and discussions

3.1 Preparation and properties of EC particles

Anti-solvent precipitation (also called the ‘‘drowning-out’’

method) is commonly applied to prepare colloidal particles,

especially for insoluble polymer materials.33 In our case, acetone

is used to dissolve EC, and water is used as the anti-solvent since

EC (ethoxyl content 48%) can hardly be dissolved in water. EC

particles are formed by the procedure described in the experi-

mental section. This process results in a stable and turbid

dispersion with no signs of sedimentation or phase separation.

Fig. 1A is a typical transmission electron microscopy (TEM)

image of ethyl cellulose particles. It can be found that in our

conditions the dispersion consist of individual EC particles with

sizes in the range of 100–200nm. It is important to note that EC

Fig. 1 (A) TEM image of ethyl cellulose particles. EC particles were

formed by adding 10 ml deionised water to 10 ml 1% EC solution in

acetone (and evaporating the remaining acetone). (B) Typical volume

based particle size distribution of a diluted EC dispersion from dynamic

light scattering.

Soft Matter

particles size strongly depends on polymer concentration in the

solvent, that is, a higher concentration leads to a bigger particles

size, in line with the findings of Plasari et al.33 The latter is easily

understood by the fact that the initial polymer concentration

determines the degree of supersaturation immediately after

acetone and water are mixed, which together with the total

number of nuclei determines the final particle concentration.

The faint connections between individual EC particles,

observed in the TEM image (Fig. 1A) are most likely artifacts

originating from the TEM-preparation procedure and/or the fact

that EC particles can melt upon local heating induced by the

electron beam. We performed dynamic light scattering (DLS) in

parallel and Fig. 1B shows a typical volume based particle size

distribution. DLS measurements show a single relatively narrow

peak, indicating a stable dispersion with no signs of particle

aggregation. The Z-average diameter of the distribution is about

130 nm, which is well in line with the sizes observed by TEM. The

observed state of the EC dispersion indicates that there is strong,

long range repulsion between the hydrophobic EC particles,

which is most likely to be of electrostatic origin.

The latter is confirmed by the z-potential of original EC-

dispersion, which turns out to be around �50 mV, indicating the

presence of a strong negative charge on the EC particle surface.

There are two possibilities to explain origin of this charge: it is

either real, i.e. EC particles have native negative charge, or

apparent, i.e. induced by the solvent. Looking at the molecular

structure of EC, it is difficult to foresee the origin of a native

negative charge.30 Thus, it can be concluded that the negative z-

potential is due to an apparent negative charge, induced by the

water molecules. One possible explanation for this could be the

adsorption of OH� ions, which is known to occur on many

hydrophobic surfaces like the surface of hydrophobic latex

particles in water, or the surface of oil droplets stabilized with

a non-ionic surfactant or no surfactant at all.34 Measurements of

the electrophoretic mobility of air bubbles in water has also

demonstrated that the air–water interface has an electrical

potential similar in sign and magnitude, thus it has been

hypothesized that adsorption of OH� ions occurs on the air–

water interface as well.35 This has led to the speculation that such

a phenomenon can be quite generic and can occur on almost any

hydrophobic surface in contact with water. It has also been

shown that adsorption of non-ionic surfactants on such surfaces

can significantly reduce this charge, as they can compete with the

adsorption to the interface with hydroxyl ions and/or to disrupt

the structure of the sub-surface water layer. The latter has lead to

an alternative speculation that the origin of this apparent charge

is due to a long range arrangement and orientation of water

dipoles near a solid surface. However experiments where strong

chaotropic agents like urea (known to break the molecular

network of water) are added to these systems, has shown that the

measured z-potential is insensitive of it, which seems to disprove

the long range order hypothesis. It has also been shown in Iva-

nov’s work34 that the measured z-potential of a xylene droplet

increased with the decrease of pH, from around �60 mV at pH 5

to around �10 mV at pH 3, thus supporting the hypothesis of

OH� ion adsorption.

In the case of EC dispersion we observed very similar trends

and behavior as shown in Fig. 2. When the pH of EC dispersion

decreases from 6.0 to 3.0, the z-potential of EC particles changes



Fig. 2 Dependence of z-potential and apparent particle size of EC on

pH of EC dispersion. Solid symbols refer to z-potential, and open

symbols refer to particle size.

Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

from �50 mV to �5 mV, accompanied by the change of the

characteristic size of EC dispersion measured by DLS, from 130

nm to several micrometres. The later is an indication of strong

aggregation in the system, supported by the fact that EC

dispersions acidified to pH 3.0, show phase separation, where

a clear liquid is observed at the top and turbid sediment at the

bottom within half an hour after acidification.

These observations seem to support two hypotheses: (i) EC

particles have an apparent charge, due to OH� ions, which is pH

sensitive and (ii) the aggregation of the EC particles at low pH

and/or high salt is a result of a decreased electrostatic repulsion.

Indeed, results of salt titrations performed at different pH values

nicely illustrate this with a stability diagram of EC dispersion as

shown in Fig. 3. There one can see that at pH below 5, the

particles aggregate, even at very low ionic strength. At higher

pH, the particles are well dispersed and stabilized by an absolute

z-potential that is clearly larger than 25mV.When, at neutral pH,

i.e. 5 to 8, the ionic strength is increased, a critical flocculation

concentration (CFC) is passed around 20 mM, which indicates

that electrostatic screening can be sufficient to destabilize the

dispersion.

