Microscopic structure and collapse of depletion-induced gels in vesicle-polymer mixtures

Abstract

We present the behavior of depletion-induced gels for vesicle-polymer mixtures when the ratio of the polymer radius of gyration to the mean vesicle radius is 0.09 and 0.27. As the polymer concentration increases, density gradients build up and an interface is developed between a highly turbid vesicle-rich phase and a polymer-rich phase. Increasing the polymer concentration further forms a gel (CP=0.3 and 0.1 wt% for Rg/a approximately 0.09 and 0.27, respectively), which subsequently collapses. This collapse is characterized by a slow initial rising for a finite delay time, a rapid collapse, and a slow final compaction to an equilibrium height. However, we observe a remarkably different polymer concentration dependence on the collapse rate. Unlike other colloidal gels, we find that the delay time for the vesicle collapse decreases with increasing polymer concentration. We show that this behavior can be accounted for by considering the permeability for solvent backflow, which is directly related to the characteristic pore area of the gel obtained using confocal microscopy.

Compressing a spinodal surface at fixed area: bijels in a centrifuge

“…The permeability of the structure describes the ease with which liquid moves through the porous medium and can be calculated from the graphs of sample height versus time. 24 We take the permeability to be…”

“…where Dr is the difference in density between the solid particles and the liquid with the lowest density, g is the centrifugal acceleration and Z s is the background solvent viscosity which is estimated by combining the viscosities of nitromethane and ethanediol in terms of the ratio 64 : 36. 24 This approach is based on the theory of sedimenting gels proposed by Buscall and White which considers the balance of the viscous drag force, the gravitational force and network stress to give the rate of change in sediment height with time. 25 The initial permeability of the bijel, determined using eqn (2), can be used to establish whether the movement of liquids throughout the structure affects the compression process.…”

Gels under stress: The origins of delayed collapse

“…Much of the work to date on gel collapse has focused on macroscopic features, typically by measuring the time evolution of the height h(t w ) of a gel, rather than on the internal structure and dynamics of a gel. However in the last few years new techniques, such as confocal scanning microscopy [15][16][17] and photon correlation imaging [4] has revealed that colloidal gels have a complex hierarchical structure, with different structural features at different length scales. So while at the individual particle level, a gel consists of dense aggregated colloids, the aggregates are organized on the micro-scale into relatively thick strands of parti-cles, which at the mesoscale are assembled into a percolating network able to transmit a stress.…”

Sudden collapse of a colloidal gel

“…Initially, the gel behaves as a solid but after a finite lag time τ d , the gel yields and catastrophically collapses. Sudden or 'delayed' network collapse is observed in a wide variety of materials [7][8][9][10][11][12][13][14][15][16] and seems to be ubiquitous at small U c /k B T . However, while sudden collapse * Corresponding author:P.Bartlett@bristol.ac.uk has been attributed to channel formation within the gel [12,13], the microscopic processes operating have never been fully established.…”