Sudden collapse of a colloidal gel

Abstract

Metastable gels formed by weakly attractive colloidal particles display a distinctive two-stage time-dependent settling behavior under their own weight. Initially a space-spanning network is formed that for a characteristic time, which we define as the lag time τ d , resists compaction. This solid-like behavior persists only for a limited time. Gels whose age tw is greater than τ d yield and suddenly collapse. We use a combination of confocal microscopy, rheology and time-lapse video imaging to investigate both the process of sudden collapse and its microscopic origin in an refractiveindex matched emulsion-polymer system. We show that the height h of the gel in the early stages of collapse is well described by the surprisingly simple expression, h(τ ) = h0 − Aτ 3 2 , with h0 the initial height and τ = tw − τ d the time counted from the instant where the gel first yields. We propose that this unexpected result arises because the colloidal network progressively builds up internal stress as a consequence of localized rearrangement events which leads ultimately to collapse as thermal equilibrium is re-established.

Numerical and experimental analysis of the sedimentation of spherical colloidal suspensions under centrifugal force

“…Many sedimentation experiments use custom charge-coupled device (CCD) camera arrangements with image processing software. 1,5 The aim is usually to track the supernatant-suspension interface and obtain its velocity, known as the separation velocity, v(t). The separation velocity characterises the rate of phase separation.…”

“…1,2 Examples of such consumer products include fabric enhancers, cosmetics, and shampoos. 3 In many of these applications, the dispersed phase has a higher density than the continuous phase.…”

Effects of vertical confinement on gelation and sedimentation of colloids

“…Under some conditions the gel can persist for years 29 , if the self-generated or gravitational stress is weaker than the yield stress 30 , but gels very often undergo sedimentation [31][32][33] . This is a poorly understood phenomenon and can sometimes be sudden in its onset -so-called delayed collapse 34 . In such delayed collapse, very little change in the macroscopic properties of the system is seen for some time, which is comparable to the timescales we consider here.…”

“…In such delayed collapse, very little change in the macroscopic properties of the system is seen for some time, which is comparable to the timescales we consider here. Then a change occurs and the system begins to sediment on a timescale of 10 5 particle diffusion times or more 34 .…”

Modeling polymeric gels: The role of chain fexibility on the structure of physical gels

“…This so called gel state can be achieved using simple model interactions provided that the range of interaction is very small when compared with the typical size of the interacting particles [6][7][8][9][10]. The physical properties of the gel are intermediate between those of a solid and a liquid and there is considerable interest on them due to their numerous industrial, analytical and domestic applications [11][12][13].…”