Session Z29: Colloids and Granular Materials

Defect dynamics of colloidal antiferromagnetic tetratic phase confined quasi-2d

We experimentally realize an antiferromagnetic tetratic phase with short-range translation order and quasi-long-range four-fold bond orientational order. The experiments employed hard-sphere micron size colloidal particles confined between parallel plates. We verify theoretical predictions (PHYSICAL REVIEW LETTERS 128, 255501 (2022)) of topological order resulting in bound pairs of oppositely-oriented dislocations and free pairs of dislocations oriented at 90 degrees. We demonstrate movement of free defect pairs in the direction of their (total) Burgers vector, as well as spontaneous formation and annihilation of oppositely oriented sets of free defect pairs. Finally, we present a simple free-volume model explaining the spontaneous defect movement.   

A Unified Kinetic Picture of Heterogeneous and Homogeneous Colloidal Crystallization from Transition Rate

Heterogeneous and homogeneous crystallization exhibit distinct phase transition kinetics. However, due to the difficulty of realizing both heterogeneous and homogeneous crystallization under one identical thermodynamic condition, an independent comparison of the two remains lacking. In a colloidal system, we experimentally realize a systematic switching from heterogeneous to homogeneous crystallization under one identical thermodynamic condition, capture the entire crystallization process of each colloidal particle with confocal microscopy, and discover a universal kinetic picture of the two situations. In the heterogeneous crystallization, we reveal an unexpected variation of critical nucleus size with boundary disorderness instead of contact angle, which violates the classical nucleation theory but enables the systematic switching. Moreover, analogous to the reaction rate in chemical reactions, we theoretically propose and experimentally measure the transition rate, which quantifies the transition probability between any two crystallization intermediate structures. To our surprise, regardless of heterogeneous or homogeneous crystallization, all 20 kinetic transition rates fall onto universal master curves throughout both nucleation and growth stages, which unifies the two distinct crystallization situations into a universal kinetic picture, i.e., all transition probabilities only depend on the local order parameter but not the heterogeneous or homogeneous mode.   

Multiscale modelling framework for the mechanical properties of illite

Clay is one the most important materials in earth's crust and has wide applications in geotechnical, environmental, and biomedical engineering. It has complex mechanical properties due to its particulate nature and complex physico-chemical interactions between primary particles. In this presentation, a multiscale framework will be presented, linking the mechanical properties of illite from atomistic up to macroscopic scales. Illite is a typical type of clay with flexible plate-like particles. The framework includes the study of inter-particle interaction at atomistic scale through free energy perturbation calculations with molecular dynamics simulations, from which the potential of mean force will be used to calibrate the coarse-grained force field to be used in meso-scale simulations. The modes of deformation and mechanical response of the mesoscopic systems are incorporated into a thermodynamically consistent constitutive model describing the small-strain elastic stiffness of illite clay at macroscopic level. The simulation results are compared with experimental results on the same material. The framework can provide good guidance on similar multiscale study on physico-chemical properties of clay to facilitate efficient material modification targeting for a greener civil engineering construction practice and has potential application in the investigation of other geomaterials.

National Science Foundation (NSF) [ACI-1548562] under the grant no. 1702689.   

Spark and glow discharges in supersonic granular flows

Supersonic granular flows often host spark and glow discharges. In nature, such phenomena are exemplified by the lightning storms that accompany volcanic eruptions. Other types of explosive particulate flows are also capable of generating discharges. When and where discharges occur seem to depend strongly on the compressible hydrodynamics of the multiphase system. For instance, experimental work has found that discharges in granular jets are generally confined to the region of rarefaction between the source (nozzle or volcanic vent) and the Mach disc. Additionally, discharges are most readily observed when large overpressures (relative to ambient) exist at the source. Surprisingly, however, these conditions are insufficient to guarantee the production of electrostatic phenomena in supersonic jets. Specifically, the chemical composition and physical properties of the solid phase in these flows appear to play key roles in the genesis of discharges. Here, we provide a review of discharge processes in supersonic flows. We first discuss the various breakdown criteria that may operate in such systems. We also describe the radio frequency signals emitted by spark discharges and how to detect them. Such signals provide information about the structure of the flows, opening the way for novel instrumentation to diagnose explosive phenomena remotely. Lastly, recent experiments with various materials—ranging from diamond particles to ground coffee—provide insight into how conductivity, ductility, and polarizability tune the capacity of a jet to manifest discharges.   

