Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
Glen McHale$, Mike Newton$, Neil Shirtcliffe$, Brian Pyatt$ & Stefan Doerr*
$School of Biomedical & Natural SciencesNottingham Trent University
*Department of Geography, SwanseaUniversity of Wales
Email: [email protected]
Wetting of Hydrophobic Beads and Sand
Glen McHale$, Mike Newton$, Neil Shirtcliffe$, Brian Pyatt$ & Stefan Doerr*
$School of Biomedical & Natural SciencesNottingham Trent University
*Department of Geography, SwanseaUniversity of Wales
Email: [email protected]
“Conversations with a soil scientist”
Overview
2. Soil as a Super-Hydrophobic Surface
3. Critical Angle for Imbibition
5. Particle Lifting and Droplet Self-Coating
1. Extreme Soil Water Repellence
4. Surface Wetting versus Porosity
Motivation
Extreme Soil Water Repellence
Extreme Soil Water Repellency
2. ProblemsVegetation difficult to re-establish (land remediation difficult)Increased run-offLand/soil erosion
3. Soil scientists use two field testsMolarity of Ethanol Droplet (MED) (i.e. critical surface tension test)(% ethanol needed for droplet to infiltrate within 3 seconds)
Water Droplet Penetration Time (WDPT)
4. Materials scientists (and soil scientists back in the lab) may
measure contact angle θmeasured
Field Conditions1. Sandy soil can become extremely hydrophobic
After forest firesAfter oil contamination
Field Conditions1. Sandy soil can become extremely hydrophobic
After forest firesAfter oil contamination
2. ProblemsVegetation difficult to re-establish (land remediation difficult)Increased run-offLand/soil erosion
3. Soil scientists use two field testsMolarity of Ethanol Droplet (MED) (i.e. critical surface tension test)(% ethanol needed for droplet to infiltrate within 3 seconds)
Water Droplet Penetration Time (WDPT)
4. Materials scientists (and soil scientists back in the lab) may
measure contact angle θmeasured
Water Droplet on Hydrophobic Sand
Shape and Packing
200 µm
Sand with139o
Soil Science Literature
A Non-Soil Scientist ViewSoil is a convoluted surface consisting of a porous/granular material
coated with hydrophobic compounds
Soil can be a super-hydrophobic surface
Extreme Water Repellence1. Soil exhibiting it is within the upper part of the soil profile2. Promoted by drying of soil3. Loose sandy soil is more prone to it 4. Forest fires or intense heating of soil is known to cause it -
volatilised (hydrophobic) waxes from organic matter subsequently condensing and coating soil particles
Super-hydrophobic Model
θe
smooth solid
(a)
Super-hydrophobic Surfaces
( )LV
SVSLe γ
γγθ −=cos
Smooth Surface Young’s equation summarises the surface chemistry
water “skating”across solid
(c)
water on solid
(b) (d)
water onsolid-liquid surface
“Rough” SurfacesIdentical surface chemistry
Wenzel
eW r θθ coscos =
Wenzel (“Sticky”)
Cassie-Baxter
)1(coscos ff eCB −−= θθCassie-Baxter (“Slippy”)
A Simple Model of Soil
Assumptions1. Uniform size, smooth spheres in a hexagonal arrangement2. Water bridges horizontally between spheres 3. Capillary (surface tension) dominated size regime
Side View Top View
θe
water
2(1+ε)R
(a)
air in gaps
2r 2R
2(1+ε)R
(b)
B
C
2rA
Reference: McHale et al, Eur. J. Soil. Sci. 56 (2005) 445-452; Hydrological Processes (2006).
Dry and Wet Soil
droplet
(a)
air in gaps
droplet
(b)
water in gaps
Droplet on Dry Sand Droplet on Wet Sand
1. Cassie-Baxter state is often a metastable state2. Water can be forced into pores by applying pressure3. Water vapour condensing can form Wenzel state whereas a droplet may
deposit in a Cassie-Baxter state
Notes
Principles of Calculation Dry Soil
Cassie-Baxter equation with composite solid-vapour surface
( )ff eCV −−= 1coscos θθ
( )ff eCW −+= 1coscos θθ
Soil with Water in Gaps
Cassie-Baxter equation with composite solid-water interface
( ) ee
efθπεθ
θε22 sin
21
13cos1
cos1)(
−+++
+=
Solid Surface Fraction
Use geometryGrains not close-packedCentre-to-centre separation
between spheres is 2(1+ε)Rwhere, ε, is a spacing constant
50
70
90
110
130
150
170
50 70 90 110 130 150 170
θ e /degrees
θ oV
/deg
rees
Dry Soil - Water Repellence Enhancement
ε= 0.677 (loose)ε = 0.452ε = 0.226ε = 0.0 (close)
Water repellence increases with
spacing of grains
Curves for packings:
ε= 0.0 (close)ε=0.226ε = 0.452ε = 0.677 (loose)
0
20
40
60
80
0 20 40 60 80
θ e /degrees
θ oL/d
egre
es
Wet Soil - Water Repellence Reduction
Water repellence decreases with
spacing of grains
Curves for packings:
Experiments on TMSCL Treated(θ ∼108o ) Glass Beads
600 µm and 126o 250 µm and 140o
Forward Tilt View Top View Packing
200 µm200 µm
Wetting versus Porosity
Raw Foam Heat Treated Foam
Switching of Super-hydrophobicity
Super-hydrophobic to Super-slurp1. Super-hydrophobic MTEOS sol-gel foam2. Switched to porous foam by heat cycle to change to hydrophilic
Imbibition into SoilSwitch to imbibition can be observed with change in liquid surface tension (rather than temperature)
Reference: Shirtcliffe et al, Chem. Comm. 25 (2005) 3135-3137 (also Nature News 20/7/05)
Raw Foam Heat Treated Foam
Imbibition into Bead Packs & Sand
Reference: Shirtcliffe et al, Hydrological Processes (2006)
Octane (72o) Heptane (65o)
Fluorocarbon Bead Packs1. Fluorocarbon coated glass beads
(size = 75 µm) on glass slides2. Range of hydrocarbon liquids3. Penetration occurs for pentane, but
not for hexane52oPentane
61oHexane
65oHeptane
72oOctane
θ θ θ θ on fluorocarbon coated glass slides / °±4
Liquid
Fluorocarbon Coated Sand
Hexane (61o)
Penetration occurs for hexane
Top View Side View
Model for Capillary Imbibition
References: Shirtcliffe et al, Appl. Phys. Lett. 89 (2006) art 094101; *S. Bán, E. Wolfram, S. Rohrsetzher 22, (1987) 301-309.
