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Using a Stokesian dynamics simulation, the microstructure of particle aggregates at an oil/water interface with an applied Couette flow is studied. The results of the aggregation are consistent with previously published experimental work demonstrating multiple regimes of behavior based on the relative strength of shear and capillary forces. In previous work, densification of aggregates at low shear rates was theorized to occur due to short time scale fragmentation/reaggregation of aggregates with rigid particle bonds. In simulations, densification is observed at low shear rates but occurs due to local reorganization of particles due to capillary torques over long time scales. Moderate shear rates create mobile bonds between particles at shorter time scales, allowing aggregates to fragment without reaggregation into smaller isolated clusters, consistent with prior experimental work. At the highest shear rates, aggregation is inhibited completely.

This manuscript quantifies the hydrodynamic drag of a spherical droplet translating in unsteady Stokes flow when the droplet has a thin, complex membrane. All forces (e.g., Stokes drag, Basset forces, unsteady memory forces, etc.) have the same functional form as the drag of a clean spherical droplet, as long as one replaces the interior viscosity with an effective (frequency-dependent) one that depends on the viscoelastic moduli of the membrane. The shear elastic moduli of the membrane do not modify the unsteady drag of the droplet – only the dilatational modes matter. We demonstrate how this result can be obtained using simple symmetry/scaling arguments. The results are written for any linearly viscoelastic membrane, but we in particular examine the cases of a purely viscous membrane, purely elastic membrane, one-mode Maxwell membrane, and one-mode Kelvin-Voigt membrane. For the latter two models, we quantify how the membrane relaxation time alters the time-dependent motion of the droplet.

This study performs Stokesian dynamics simulations of suspensions of rigid spheres to determine the conditions under which swollen starch granules can be considered rigid for rheology measurements. The first part of the study benchmarks the numerical simulations to experimental measurements of storage modulus (G′) for monodisperse polystyrene suspensions. The second part of the study compares the simulations to experimental measurements of G′ for waxy maize and normal rice starch suspensions swollen to volume fractions 0.4 0.5. These results indicate that granule deformability plays an important role when φ > 0.5, and indicates the range where one can apply theories of rigid suspension rheology.

Hydrophobic/hydrophilic mixtures of latex particles at an air/water interface self-assemble, creating space filling, interconnected aggregates as the relative surface fractions of the dissimilar particles approach 0.5, which is reflected both in qualitative observation and fractal dimension of the microstructure. It is hypothesized that this change in microstructure occurs due to an asymmetry in the electrostatic interaction between similar and dissimilar particles caused by polarization of hydrophilic particles by hydrophobic particles. The changes in both microstructure and interparticle interactions significantly impact the interfacial viscoelasticity. As greater shape complexity is observed, interfacial complex moduli can increase by as much as 3 orders of magnitude and interfaces become more elastic.

Hypothesis Aggregation of particles on a liquid interface is controlled by inter-particle forces and hydrodynamic interactions. Previous experimental work has shown atypical structures despite diffusion limited cluster aggregation like behavior. It is likely that this is primarily due to the role of capillary quadrupoles in allowing particle repositioning after aggregation, which is tested here. Experiments Using Stokesian dynamics and inter-particle forces unique to particles at liquid interfaces, aggregation of particles adsorbed to a liquid interface is studied. Simulations’ parameters are adjusted to control hydrodynamic interaction strength, initial particle position, and inter-particle forces magnitudes to compare to existing experimental results and hypothesis. Findings It is found that initial particle position plays a small role on equilibrium interfacial microstructure but has a significant impact on aggregation kinetics. Interfacial hydrodynamic interactions and inter-particle forces have a strong impact on equilibrium microstructure by altering the amount particles can reposition, which is consistent with published results. Capillary forces that allow significant repositioning after contact appear to play a key role in previously observed fractal dimensions of particle laden interfaces.

Toxin retention is felt to be a major contributor to the development of uremia in patients with advanced chronic kidney disease and end-stage renal disease (ESRD). Uremic retention compounds are classically divided into 3 categories: small solutes, middle molecules, and protein-bound toxins. Compounds comprising the first category, for which the upper molecular weight limit is generally considered to be 500 Da, possess a high degree of water solubility and minimal or absent protein binding. The second category of middle molecules has largely evolved now to be synonymous with peptides and proteins that accumulate in uremia. Although not precisely defined, low-molecular weight proteins as a class have a molecular weight spectrum ranging from approximately 500 to 60,000 daltons. The final category of uremic retention compounds is protein-bound uremic toxins (PBUTs). As opposed to the above small, highly water-soluble toxins, which are largely by-products of protein metabolism, PBUTs have diverse origins and possess chemical characteristics that preclude the possibility of circulation in an unbound form despite being of low molecular weight. This review is the first in a series of papers designed to provide the current state of the art for extracorporeal treatment of ESRD. Subsequent papers in this series will address membranes, mass transfer mechanisms, and future directions. For small solutes and middle molecules, particular emphasis is placed on the important clinical trials that comprise the evidence base regarding the influence of dialytic solute removal on outcome. Because such trials do not exist for PBUTs, the discussion here is instead focused on solute characteristics and renal elimination mechanisms.

Hypothesis It is possible to control the absolute and relative magnitude of repulsive and attractive interactions and hence microstructure of interfacial particles at and air/water interface by adjusting subphase composition. It should be possible to modify interfacial viscoelasticity from elastic to viscous behavior through these changes to interfacial microstructure. Experiments Particle laden interfaces are made from micron sized polystyrene at an air/water interface. The inter-particle interactions are controlled by the subphase salt concentration and addition of both non-ionic and ionic surfactants. These interfaces are then characterized using an interfacial rheometer with a custom visualization system. Findings Three distinct microstructures are observed. Low repulsion and high attraction systems exhibit a soft glassy rheology with a disordered but dense microstructure. Creating high repulsion results in a dense hexagonal crystal. Finally, in systems with reduced repulsion and attraction, a hexatic phase can be observed. Each of these microstructures exhibit unique interfacial viscoelastic behavior. These results indicate that control over the properties of these interfaces, and hence Pickering emulsions, is possible through manipulation of interparticle forces.

A Stokesian dynamics simulation of the effect of surface Couette flow on the microstructure of particles irreversibly adsorbed to an interface is presented. Rather than modeling both bulk phases, the interface, and particles in a full 3D simulation, known interfacial interactions between adsorbed particles are used to create a 2D model from a top down perspective. This novel methodology is easy to implement and computationally inexpensive, which makes it favorable to simulate behavior of particles under applied flow at fluid-fluid interfaces. The methodology is used to examine microstructure deformation of monodisperse, rigid spherical colloids with repulsive interactions when a surface Couette flow is imposed. Simulation results compare favorably to experimental results taken from literature, showing that interparticle forces must be 1 order of magnitude greater than viscous drag for microstructure to transition from aligned particle strings to rotation of local hexagonal domains. Additionally, it is demonstrated that hydrodynamic interactions between particles play a significant role in the magnitude of these microstructure deformations.