As discussed earlier, similar adsorption of hydroxyl ions is

known to occur at the air–water interface leading to negative z-

Fig. 3 Stability diagram of a dispersion of 1 wt% EC particles as

a function of pH and ionic strength (adjusted with NaCl). Red symbols

refer to unstable EC particle dispersion and blue symbols refer to a stable

dispersion.


potential of the same order.35 Thus one can expect that in this

case both air–water and EC–water interface will be negatively

charged and there will be a strong repulsion between EC particle

and the air–water interface, when an EC particle approaches the

air bubble’s surface. As is well known in DLVO theory such

repulsion will lead to the formation of an adsorption barrier of

EC particles at the interface. Since both air–water and EC–water

interface charge is governed by the same process, e.g. hydroxyl

ion adsorption on the hydrophobic surfaces, one can speculate

that the particle adsorption barrier will be also sensitive to pH

and ionic strength changes. Thus, when pH is decreased or ionic

strength increased, one can expect: (i) easier particle adsorption

at the air–water interface due to a reduced adsorption barrier; (ii)

formation of denser particle layers due to reduced inter-particle

repulsion at the interface (and thus better foam ability and

stability) and (iii) destabilisation of EC particle dispersion, due to

a reduced inter-particle repulsion.

In addition, the presence of an apparent surface charge can

also change the particle contact angle at the air–water interface,

due to the electrostatic repulsion between bare air–water and

EC–water interfaces near the point of contact line at the EC

particle surface. Therefore one can also expect that EC particles

are becoming effectively ‘‘hydrophilized’’ by OH� and thus salts

and acids can act as ‘‘hydropobization’’ agents for many types of

hydrophobic colloidal particles, which do not possess an intrinsic

charge, but have an apparent charge due to hydroxyl ion

adsorption.36 To estimate the importance of this effect, we have

measured the contact angle of water droplet in air on a glass slide

where 10 wt% EC in ethanol solution has been spin-coated to

form a thin film. The contact angle of water drops on the EC film

was carried on a Kr€uss DSA apparatus, where at least 3 sepa-

rated drops were used for measurement and the precision of the

measurement was around �3�. Herein deionized water was used,

and the pH of water was adjusted using HCl. The results show

that indeed the contact angle changes from 68� at pH ¼ 7 to

around 80� at pH¼ 5, but then seems to decrease back to around

75� at pH ¼ 3. Since the effects we observed are relatively small

and in the limit of the experimental accuracy of our method, we

could not distinguish if there is maximum around pH ¼ 5 or its

plateaus off around 75–80� at low pH as one can expect.

However, what could be concluded is that if there are effects of

pH and ionic strength on the contact angle (in the range of pH 3–

7), these effects are relatively small (less than 10�). We will also

assume that a flat EC surface and spherical EC particle have

similar contact angles due the following reasons: (i) from SEM

and TEM images it is clear that EC particles are relatively

smooth, and although we do not have data to support it we can

assume that EC spin-coated films are equally smooth; (ii) from

the methods used to prepare spin-coated surfaces and EC

particles, we can expect that EC surface properties in both cases

are very similar; (iii) it is reasonable to neglect the line tension

effects for 130 nm particles.37 If above is true, then we can esti-

mate that the adsorption energy of 130 nm EC with contact angle

of 68� is around 51 000 kT (here k is Boltzmann constant and T is

absolute temperature taken to be 295 K), while the adsorption

energy for the same size particle with contact angle of 80� is

89 000 kT. Though it might seem that the relative changes of the

adsorption energy are large, one should realize that in absolute

values these energies are also very large so that in both cases

Soft Matter


Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

particles should be irreversibly attached at the air–water interface

and stable foams should be produced. As it will be shown in the

next section, the main reasons for poor foam ability and stability

at high pH of the system is not due to changes of the surface

wettability of the particles, but due to the presence of a strong

adsorption barrier of electrostatic origin, which could not be

overcome by the typical forces applied during the aeration

process. At low pH and/or high salt, the particle attachment

barrier is lower and therefore EC particles become an effective

Pickering stabilizer. At those conditions one also expects that EC

dispersions become unstable, as well.

Fig. 5 Foam volume of EC foam (produced by hand shaking) without

acid and with acid at different times. The inset figure is the curve for the

first two hours.

3.2 Foamability and foam stability of EC dispersion at neutral

and acidic pH

The foamability and foam stability of 2.0 wt% EC dispersion

with or without acid are investigated and compared as shown in

Fig. 4 and Fig. 5. Bubble morphologies as probed by light

microscopy and cryo-SEM are presented in Fig. 6 and 7,

respectively.

At neutral pH, EC dispersions show reasonable foamability

and foam stability around few hours, which is quite similar to

common surfactants (Fig. 4 and 5). In addition, the liquid which

drains from the foam upon storage is a turbid and stable EC

dispersion. This is an indication that most of the particles are not

able to attach/adsorb at the air–water interface and thus to

stabilize the foam. To explain the reasonable foam ability and

poor stability, we presume that it is due to the presence of a water

soluble surface active component in the EC dispersion. This is

supported by the fact that when EC dispersion is dialyzed and

this component is removed, the foam-stability of EC dispersions

at low pH remains unchanged, while foamability is slightly

decreased, however at natural pH dialyzed EC dispersions do not

show any significant foamability. We speculate that this surface

active component, which decreases the interfacial tension of

native EC dispersion to 36 mN m�1, is most likely a low molec-

ular weight fraction of EC polymer, but have not confirmed this

conclusively.

Light microscopy shows that the morphology of the bubbles is

spherical (Fig. 6A) with an average size much larger than 100 mm.