Assumptions1. Spherical particles2. Fixed & hexagonally packed3. Planar meniscus with Young’s
law contact angle, θe
4. Minimise surface free energy, F
Results for Close Packing1. Change in surface free energy with
penetration depth, h, into first layer of particles
2. Equilibrium exists provided liquid does not touch top particle of second layer
hR
hRF eLV ∆
−+−=∆ 1cosθγπ
3. If liquid touches second layer at depth, hc, then
complete imbibition occurs
4. Critical contact angle, θc, when hc reached
RRhc 63.13
8 ==
θc=50.73o
Consistent with experiments*
50
70
90
110
130
150
170
50 70 90 110 130 150 170
θ e /degrees
θ oV
/deg
rees
Minimum Hydrophobicity to Support Liquid when Grains are Loose Packed
Recall Soil Graph
3
2221cos
2min εεθ −−+−=e
Minimum Hydrophobicity
i.e. Solid point at start of each curve
εmax=√3-1=0.732
Separation when bead pushes up through hole is
Droplet Self-Coating
Liquid Marbles
Reference: Aussillous, P.; Quéré, D. Nature 411, (2001), 924-927 Acknowledgement: David Quéré
solid
vapour
waterMinimise
Energy
( )22 cos1 eLVRF θγπ +−=∆
Loose Surfaces1. Loose sandy soil – grains are not fixed, but can be lifted2. Surface free energy favors solid grains attaching to liquid-vapor interface3. A water droplet rolling on a hydrophobic sandy surface becomes coated
and forms a liquid marble
water
vapour
solid
Surface Free Energy
Energy is always reduced on grain attachment
Particle Lifting
75 µm silica spheres and hexane
Water Droplet Evaporation on Hydrophobic Sand
Evaporatively Driven Coating
Reference Shirtcliffe et al., to be submitted to APL (2006). See also reports on drying and buckling: Tsapis, et al.,Phys. Rev. Lett. 94, 018302-1 (2005); Schnall-Levin, et al., Langmuir 22, 4547-4551, (2006);.
Water on Hydrophobic Sand
Water on Hydrophobic 75 µm Silica Beads
Evaporatively Driven SortingSurface Free EnergiesWhen two particles of the same size, but different wettabilities, compete for
a reducing air-water interface the one with its contact angle θe closest to 90o
should win and remain at the interface
Experimental Test1. Bed of blue hydrophobic (115o)
spheres of diameter 500 µm and transparent hydrophilic (17o)
spheres of diameter 700 µm2. Allow droplet to evaporate and
clump to form
( )22 cos1 eLVRF θγπ +=∆Ejection: Surface–into-Air
( )22 cos1 eLVRF θγπ −=∆Ejection: Surface–into-Liquid
After evaporation blue particles are on outside of clump
Conclusions
1. Porous Material versus Super-hydrophobic SurfaceS/H predicts hydrophobicity enhancements on sand/beads
Extreme soil water repellence is an example
2. Imbibition of LiquidsCritical contact angle is 50.73o on hexagonal bead packs
For hydrophobic sand this increases to 61o-65o
3. Droplet Self CoatingSubstrate features may not be fixed
Grains can re-arrange – droplets become liquid marbles
Evaporation drives self-coating and grain sorting
The End
Funding BodiesEPSRC GR/R02184/01
Super-hydrophobic & super-hydrophilic surfaces (GM, MIN, NJS)EPSRC EP/C509161/1
Extreme soil water repellence (GM, FBP, MIN, NJS)NERC NER/J/S/2002/00662
Advanced Fellowship for Dr Stefan Doerr (SD)NERC NEC003985/1
Fundamental controls on soil hydrophobic behaviour (SD)
Acknowledgements
Particle Lifting Data
Reference Shirtcliffe et al., to be submitted to APL (2006).
1. Evaporation of water droplet on 75 µm diameter silica bead “free” pack 2. Droplet spherical radius (xxx) and height of a skin of silica beads (+++)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 20 40 60 80
t /minutes
h ski
n, R
dro
ple
t/mm
Solid line is product
hskinRdroplet
If skin is constant in area then product of these should tend to a constant