The colloid crunch: fighting gravity
00:00:00
(1)
University of Bristol, UK
(2)
Bayer CropScience, Germany
Lisa Teece (1) , Malcolm Faers (2), and Paul Bartlett (1)
Outline of talk
| Collapse of weak gels
| A novel mechanism of collapse
| Switch to conventional (linear) collapse at small initial heights
| Why?
00:00:01
Collapse of strong gels
4 days
Manley et al. Phys. Rev. Lett. 94, 218302, (2005)
Buscall , Colloids Surf. 5, 269, (1982) | Settling starts immediately.
| Linear collapse 00:00:02
London, April 20, 2009 [3 / 19]
Depletion‐based emulsion gel
PDMS emulsion
stabilised by mixture of nonionic
and anionic surfactants
Index‐matched solvent mixture
Ethylene glycol and water
Non‐adsorbing polymer
Semi‐dilute solution of xanthan –
high molecular wt. polysaccharide
00:00:05
Solvent fluorescently labelled
Time‐lapse video microscopy
00:00:05
X 7600 ‐ 48 hours
Time‐lapse video microscopy
Accuracy: ± 0.25 mm
| Three‐stage settling process
| Mechanism totally unknown
00:00:06
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Vary initial height h(0), [polymer], nature of walls
00:00:07
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Time‐dependent gel collapse
Compressed exponential
00:15:58
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Height‐independent scaling
00:15:58
tall samples h > w
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Universal time‐dependent collapse
00:16:00
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Accelerated collapse
00:16:51
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Linear collapse
00:16:51
Thin samples h < 1 mm
London, April 20, 2009 [12 / 19]
Summary of key observations
Accelerated collapse
| Fast | Tall samples
Linear collapse
| Slow | Short samples
00:17:53
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Origin of t3/2 scaling
Gravitational collapse?
| Linear collapse 00:21:09
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Origin of t3/2 scaling
Stress dipoles
Aging of gel
Micro‐collapse
Point dipole of intensity Q(t)
Assume dipole intensity is approximately linear in time
Cipelletti et al. Phys. Rev. Lett. 84, 2275, (2000)
00:24:34
London, April 20, 2009 [15 / 19]
Origin of t3/2 scaling
Purely elastic medium
Gel shrinkage:
Elastic gel
00:25:24
What is missing?
London, April 20, 2009 [16 / 19]
Origin of t3/2 scaling
Poroelasticity of gel
Propagation of stress by diffusion
Stress diffusion constant determined by elastic and hydraulic properties of network
Fluid‐filled elastic matrix
For current gels estimate
Equilibration of stress over 10 mm take about 70 hours
00:27:41
London, April 20, 2009 [17 / 19]
Origin of t3/2 scaling
Stress diffusion in gels
Fluid‐filled elastic matrix
00:28:31
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Conclusions
|
Novel mechanism of collapse in weak gels
|
Internal stress inhomogeneites lead to accelerated collapse of gels
|
At small heights switch in mechanism to conventional linear collapse
00:33:01
London, April 20, 2009 [19 / 19]