Session A27: Colloids and Granular Materials

Multi-scale structure in SeSP Dispersions

We use Monte-Carlo methods to study the structural properties of dispersions of Superellipse Sector Particles (SeSPs). Superellipses are generalizations of ellipses that allow for convexity and sharp corners; SeSPs expand the range of particle shapes by not requiring the full 2π arc. SeSPs thus allow for complete control over opening aperture, aspect ratio, and corner sharpness. Simulations generate random, dense dispersions of non-overlapping monodisperse particles and radial, bond-orientation, and spatio-orientational distributions are compared across different parameter values. We also explore the pairwise interaction of two isolated SeSPs and compare the allowable orientations with those found in the bulk dispersion. Compared with the pairwise interactions, particles in dense dispersions are more likely to be found with close nearest neighbors, with the interaction between non-overlapping radial and orientational positions apparent in the spatio-orientational distribution. This is also seen in more traditional radial distribution functions, where dispersions show a peak at small separations, compared with a plateau found in the pairwise distribution.   

Rigidity and Packing in SeSP Dispersions

We use athermal molecular dynamics simulations to study coupling, entanglement, and rigidity in dispersions of Superellipse Sector Particles (SeSPs) above and below jamming. SeSPs are segments of superelliptical curves, forming a tunable set of hard-particle shapes that exhibit complicated coupling between particle position, orientation, and separation. We use constraint counting to identify rigid clusters of particles and determine the rigidity percolation point, as well as an upper bound for random loose packing and a lower bound for random close packing. We systematically vary SeSP angularity and opening angle to investigate the impact on critical packing fractions and correlations with local-to-intermediate scale structure. We find that the random close packing fraction varies non-monotonically with angularity, unlike the random loose packing fraction, which decreases monotonically.   

Capillary bridge retention between two particles

A droplet on a flat surface, which has a convex air-liquid interface, evaporates linearly according to the time. A concave air-liquid interface in a microscopic length scale has been revealed to have different evaporation dynamics due to the negative curvature. According to the Laplace-Young equation, the low pressure inside the air-liquid interface is expected to suppress the evaporation rate, so there could be a retention effect. Significantly, the two-particle system is the representative case for that phenomenon. Here, we show a capillary bridge retention dynamics as a thermodynamic equilibrium state between two particles. We observe the capillary bridge using an optical microscope and X-ray microscope to visualize the interface line. We experimented with several micro-radius-scale systems from 10⁰ um to 10² um to find if the evaporation dynamics and thermodynamic equilibrium depend on the internal pressure induced by the air-liquid interface's negative curvature. This comprehension of capillary bridge retention would be essential in the fundamental fields such as cloud seeds, sand, soil, and industrial fields such as inkjet printing, painting, and cosmetics.   

Dynamic properties of a 2D granular analogue of a liquid puddle predicted through a 'granular capillary length'

The structure of an accumulation of granular material, such as a pile of sand, can be characterized by the angle of repose, which is dependent on the balance between gravity and inter-grain friction. In contrast, for the case of a continuum material like a simple liquid, the height of a puddle is dictated by the capillary length which balances gravity and surface tension. Here we present an experiment of a 2D pile of monodisperse microscopic oil droplets. The droplets are buoyant, adhesive, and friction is negligible. Oil droplets are deposited within a chamber and accumulate at a barrier under the influence of buoyancy. In our experiments, the structure of the pile determined by a balance between buoyant and adhesive forces, reminiscent of the spreading of a liquid puddle, even though the pile is granular and 2D in nature. We define a parameter that can describe the structure of the piles, the 'granular capillary length', analogous to the capillary length in liquids. Additionally, as droplets are being added to the pile, collapsing events occur which spread the material across the barrier. The frequency of the collapses is a function of the defined granular capillary length.   

Grain splitting: a novel mechanism for grain coarsening

Grain coarsening, the process by which crystal grains grow and merge, determines the evolution of crystal structure in metals and self-assembled colloidal materials alike, ultimately governing material properties ranging from yield strength to electrical conductivity. We study hard sphere colloidal polycrystals and find that, contrary to the current paradigm in which grains disappear only by shrinking or rotating as a single rigid object to align with a neighbor, grains may split into a pair of counterrotating regions to match the orientations of two adjacent grains. Each of these counterrotating regions is itself composed of smaller, independently rotating granules. This discovery reveals a new route for grain growth, which may provide insight to grain coarsening and aging in a wide range of polycrystalline materials.   

