Structure and dynamics of colloidal depletion gels: Coincidence of transitions and heterogeneity

Abstract

Transitions in structural heterogeneity of colloidal depletion gels formed through short-range attractive interactions are correlated with their dynamical arrest. The system is a density and refractive index matched suspension of 0.20 volume fraction poly(methyl methacyrlate) colloids with the non-adsorbing depletant polystyrene added at a size ratio of depletant to colloid of 0.043. As the strength of the short-range attractive interaction is increased, clusters become increasingly structurally heterogeneous, as characterized by number-density fluctuations, and dynamically immobilized, as characterized by the single-particle mean-squared displacement. The number of free colloids in the suspension also progressively declines. As an immobile cluster to gel transition is traversed, structural heterogeneity abruptly decreases. Simultaneously, the mean single-particle dynamics saturates at a localization length on the order of the short-range attractive potential range. Both immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian distributions of single particle displacements reveal enhanced populations of dynamical trajectories localized on two different length scales. Similar dependencies of number density fluctuations, free particle number and dynamical length scales on the order of the range of short-range attraction suggests a collective structural origin of dynamic heterogeneity in colloidal gels.

Aqueous Colloid + Polymer Depletion System for Confocal Microscopy and Rheology

“…Upon addition of polymer at concentrations greater than or equal to 5.36 mg mL −1 , the particles became dynamically arrested. These measurements confirm that the TFEMA-co-tBMA/PAM depletion system exhibits the dynamic arrest observed for colloidal gels in earlier microscopic studies of PMMA/PS depletion mixtures [23,40,67]. …”

“…The particles behaved approximately as hard spheres when [NaCl] = 20 mM, but became unstable and aggregated when the concentration of NaCl was further increased. At a fixed salt concentration of 20 mM, changes in the particle structure and dynamics with increasing depletant (PAM) concentration observed at two particle volume fractions (φ = 0.05 and 0.3) followed the trends reported in earlier experiments on depletion-driven gelation of nearly hard spheres [22,23]. To demonstrate the suitability of this system for rheology, we measured the viscosity and first normal stress difference N 1 of two suspensions at φ = 0.4 with and without added polymer.…”

“…Inducing an attraction between particles, for example by adding a non-adsorbing depletant [19], shifts the equilibrium phase boundaries [20]. Depletion interactions can also generate other kinds of nonequilibrium solids such as attractive glasses [21] and colloidal gels [22,23], whose mechanical properties depend upon the strength and nature of the interparticle attractions. Imaging experiments, in this context, allow the microscopic particle structure to be linked to the macroscopic mechanical properties [24,25].…”

Spatially and temporally reconfigurable assembly of colloidal crystals

“…For the assembly kinetics, time series of 100-150 frames (512 Â 512 pixels) with spatial resolution of 49 nm pixel À 1 were collected at a frame rate of 1.6 s. For the assembly structure, 3D image volumes of 25 Â 25 Â 10 mm 3 were acquired at the voxel size of 49 Â 49 Â 46 nm 3 . Centroidal coordinates of all particles were determined by methods discussed in Crocker et al and characterized in Dibble et al 34,35 The image analysis yielded centroidal coordinates resolved at the scale of ± 35 nm in the object plane and ± 45 nm in the axial direction. From these centroidal coordinates, we characterized the crystallization kinetics by determining the number of particles identified at each time frame.…”

A review of solid–fluid selection options for optical-based measurements in single-phase liquid, two-phase liquid–liquid and multiphase solid–liquid flows

“…Of note is the fact that PS is vulnerable to a number of chemicals, such as cyclohexyl bromide and decalin (Dibble et al 2006), and its use as a RIM solid in the literature is rare. However, one application example by Koh et al (1994) had particles made from PS with divinylbenzene cross-linkage that were both density and RI (1.600) matched to a mixture of 1-methylnaphthalene, 1-chloronaphthalene, and polyalkylene glycol.…”