Fig. 7 depicts representative cryo-SEM images of EC foam

produced by hand shaking at neutral and acidic conditions.

Panels A to C represent foams made at neutral pH and these

show large bubbles ([10 mm). When zoomed in, loose struc-

tures can be observed, which must resemble EC particles clus-

tered together by ice crystallization, an artifact from the

Fig. 4 Typical procedure of ethyl cellulose foam produced by hand-shaking w

EC dispersion, and the second step is to shear EC dispersion for foaming. N

Soft Matter

preparation procedure. One can see from these SEM images that

large part of EC particles seen on the picture are in the bulk,

rather than accumulated at the interface. In the view from the

bubble interior (Panel C) a loose structure of EC-particles can

also be observed and no indications of a strong surface layer are

present. The poor stability of the foam, the absence of any non

trivial bubble morphology and low representation of particles at

the bubble surface confirm that EC particles at neutral pH and

without salt are not very effective as Pickering stabilizers. This is

in line with the expectation that EC particles could not adsorb at

the interface due to the presence of a strong electrostatic barrier

(repulsion) between the bare air–water and EC–water interfaces

at normal pH and low ionic strengths, and that the foamability in

this case is due to the presence of soluble surface active compo-

nent, co-existing with EC particles.

At pH 3.0, the foam can reach an air phase volume fraction of

0.5 in the first seconds of shaking (Fig. 4). After shaking, the

bubbles are creaming under the influence of gravity and as

a result the foam volume will decrease in the first several hours

(Fig. 5). It is noteworthy that the subnatant is clear, opposite to

the pH-neutral case (Fig. 4). Nevertheless, the foam volume will

keep constant for more than one month after a creaming balance

has been reached (Fig. 5). In addition, there is no obvious bubble

size growths after 2 days. Fig. 6B shows a typical optical

microscopy image of EC bubbles produced by hand-shaking.

The bubbles are polydisperse, ranging from 10 mm to 100 mm.

ithout (A) and with acid (B, pH 3.0). The first step is to tune the pH of the

ote: foam can be sheared and formed immediately once acid is added.



Fig. 6 Optical microscopy of EC bubbles prepared from neutral disper-

sion (panel A) and acidified dispersion (panel B). EC bubbles were

produced byhand shaking 2%ECdispersion for 2min. Scale bar¼ 100mm.

Fig. 7 Cryo-SEM images of EC bubbles. A–C, foam prepared at neutral

pH, D–F, bubbles prepared at pH3. EC bubbles were formed by hand

shaking 2.0 wt% EC dispersion for 2 min. Images A, D, are images of

a cut face of a frozen aqueous foam; images B and E are the magnified

picture of images A and D; images C and F show the bubble surface seen

from the bubble interior. Image G is the Cryo-SEM image of EC particles

stabilized foam, diluted in 0.25 wt% xanthan solution, kept at pH 3. Here

a cross-sectional view on the bubble surface is shown, with the etched

liquid phase on the left and the air bubble to the right.

Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

Some bubbles are non-spherical and have a wrinkled surface,

which is a typical characteristic for particle stabilized bubbles,

and are often denoted as ‘‘armoured bubbles’’.17


From the findings above, it can be hypothesized that these

bubbles are stabilized by EC particles which is further proved by

cryo-SEM. Here, individual polydisperse bubbles with a diam-

eter of few tens of micrometres were cut and observed (Fig. 7D–

F). Fig. 7E shows the cross section of few cracked bubbles under

cryo-SEM, indicating that EC particles form a multilayer around

air bubbles (further magnification can be seen on Fig. 7G). It is

also clear from these images that the amount of EC particles

which don’t attach on the bubbles surface is quite low. At the

same time in Fig. 7F, which is a zoomed in internal image of

a cracked bubble, shows EC particles close packed at the bubble

surface, holes and traces of cracking lines, which is indicative of

the attraction between the particles in the packed layer, just as

the particles in the bulk.38 These experimental results are in line

with the modeling of packing of adsorbed mono- and poly-

disperse sticky, elastic particles in 2D done by Groot et al.38,39

Next we want to estimate what is the maximum overrun

(OVmax[%] ¼ 100f/(1 � f), where f, is the gas volume fraction),

that is possible to achieve for a given particle concentration, by

assuming that all EC particles are adsorbed at the air–water

interface of the foam in a close packed mono-layer in a hexagonal

lattice. This could be done by making a simple calculation of the

total surface area per unit volume of foam for a given bubble size

and overrun (OV), followed by counting the number of particles

needed to adsorb on this surface in close packed lattice and then

converting this amount into weight fraction of particles in the

initial solution. Since this is a linear expression, by solving with

respect to the OV we get:

OVmax½%� ¼ wtEC ½%� sinðwÞ2

a

rl

rEC

Rbubble3;2

REC3;2

(2)

where, wtEC ¼ 2 wt% is the weighted fraction of EC particles in

the initial dispersion, rl� 1 kg l�1 is the density of the non aerated

dispersion, rEC¼ 1.3 kg l�1 is the density of EC particles assumed

to be equal to the density of EC material, Rbubble3,2 � 50 mm is the

volume to surface averaged (Sauter mean) bubble radius andREC3,2

� 65 nm is the volume to surface averaged radius of EC particles,

while a is the surface coverage fraction, which for hexagonally

packed mono-disperse spheres is �0.91.