Adsorption of surfactant on colloidal particles

For a smooth spherical or cubic particle, adsorption should scale with the square of the particle size.  Work in our lab with three different particle compositions that have been reduced in size via ball milling indicates that such scaling is not followed where surfactant adsorption is measured, adsorption scales linearly with particle size.   The particles show no obvious anisotropy; so macroscopic particle shape is not an explanation as could be the case for cylindrical particles for example. An explanation for this scaling behavior will be presented, along with a comparison of the results from nitrogen adsorption which reveals something beyond surfactant adsorption.   

Relaxation of a 2D oil droplet raft at a curved surface

When continuum materials with cohesive forces are perturbed from an equilibrium configuration, they relax over time tending toward the lowest energy shape. We are interested in studying the physics of a similar ageing process in a two-dimensional granular system in which individual particle rearrangements can be directly observed. We present an experiment in which a two-dimensional raft of microscopic cohesive oil droplets is elongated then allowed to relax back to a preferred shape. As the droplet raft is gently confined by a curved meniscus, we can study the relaxation toward equilibrium for hours to days. Over sufficiently long times, coalescence plays a crucial role introducing disorder in the system through local defects, and promotes particle rearrangements. Varying the size of droplets and strength of cohesive forces, we investigate the geometry and dynamics of short- and long-term structure ageing due to large-scale relaxation and local coalescence events.   

Microgels, their ionic cloud and some neutron scattering experiments

This talk will focus on colloidal hydrogels -microgels- and emphasize their significance and what new ingredients they bring compared to more conventional colloids. For instance, colloidal hydrogels can spontaneously deswell to allow crystallization of binary mixtures without defects. The key to this behavior relies on the existence of peripheric charged groups, responsible for providing colloidal stability when deswollen, and the associated counterion cloud. When in close proximity, clouds of different particles overlap respectively freeing the associated counterions, which are then able to exert an osmotic pressure that can potentially shrink the particles. Up to now, however, no direct measurement of this ionic cloud exists, perhaps even also for hard colloids, where it is referred to as an electric double layer. We will present recent small-angle neutron scattering experiments with contrast variation with different ions to isolate the change in the form factor directly related to this counterion cloud, to further obtain its geometric properties. Our results highlight that the modeling of colloidal hydrogel suspensions must unavoidably and explicitly consider the presence of this cloud, which exists for nearly all colloidal hydrogels synthesized today.   

The interplay between defects and collective motion in a 2D Yukawa crystal

2D materials offer an ideal platform to quantify collective motion. While defects are important for rearrangements in crystals, the role of collective motion is often overlooked. To understand the interplay between defects and collective motion, we perform molecular dynamics simulations of a charged colloidal crystal—a model extendable to 2D dusty plasma crystals. To unambiguously distinguish thermal vibrations from rearrangements, we map configurations from the real trajectories to the nearest energy-minimized configuration, or inherent structure (IS). In the resulting IS trajectory, the crystal has extended periods in the defect-free ground state interrupted by brief transitions to excited energy states where defects spawn replacing particle clusters with string-like geometries; small collective ring exchanges can also occur in the ground state without any defects. Defects, identified through the Voronoi tessellation as particles with 5 or 7 neighbors, must occur in pairs, and usually form clusters of 4 or 6 neighboring defects; these defect clusters also come in pairs and are randomly distributed in space. Collective particle exchanges link the defect clusters, ultimately healing them. Thus, a picture emerges where diffusion is primarily a result of collective motion, facilitated by defect clusters.   

