Dynamics Of Dense Suspensions Research Articles

Driven dynamics in dense suspensions of microrollers.

We perform detailed computational and experimental measurements of the driven dynamics of a dense, uniform suspension of sedimented microrollers driven by a magnetic field rotating around an axis parallel to the floor. We develop a lubrication-corrected Brownian dynamics method for dense suspensions of driven colloids sedimented above a bottom wall. The numerical method adds lubrication friction between nearby pairs of particles, as well as particles and the bottom wall, to a minimally-resolved model of the far-field hydrodynamic interactions. Our experiments combine fluorescent labeling with particle tracking to trace the trajectories of individual particles in a dense suspension, and to measure their propulsion velocities. Previous computational studies [B. Sprinkle et al., J. Chem. Phys., 2017, 147, 244103] predicted that at sufficiently high densities a uniform suspension of microrollers separates into two layers, a slow monolayer right above the wall, and a fast layer on top of the bottom layer. Here we verify this prediction, showing good quantitative agreement between the bimodal distribution of particle velocities predicted by the lubrication-corrected Brownian dynamics and those measured in the experiments. The computational method accurately predicts the rate at which particles are observed to switch between the slow and fast layers in the experiments. We also use our numerical method to demonstrate the important role that pairwise lubrication plays in motility-induced phase separation in dense monolayers of colloidal microrollers, as recently suggested for suspensions of Quincke rollers [D. Geyer et al., Phys. Rev. X, 2019, 9(3), 031043].

Translational and rotational dynamics in dense suspensions of smooth and rough colloids

We demonstrate that colloidal particles with surface roughness exhibit hindered rotational diffusion in quiescent dense suspensions. This is accomplished by the use of confocal microscopy and particle tracking to follow the translational and rotational dynamics of smooth and rough colloids suspended in a refractive index and density matched organic solvent. Measurement of the three-dimensional rotational diffusion is enabled by the addition of inert Janus tracers made of native colloids coated with a thin layer of aluminum. These experiments show that the mean square displacement (MSD) is unaffected by particle roughness, while the mean square angular displacement (MSAD) decreases for rough colloids at high volume fractions. Our results quantify the slowdown in the rotational dynamics of rough colloids, which is evidently due to steric frustration caused by the surface topography of the particles.

Kinetic arrest, dynamical transitions, and activated relaxation in dense fluids of attractive nonspherical colloids

The coupled translation-rotation activated dynamics in dense suspensions of attractive homogeneous and Janus uniaxial dicolloids are studied using microscopic statistical mechanical theory. Multiple kinetic arrest transitions and reentrant phenomena are predicted that are associated with fluid, gel, repulsive glass, attractive glass, plastic glass, and novel glass-gel states. The activated relaxation rate is a nonuniversal nonmonotonic function of attraction strength at high volume fractions due to the consequences of a change of the transient localization mechanism from caging to physical bonding.

Dynamics in dense suspensions of charge-stabilized colloidal particles

The dynamic behavior of charge-stabilized colloidal particles in suspension was studied by photon correlation spectroscopy with coherent X-rays (XPCS). The short-time diffusion coefficient, D(Q) , was measured for volume concentrations phi < or = 0.18 and compared to the free particle diffusion constant D(0) and the static structure factor S(Q) . The data show that indirect, hydrodynamic interactions are relevant for the system and hydrodynamic functions were derived. The results are in striking contrast to the predictions of the PA (pairwise-additive approximation) model, but show features typical for a hard-sphere system. The observed mobility is however considerably smaller than the one of a respective hard-sphere system. The hydrodynamic functions can be modelled quantitatively if one allows for an increased effective viscosity relative to the hard-sphere case.

Structure and Dynamics in Dense Suspensions of Micellar Nanocolloids

We present an experimental description of the dynamics of a dense colloidal suspension of model hairy nanoparticles using dynamic light scattering. These colloids were obtained by cross-linking the poly(2-cinnamoylethyl methacrylate) cores of polystyrene-block-poly(2-cinnamoylethyl methacrylate), copolymer micelles. Because of their small size and the small spacing of their liquidlike structure, as detected with small-angle neutron scattering, these particles enable the systematic investigation of the low-scattering wavevector (q) fraction of the dynamic structure factor (S(q,t)) away from the peak, which relates to the osmotic modulus of the suspension. The two relaxation processes contributing to S(q,t) are the fast cooperative diffusion of the concentration fluctuations (hair interactions) and the self-diffusion of the slightly polydisperse cores (incoherent contribution). The former speeds up and loses intensity with increasing concentration, analogous to linear and hyperstar polymer solutions, whereas the latter slows down and exhibits virtually increasing intensity with concentration as a consequence of the correlation hole at low q's, much like the hard sphere colloids. The study of these hairy particles contributes to the overall picture of the dynamic response of concentrated colloidal suspensions sterically stabilized by grafted macromolecules.