If the experimentally measured OV is smaller than the pre-

dicted value, it means that not all particles are adsorbed on the

interface. If most of EC particles are not in the bulk, then the

ratio between predicted vs. measured OV can be used as indica-

tion of the number of EC layers formed around a single foam

bubble. Calculation using eqn (2) shows that the maximum

possible OV in this case should be around 325%. While from

Fig. 4B it is clear that after shaking 10 ml of 2 wt% EC dispersion

has produced around 25 ml of foam, thus OV ¼ 100 � (25 � 10)/

10 ¼ 150%. From this calculation, it is clear that particles are in

excess in this case and that a large part of them should be in the

bulk, trapped within the Gibbs–Plateau borders, or present in the

drained liquid, or forming multi-layers around the bubble

surfaces. The fact that after creaming the serum is transparent,

indicating that EC particle concentration in the drained liquid is

very low. Indeed from the cryo-SEM depicted in Fig. 7G, we can

see that only very few free particles are observed in the bulk while

the majority of the particles are adsorbed/attached to the bubble

surface and forming multiple layers around it. This is well inline

Soft Matter


Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

with the estimation, which is also quite sensitive to the bubble

diameter as well. However, it is worth noting that the average

number of layers around the bubbles can vary from a single layer

as in the case observed in polystyrene particles stabilized

bubbles,40 to multi-layers, depending on the type of the aeration

procedure used, EC particle concentration, pH and ionic

strength. In summary, it can be concluded that the stability of EC

bubbles in the hand shaken foams can be mainly attributed to the

EC particle (multi) layer, which provides a steric hindrance

against bubble coalescence. In addition, cryo-SEM images show

that only a few EC particles are present in the continuous phase,

and most EC particles are used to stabilize bubbles. Together

with the high foam stability and the non-spherical ‘‘armored’’

bubble morphology, cryo-SEM confirms that EC particles are an

effective Pickering stabilizer for foams.

In order to further confirm how pH affects foamability and

foam stability, the pH of the EC dispersion was tuned by adding

desired amounts of aqueous HCl solution, followed by standing

for half an hour. The resulting suspension was shaken for 2 min

by hand. From the pictures shown in Fig. 4, it can be found that

initially stable EC dispersion tends to aggregate after changing

the pH from 6.8 to 3.0 by adding HCl. However after shaking,

these particle aggregates are easily broken, and can contribute to

foam stabilization. Fig. 8 shows digital pictures of EC foam with

different pH produced by hand-shaking. The results in Fig. 8 can

be divided into 3 regions: pH < 4.0; 4.0 < pH < 12.0; pH > 12.0.

When the pH of the EC dispersion is between 4.0 to 12.0, only

a few visible bubbles can be found, and they disappear after

several hours. The liquid phase separated from the foam due to

creaming is highly turbid, which indicates that a large part of the

EC particles are not adsorbed at the air–water interface and thus

do not contribute to the foam stabilization. Once the pH value

reaches 4.0, foamability increases significantly and a little

amount of translucent water solution drains from the foam. The

translucent water indicates that it is a diluted dispersion of EC

particles and the majority of the particles have been adsorbed to

the air–water interface. However, at lower pH (normally less

than 3.7) or higher pH (normally higher than 13.0), stable foam is

formed and the water separated from the foam is clear. For low

pH, this phenomenon can be explained by the pH dependence of

z-potential and size of EC particles, shown in Fig. 2. For

extremely high pH, the surface charge is still expected to be

highly negative, but the overall ionic strength (approx. 100 mM

at pH 13.0), could induce a screening of the charge, leading to

Fig. 8 Pictures of EC foam produced by hand-shaking for 2 min at

different pH. The pictures were taken 30 min after the foams were

formed. The pH values of the EC dispersions are 1.0, 2.9, 4.0, 6.5, 12.0,

13.5 respectively from A to F.

Soft Matter

reduced repulsion barrier between particles and the air–water

interface.

3.3 Effect of the electrolyte concentration on the foamability

Similar to acid and base, electrolytes such as NaCl can screen the

surface charge of particles efficiently, and decrease both the EC

particle adsorption barrier at air–water interface and the inter-

particle repulsion. Thus it is therefore expected that NaCl can

enhance the foamability and the foam stability of EC dispersions,

but will also destabilize the dispersions as shown and discussed

before. Fig. 9 shows the digital pictures of EC foams with

different concentrations of NaCl produced by hand-shaking.

Again, without salt, the EC dispersion can only produce some

visible bubbles which disappear in several hours. The foamability

and foam stability of the EC dispersion can be improved by

adding a small amount of NaCl. Finer bubbles could be formed

when 0.01 M NaCl was added into the EC dispersion. However,

we observed a slightly bluish appearance in the water separated

from the foam, indicating that a portion of the EC particles did

not stabilize the bubbles at this condition. With increasing salt

concentration, EC dispersions show improved foamability and

foam stability. In summary as the concentration of salt increases,

the absolute value of the z-potential decreases and the EC

particles aggregate, while these dispersions can produce stable

foams.

Finally, various electrolytes, acids, and bases, including HCl,

H2SO4, HAc, citric acid, NaOH, NaCl, CaCl2 and KCl were

added into the EC dispersion. Results confirm that all these

electrolytes can enhance the foamability and foam stability of EC

dispersion, in line with the analysis in the previous section. It is

worthwhile noting that EC particles show similar foaming

behaviors to that of polystyrene particles12 and silica particles.20

3.4 Effect of the foaming method on the bubbles’ size and foam

stability

From practical point of view, it is very important to control the

bubble size and size distribution, which can for example affect

the texture and appearance of food products. Traditionally,

foams can be produced by agitation (shaking) or mechanical

whipping of the colloidal systems. To represent this variability of

different methods and to obtain bubbles with different sizes, we

Fig. 9 Digital pictures of EC foam produced by hand-shaking for 2 min

at different concentration of NaCl. The NaCl concentration of EC

dispersions are 0 M, 0.01 M, 0.05M, 0.1 M, respectively from A to D, the

pH of all samples being neutral.



Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

produced EC foams by using: hand-shaking, Kenwood Kitchen

mixer or hand-held cappuccino/frappe mixer (Aerolatte), all of

which were expected to produce relatively large bubbles. In order

to obtain small EC bubbles, two high shear mixers, an Ultra

Turrax and Silverson, were used as well. The resulting foam

microstructures are shown in Fig. 10.

For 1.0 wt% EC dispersion (pH¼ 3.0), aerated using Aerolatte

and Kenwood mechanical mixers, we observed that the final

foam volume can be from 2 up to 11 times larger than the volume

of initial solution, depending on the time of whipping. This

indicates that the maximum overrun that can be achieved using

these methods is around 1000%. The latter is confirmed by the

fact that the same overrun has been achieved at two different

mixing speeds with the Kenwood (mid and high), given that

sufficient time is given in the case of lower speed. What changes

in these cases is the time needed for the system to achieve the

same plateau value of the overrun. The maximum overrun is also

independent of the initial volume of the solution in the range

150–250 ml. At larger liquid volumes the maximum foam volume

is limited by the volume of the mixing bowl and the fact that

whipping conditions change when the whisks get completely

immersed in the foam.

Resulting foams prepared by Kenwood are composed from

relatively large air bubbles with sizes ranging from 100 mm to

millimetre scale (picture not shown). Using eqn (2) for the

maximum overrun and taking values for the volume to surface

averaged bubble radius R3,2 ¼ 300 microns, EC concentration is

1.0 wt%, we estimate that maximum theoretically predicted

overrun for this system is around 10 00%. As one can see both

experimentally and theoretically predicted OV are close, indi-

cating that for the case of larger OV, almost all particles are used

to stabilize the bubbles, forming most likely monolayer. The

decrease of the number of adsorbed particle layers in mechan-

ically whipped foams, when compared to hand shaking ones, is

expected. Indeed, the mechanical devices can provide sufficient

mechanical energy density, allowing incorporating much larger

Fig. 10 Optical images of bubbles from different aeration methods.

Foam was produced from 1% EC dispersion (pH ¼ 3.4): A, by hand-

shaking for 2 min or B, by using Silverson (9500 rpm) or C, by Ultra-

Turrax(13 500 rpm) or D, by Ultra-Turrax(21 500 rpm). A, C, D, scale

bar ¼ 200 mm; B, scale bar ¼ 50 mm.


amount of gas, with bigger total surface area, which in turn

requires a larger amount of particles to fully cover. In this case

the maximumOV is mainly limited by the particle concentration,

given that energy density, whipping time and bowl volume is

sufficient. While in the case of manual shaking, energy density is

low and a much lower amount of gas can be trapped, thus the

total surface area of the foam is smaller and the excess particles

can formmulti-layers at the interface and theOV is limited by the

mechanical energy input that can be supplied by shaking as well

the volume of the cylinder and its geometry (determining energy

dissipations).

Interestingly when even higher shear devices such as the Sil-

verson are used, the bubbles’ size in the foam turns out to be

much smaller than with other methods, i.e. in the order of 10 mm.

At first it might seem that this confirms the well known principle

that higher energy input or higher shear stress would lead to

smaller bubbles, due to the higher energy input allowing

breaking initially big bubbles into small ones.41–44 However, the

step-change in bubble size could also suggest a different mech-

anism of bubble formation is in place for this device. In hand

shaking and in other ‘‘lower’’ shear devices such as the Kenwood

kitchen mixer, gas is mechanically entrapped and large pockets/

bubbles are consequently broken down into smaller bubbles. In

high shear devices, fluid velocity in the narrow gaps, can reach

much higher values. So the hydrodynamic cavitation can occur

due to hydrodynamic pressure drop. This will lead to nucleation

and growth of bubbles at high velocity zone, which normally will

collapse when the fluid exits the high shear zone and the velocity

drops, thus hydrodynamic pressure increases. In the absence of

surfactants, the cavitated bubbles will collapse, leading to the

typical cavitation sound and also to high stresses in the zone of

collapse, which can lead to significant wear and damage of all the

materials and surfaces in contact. However in the presence of

particles, one can speculate that expanding bubbles, in a low

shear zone can sweep particles. Due to their high attachment

energy these particles remain on the surface when bubbles enter

the high pressure zone and start to collapse, which in turn can

prevent the complete bubble collapse. Upon re-circulation in the

system, these bubbles will continuously expand and shrink,

which will lead to accumulation of the particle at their surface,

and compactification of the layer.

This hypothesis is confirmed by visual and audio observations:

at low shear rates of the mixer bubbles are not formed and the

foam formation starts only when certain critical velocity is

reached and the typical cavitation noise is heard, at the same time

the solution becomes more turbid and its volume starts to rise.

We also observed that the maximum OV in this system is limited

to about 40%, unless the mixing head is raised near the solution

surface or RPM increased, which in both cases leads to pertur-

bation of the upper solution surface and incorporation of air in

the form of larger bubbles. Indeed in the latter we observe two

populations of bubbles, small ones with size around 5 microns

which are likely formed due to cavitation and larger ones of order

of hundreds of microns. When, however, additional air is not

incorporated (and thus system is limited to about 40%OV) and

only small bubbles are observed.