Direction-dependent dynamics of colloidal particle pairs and the Stokes-Einstein relation in quasi-two-dimensional fluids

Hydrodynamic interactions are important for diverse fluids, especially those with low Reynolds number such as microbial and particle-laden suspensions, and proteins diffusing in membranes. Unfortunately, while far-field (asymptotic) hydrodynamic interactions are fully understood in two- and three-dimensions, near-field interactions are not, and thus our understanding of motions in dense fluid suspensions is still lacking. In this contribution, we experimentally explore the hydrodynamic correlations between particles in quasi-two-dimensional colloidal fluids in the near-field. Surprisingly, the measured displacement and relaxation of particle pairs in the body frame exhibit direction-dependent dynamics that can be connected quantitatively to the measured near-field hydrodynamic interactions. These findings, in turn, suggest a mechanism for how and when hydrodynamics can lead to a breakdown of the ubiquitous Stokes-Einstein relation (SER). We observe this breakdown, and we show that the direction-dependent breakdown of the SER is ameliorated along directions where hydrodynamic correlations are smallest. In total, the work uncovers significant ramifications of near-field hydrodynamics on transport and dynamic restructuring of fluids in two-dimensions.   

Highly Polydisperse Colloidal Gels

We experimentally study colloidal gels composed of highly polydisperse particles. Much prior work has focused on gels composed of mono- or bi-disperse particles as the backbone, perhaps with a third species acting as a depletant. Our samples are made from highly polydisperse particles (the ratio of sizes between largest and smallest particle being approximately 10:1)  suspended in a density matched organic solvent, in addition to a polymer used to produce a depletion force. We study the effect polydispersity has on the dynamics and structures that form. Larger particles are more sensitive to the depletion force, and the depletion force acting on a small particle in proximity to a large particle is larger still (an effect similar to the enhanced depletion for particles near a wall). Thus, large particles act like nodes around which clusters of smaller particles will congregate. For our largest particles (~10 μm diameter) diffusive motion is reduced compared to that of our smallest particles (~1 μm diameter). Our work demonstrates the role that polydispersity plays in the behavior of colloidal systems.

    

Interaction dynamics between concave and circular granular particles undergoing cyclic shear

We study the dynamics of superellipse sector particles (SeSPs) in a bidisperse monolayer of disks undergoing oscillatory cyclic shear. The SeSP generating function allows for continuous parameterization of corner sharpness, aspect ratio, and opening aperture, reproducing a wide variety of particle shapes including rods, discs, U-shaped staples and, in our study, C-shaped semi-circles. SeSPs are placed in an annular planar Couette cell, oriented vertically ad sheared from the top. We minimize SeSP-SeSP interactions by keeping the number of SeSPs small and focus our attention on the behavior of the SeSPs in response to the motion of the surrounding circular disks. Particles translational and rotational motions are tracked with video analysis, and we look for reversible and irreversible motions and correlate these with the surrounding particle configurations.   

Bicontinuous particle-stabilized emulsion gels in magnetic fields: A Lattice Boltzmann simulation study

Bicontinuous interfacially stabilized emulsion gels (bijels) are particle stabilized emulsions with a bicontinuous percolating domain morphology. They can serve as templates for the fabrication of functional porous materials with tailored microstructure, including membranes, catalyst supports, and drug delivery systems. Using Lattice Boltzmann simulations, we demonstrate how bijels stabilized by anisotropic magnetic particles respond to magnetic stimuli. The simulation results show that the domain size increases with the applied field strength. We investigate the interplay of orientational order of interface-embedded particles and the domain formation by analyzing the nematic order tensor. The results indicate that the domain structure becomes anisotropic which affects the permeability and tortuosity of the microstructure. We further investigate the stability of the bijel morphology and the relaxation behavior upon switching the magnetic field off. Preliminary results suggest that the formation rate differs from the relaxation rate, and the response to magnetic fields may exhibit hysteresis. We discuss the potential application of these effects in fabricating emulsion systems with tunable/switchable domain morphology.