Gelation on the microscopic scale

Abstract

Particle-tracking methods are used to study gelation in a colloidal suspension of Laponite clay particles. We track the motion of small fluorescent polystyrene spheres added to the suspension, and obtain the micron-scale viscous and elastic moduli of the material from their mean-squared displacement. The fluorescent spheres move subdiffusively due to the microstructure of the suspension, with the diffusive exponent decreasing from close to one at early times to near zero as the material gels. The particle-tracking data show that the system becomes more heterogeneous on the microscopic scale as gelation proceeds. We also determine the bulk-scale moduli using small-amplitude oscillatory shear rheometry. Both the macroscopic and microscopic moduli increase with time, and on both scales we observe a transition from a primarily viscous fluid to an elastic gel. We find that the gel point, determined as the time at which the viscous and elastic moduli are equal, is length-scale dependent--gelation occurs earlier on the bulk scale than on the microscopic scale.

A microrheological study of hydrogel kinetics and micro-heterogeneity

“…It should be noted that, for each of the samples studied here, none of the three metrics used reports a significant rise in heterogeneity after the gel point. This is in direct contrast to data systems that are more heterogeneous, where heterogeneity is observed to continue rising after the gel point [18]. It should also be noted that due to the evolving nature of the hydrogel, it is not possible to study heterogeneity at particularly long lag times as individual microrheology measurements must be short enough to ensure that the sample does not change significantly during measurement.…”

“…Early work on using microrheology to probe heterogeneity focussed on biologically relevant materials such as actin, both as filaments and bundles [27] and as α-actinin cross-linked networks [28]. There are also reports of microrheological studies of the heterogeneity of solutions of chromosonal DNA [29] and of industrially relevant materials such as Laponite [18] and wheat gliadin suspensions [30]. A number of different metrics have been developed by various research groups to quantify micro-heterogeneity [31][32][33], although few studies have compared these different approaches.…”

“…Important factors in designing hydrogels for specific applications include the kinetics of formation [18] and the spatial heterogeneity of the hydrogel network [19]. Understanding the former is essential when designing socalled smart systems that gel under pre-specified conditions [20], whereas the latter is highly relevant to a number of biological applications.…”

On the kinetics of acid sodium caseinate gelation using particle tracking to probe the microrheology

“…That is, it is assumed that the sample is homogeneous and that there is no local variation in the structure. Although this assumption is valid for simple fluids, most colloidal systems are more complex and commonly present spatial heterogeneities [15][16][17][18][19]. In order to understand the origins of the overall response, it is therefore necessary to probe rheology over shorter length scales and particle tracking provides this possibility.…”

Vibrating wire rheometry

“…Measurements in Newtonian fluids and a homogeneous viscoelastic polymer solution are presented to demonstrate the efficacy and accuracy of the device. We also show how the device can be used to probe the time-dependent changes in microstructure of an aging suspension of Laponite clay [16,17] .…”