Again a simple estimation of the maximumOV shows that, for

1wt% dispersion and bubbles with radius of 5 mm, the expected

maximum OV should be around 50% indicating that in this case

Soft Matter


Fig. 12 10 cm tall snow man made from EC foam, which was stable for

a period of more than 1 year long after the foam has been dried

completely, indicating that EC particles form a self-supporting structure

that can be further templated for various applications.

Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

almost all of the particles are used to form bubbles. However

when the particle concentration is increased to 2 wt% (shown in

Fig. 11), while keeping the same RPM in the mixer, the

maximum OV remains almost the same (again unless external air

is trapped form the upper surface of the solution), while bubble

size is reduced by a factor of 2. If the gas supply was not the

limiting factor, one would expect that an increase of the

concentration at a fixed energy input could lead to the formation

of foams with the same bubble sizes (determined by the energy

density in the system) and twice as large OV. The latter seems to

be an indication that either the gas supply is limited in this case or

that insufficient aeration time has been applied at least for the

case of low concentration. However when we varied the foam

formation time, we observe that the maximum foam volume and

the final bubble size are achieved in the first few minutes and then

they level off. The last observation confirms that in this case the

maximum OV is likely to be determined by the limited supply of

gas available for production of stable cavitating microbubbles,

i.e. linked most likely with the equilibrium amount of dissolved

gas (nitrogen) in the solution. In addition, the increase of the

particle concentration from 1.0 to 2.0 wt%, increases slightly the

viscosity of the solution at low pH or high salt concentrations,

which also facilitates a smaller bubble formation, but we think

that this is a secondary effect in this case.

It is observed that the Silverson and Ultra-Turrax show similar

trends and the obtained foams were composed of small bubbles

and were super stable as well, similar to the foams formed by low

shear mixers and hand shaking, which led to larger bubbles.

What is also interesting is that these two types of foams can be

mixed and remain stable nevertheless they are composed of very

small and big bubbles at the same time.

Fig. 12 shows an image of a ‘‘never-melting’’ snowman made

of creamed EC foam obtained using the Kenwood mixer. The

fresh foam was left to drain for 3 days to drain and then the

relatively dry EC foam on top was taken out for shaping. The

picture of this never-melting snow man has been taken 5 months

after it has been made and it is very stable and standing for more

than a year in the open atmosphere.

To our knowledge, this is the first time to prepare so small

bubbles stabilized by particles. From the observation of the EC

foam, shown in Fig. 11, EC foam produced using Ultra-Turrax

Fig. 11 Optical microscopy of EC bubbles. EC bubbles were produced

by homogenizing 20 ml 2% EC dispersion (pH ¼ 3) at 21 500rpm for 2

min by Ultra-Turrax, scale bar ¼ 20 mm.

Soft Matter

homogenizer appears very fine and creamy, indicating that even

by visual observations, these foams are made of extremely small

bubbles.

3.5 Disproportionation stability of foams prepared from EC

dispersion

Apart from the foam stability probed in pure water, which

comprises a combination of drainage, coalescence, and dispro-

portionation processes, we measured the evolution of the average

bubble size in thickened foams using turbidimetry. The advan-

tage of this method is that we can study disproportionation only

and make an attempt to quantify the coarsening time.

When mixing foams obtained by the EC dispersion from

various pH values, we found that the foam formed at neutral pH

without salt yielded foam with bubbles too large to be mixed

properly into the xanthan gum aqueous solution, which provides

yield stress sufficient to keep bubbles suspended. Foams formed

using a Kenwood kitchen mixer or Silverson high shear mixer at

acidic conditions or in the presence of 20 mM MgCl2 formed

homogeneous foams in xanthan and were analyzed further.

Fig. 13 shows the time evolution of the squared bubble size

diameter. The curves can be reasonably fitted with the linear

model over the range of approximately 3 days (see inset). This

provides an estimate of the coarsening times of EC-foams, which

are approximately 7 and 9 days for salt and acid respectively.

Such a coarsening time is already about two orders of magnitude

higher than coarsening times found for some reference materials

such as milk proteins.32 This result indicates that the surface layer

formed under these conditions by EC particles provides

substantial resistance against shrinkage and/or growth of

bubbles in foam. It is also worthy to note that foams formed

under high salt conditions coarsen slightly faster than the foams

formed under acidic conditions, indicating that either the particle



Fig. 13 Relative squared bubble diameter d2(t)/d2(0) of 50 vol% air foams

in a xanthan solution. The foams were generated from aqueous solutions

containing 2 wt%ECusing aKenwoodmixer (A) or Silversonmixer, (:)

and the resulting foams were transferred into 0.5 wt% xanthan solution.

Solid symbols refer to acid induced foams, open symbols to salt induced

foam. The inset shows the same data set in detail over the first 3 days.

Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

detachment energy or the layer strength is slightly higher for the

acid-induced case. This can be understood from the fact that

acidification leads to a reduction of the surface charge, whereas

salt only screens the charge at the surface.45

The foam prepared by the Silverson mixer, consists of bubbles

in the range of 5–10 mm and shows a similar size evolution to the

foams prepared in the Kenwood mixer (initial bubble size �150

mm) over the first few days. However, after that, the foam

prepared in the Kenwood mixer drains faster, since the yield

stress of the xanthan solution is not sufficient to suspend such big

bubbles. For this reason we had stopped further monitoring the

sample bubble size distribution. Nevertheless the stability of the

snow-man shown in Fig. 12 is an indication that these foams are

ultra stable as well and can remain for a period of months as well;

even when they are completely dry.

The foams prepared in the Silverson mixer did not show much

creaming, apart from a few visible bubbles. When we analyzed

the foam over prolonged time, we observed an apparent decrease

of the average bubble diameter, suggesting that in the majority of

the foam, large bubbles disappeared and small ones remained.

This observation confirms the bubble stability over months and

indicates that the initial coarsening times represent a limited

stage of initial coarsening. During this stage two things happen:

(1) small bubbles will reinforce because of further consolidation

of the EC particle layer and (2) large bubbles grow and, given the

cracks in the particle layers observed by SEM (Fig. 7F), the EC-

layer may rupture. As a result of this uncovered areas on the

bubble surface appear and bubbles may coalesce and eventually

cream or even escape to the headspace, as suggested from a 20%

loss of air after two months. Hence, the coarsening time

measured over the first three days is an underestimate of the

coarsening time of the small bubbles, which have a lifetime of

more than 2–3 months.

4 Conclusions

We have shown that aqueous foams with long term stability can

be formed using particles derived from ethyl cellulose using

a simple anti-solvent precipitation method. The foamability and


foam stability of EC dispersions is found to depend greatly on

the surface charge of the EC particles, which can be tuned by

adding some electrolytes, such as acid, base or salt. When the

absolute value of the z-potential of EC particles is well below

25 mV, i.e. at pH 4 or ionic strength >20 mM, EC particles

aggregate and foam prepared from these aggregated particles can

be stable for months. We explained this behavior to be due to the

adsorption of hydroxyl ions at hydrophobic surfaces—EC–water

and air–water in this case. This leads to the formation of an

apparent negative charge on these surfaces, which in turn leads to

a strong electrostatic repulsion between the particles and particle

and air–water surface. Due to their electrostatic origin these

repulsions can be suppressed by a decrease of pHwhich decreases

the concentration of OH- ions and thus the surface charge at

both air–water and EC–water surfaces and/or via addition of

electrolyte which screens it. We believe that this could be

a universal phenomenon occurring in many particle-stabilized

systems like hydrophobic latex or modified silica particles, which

do or do not have native charge on their surface. In this case the

dispersion destabilization is indicative of good foam stability, but

is not the cause of it. There could also be changes of the particle

contact angle caused by changes of pH or ionic strength, thus

making the particles more hydrophobic. However at least for the

case of EC, we have shown that this is the secondary order effect,

when compared to changes of particle adsorption barrier, which

governs whether particles can be adsorbed at the air–water

interface during the foaming processes.

In the conditions where we obtain super stable foams (low pH

and/or high or moderate ionic strengths), cryo-SEM images of

the foam reveal that EC stabilized foams are covered by a closely

packed mono or multi layer of EC particles, which provides

a steric hindrance against bubbles coalescence and a mechanical

resistance against bubble shrinkage. Bubbles with various kinds

of size, from several microns to hundreds of microns, are

obtained by changing the dispersion method or the shear rate.

Even small bubbles (d < 10 mm) also show exceptional stability

against disproportionation. To monitor the bubble size evolution

of these foams, we mix them with aqueous xanthan solution,

which provides a weak yield stress sufficient to keep bubbles

suspended and thus slowing the liquid drainage. We observe that

the bubble size evolution with time follows a two-step process

with an initial stage having a coarsening time of approximately 1

week, followed by a plateau, where the coarsening has been

arrested for a period of months. In the first stage, small bubbles

consolidate and large bubbles grow, becoming vulnerable to

creaming and coalescence. In the second stage, consolidated

small bubbles remain and no changes occur for at least 2–3

months. Because of its tunable bubble size and exceptional

stability, foam stabilized by EC particles can find many practical

applications. EC particle stabilized foams differ from most other

particle stabilized foams in that the initial EC material has food

grade status for specific applications in several markets and thus

may offer scope for the stabilization of food foams.

Acknowledgements

We would like to thank Mr. Mark Kirkland and Dr Jaap Nijsse

for Cryo-SEMmeasurement and Dr Rob Groot and Dr Andrew

Cox for the stimulating discussions. MK and AC are from

Soft Matter


Dow

nloa

ded

by U

NIL

EV

ER

RE

SEA

RC

H L

AB

OR

AT

OR

Y o

n 22

Dec

embe

r 20

11Pu

blis

hed

on 2

2 D

ecem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1SM

0651

8A

View Online

Unilever R&D, Colworth in UK, JN and RG are from Unilever

R&D, Vlaardingen in The Netherlands. We also would like to

thank to the referees of this paper for their critical and

constructive comments, which helped to improve the quality of

the manuscript significantly.

References

1 D. Myers, Surfaces, Interfaces, and Colloids: Principles andApplications, Second Edition, Wiley-VCH, New York, 1999.

2 S. S. Shankar, U. S. Patil, B. L. V. Prasad and M. Sastry, Langmuir,2004, 20, 8853–8857.

3 A. R. Studart, U. T. Gonzenbach, E. Tervoort and L. J. Gauckler, J.Am. Ceram. Soc., 2006, 89, 1771–1789.

4 U. T. Gonzenbach, A. R. Studart, E. Tervoort and L. J. Gauckler,Angew. Chem., Int. Ed., 2006, 45, 3526–3530.

5 T. N. Hunter, R. J. Pugh, G. V. Franks and G. J. Jameson, Adv.Colloid Interface Sci., 2008, 137, 57–81.

6 P. Walstra, Physical Chemistry of Foods, Marcel Dekker, New York,2006.

7 H. Wildmoser, J. Scheiwiller and E. J. Windhab, LWT–Food Sci.Technol., 2004, 37, 881–891.

8 J. R. Lindner, Nat. Rev. Drug Discovery, 2004, 3, 527–532.9 W. Drenckhan, Angew. Chem., Int. Ed., 2009, 48, 5245–5247.10 J. I. Park, E. Tumarkin and E. Kumacheva, Macromol. Rapid

Commun., 2010, 31, 222–227.11 E. Dressaire, R. Bee, D. C. Bell, A. Lips and H. A. Stone, Science,

2008, 320, 1198–1201.12 J. C. Wilson, PhD Thesis, University of Bristol, UK, 1980.13 B. P. Binks, Curr. Opin. Colloid Interface Sci., 2002, 7, 21–41.14 Z. Du, M. P. Bilbao-Montoya, B. P. Binks, E. Dickinson, R. Ettelaie

and B. S. Murray, Langmuir, 2003, 19, 3106–3108.15 R. G. Alargova, D. S. Warhadpande, V. N. Paunov and O. D. Velev,

Langmuir, 2004, 20, 10371–10374.16 B. P. Binks and T. S. Horozov, Angew. Chem., Int. Ed., 2005, 44,

3722–3725.17 A. B. Subramaniam, M. Abkarian, L. Mahadevan and H. A. Stone,

Nature, 2005, 438, 930.18 S. Fujii, P. D. Iddon, A. J. Ryan and S. P. Armes, Langmuir, 2006, 22,

7512–7520.19 U. T. Gonzenbach, A. R. Studart, E. Tervoort and L. J. Gauckler,

Langmuir, 2006, 22, 10983–10988.20 B. P. Binks, B. Duncumb and R. Murakami, Langmuir, 2007, 23,

9143–9146.

Soft Matter

21 A. Cervantes Martinez, E. Rio, G. Delon, A. Saint-Jalmes,D. Langevin and B. P. Binks, Soft Matter, 2008, 4, 1531–1535.

22 A. L. Campbell, B. L. Holt, S. D. Stoyanov and V. N. Paunov, J.Mater. Chem., 2008, 18, 4074–4078.

23 B. P. Binks, M. Kirkland and J. A. Rodrigues, Soft Matter, 2008, 4,2373–2382.

24 A. L. Campbell, S. D. Stoyanov and V. N. Paunov, Soft Matter, 2009,5, 1019–1023.

25 J. I. Park, Z. Nie, A. Kumachev, A. I. Abdelrahman, B. P. Binks,H. A. Stone and E. Kumacheva, Angew. Chem., Int. Ed., 2009, 48,5300–5304.

26 W. Zhou, J. Cao, W. Liu and S. Stoyanov, Angew. Chem., Int. Ed.,2009, 48, 378–381.

27 E. Dickinson, Curr. Opin. Colloid Interface Sci., 2010, 15, 40–49.28 21 CFR172.868, U.S. Food and Drug Administration, 2010.29 Eur. Food Standards Agency J., 2004, p. 1.30 E. Mathiowitz, Non-degradable polymers for drug delivery in:

Encyclopedia of controlled drug delivery, 2 Volume Set, Wiley,1999.

31 J. Raut, S. D. Stoyanov, C. Duggal, E. G. Pelan and L. N. Arnaudov,Hydrodynamic Cavitation: A Bottom-Up Approach to LiquidAeration, Soft Matter, 2011, submitted.

32 T. B. J. Blijdenstein, P. W. N. de Groot and S. D. Stoyanov, SoftMatter, 2010, 6, 1799–1808.

33 E. Plasari, P. Grisoni and J. Villermaux, Chem. Eng. Res. Des., 1997,75, 237–244.

34 K. G. Marinova, R. G. Alargova, N. D. Denkov, O. D. Velev,D. N. Petsev, I. B. Ivanov and R. P. Borwankar, Langmuir, 1996,12, 2045–2051.

35 A. Graciaa, G. Morel, P. Saulnier, J. Lachaise and R. S. Schechter, J.Colloid Interface Sci., 1995, 172, 131–136.

36 L. G. J. Fokkink and J. Ralston, Colloids Surf., 1989, 36, 69–76.37 L. N. Arnaudov, O. J. Cayre, M. A. S. Cohen, S. D. Stoyanov and

V. N. Paunov, Phys. Chem. Chem. Phys., 2010, 12, 328–331.38 R. D. Groot and S. D. Stoyanov, Soft Matter, 2010, 6, 1682–

1692.39 R. D. Groot and S. D. Stoyanov, Soft Matter, 2011, 7, 4750–4761.40 D. Dupin, J. R. Howse, S. P. Armes and D. P. Randall, J. Mater.

Chem., 2008, 18, 545–552.41 G. I. Taylor, Proc. R. Soc. London, Ser. A, 1932, 138, 41–48.42 H. P. Grace, Chem. Eng. Commun., 1982, 14, 225–277.43 J. M. H. Janssen and H. E. H. Meijer, J. Rheol., 1993, 37, 597–608.44 U. T. Gonzenbach, A. R. Studart, E. Tervoort and L. J. Gauckler,

Langmuir, 2007, 23, 1025–1032.45 R. J. Hunter, Foundations of colloid Science, Volume I, Clarendon

Press, Oxford, 1987.



Documents

Super stable foams stabilized by colloidal ethyl cellulose particles