The free energy barrier to grain splitting

Crystal grain coarsening is widely thought to proceed via grain rotation and grain shrinkage, but recent experimental studies have revealed a third mechanism, in which a single crystal grain splits into counterrotating regions themselves composed of small, individually rotating granules. These granules are so small that the Read-Shockley model for grain boundary energy does not apply to them. Instead, we present a particle-scale theoretical description of the free energy of the polycrystal during the grain splitting event. The free energy is calculated from the entropy, which in our hard sphere system is directly computed from the free space available to each particle. We determine that there is a free energy barrier to grain splitting, and that the barrier height decreases with decreasing grain size. Consequently grain splitting is likely to play an important role in polycrystalline systems with small grains, a broad-reaching discovery that can inform future mesoscale modeling of grain coarsening, in both colloidal and atomic polycrystals.   

High precision measurement of short-ranged colloidal interactions with digital holographic microscopy

Though short-ranged attractive interactions such as the depletion interaction are widely used in the field of colloidal self-assembly, direct characterization of the interaction potential is currently limited by the precision of measurement techniques. We show that short-ranged colloidal interactions can be measured optically at separation distances as small as a few nanometers. We do this by combining digital holographic microscopy and forward modeling, which allows us to precisely track pairs of interacting colloidal particles in three dimensions and at high frame rates. Through a Bayesian approach that accounts for uncertainties in the sizes and positions of the particles, we infer the pair potential from measurements of the fluctuations in separation distance. We validate the results by comparison with indirect measurements of the interaction.   

Metallization of colloidal crystals

Size asymmetric binary colloidal crystals with structures analogous to ionic atomic crystals undergo a transition where the smaller component delocalizes as the temperature increases, generating "metallic" colloidal crystals. The small particles roam among the large particles holding together the lattice, as electrons do in metals. In this talk, I will describe the nature of this localized-delocalized sublattice transition in size asymmetric colloidal mixtures and make analogies with an insulator-metal transition that is largely driven by phonons of different crystal phases.   

Charged Nanoparticle Assemblies in Extremely Saline Solutions

Electrostatic interactions manifest in counterintuitive ways in highly concentrated salt solutions.  In this study, we utilize assemblies of highly charged nanoparticles as responsive probes for the changes in the strength and spatial extent of electrostatic interactions over a very wide range of ionic conditions. Gold nanoparticles functionalized with monodisperse, non-base-pairing DNA are aggregated in solutions containing calcium ions. Our small-angle X-ray scattering (SAXS) measurements reveal that raising the solution ionic strength induces transitions from FCC, to body-centered cubic (BCC), to amorphous structures. Furthermore, the nearest-neighbor distances in the lattices begin to increase above a threshold salt concentration, suggesting an increase in screening length. This anomalous lattice expansion is coincident with the appearance of short-ranged correlations between ions in the bulk solution, as revealed by in situ wide-angle X-ray scattering. The generality of the phase transitions and the lattice expansion will be discussed. This work demonstrates how interactions between charged objects continue to evolve past the regime where classical theory predicts electrostatics to become negligible.   

A multithreaded extension to Voro++ for rapid analysis of particle systems

The Voronoi tessellation is a widely used technique in computational geometry, and has proven effective in many branches of physics for analyzing systems of grains, particles, or atoms. Voro++ is a C++ software library for computing the Voronoi tessellation that was released in 2009. Here, we take advantage of modern computer hardware, and extend the original serial version to allow for parallel computation of Voronoi cells on multi-core computers. We look at the best parallel strategies to generate Voronoi diagrams for different particle distributions (e.g. homogeneous, clustered, or localized), and we show near-perfect parallel efficiency in the performance in many cases, allowing systems with millions of particles to be analyzed in a reasonable timeframe. We demonstrate an example usage of the library to characterize high temperature crystalline systems using a topological analysis of Voronoi cells.   

Holographic Tracking of Jeffery Orbits in Colloidal Dimers and Ellipsoids

An in-line hologram of a colloidal sphere can be analyzed with the Lorenz-Mie theory of light scattering to measure the sphere's three-dimensional position with nanometer-scale precision while also measuring its diameter and refractive index with part-per-thousand precision. Applying the same technique to aspherical or inhomogeneous particles yields the position, diameter and refractive index of an effective sphere that represents an average over the particle's geometry and composition. This effective-sphere interpretation has been applied successfully to a variety of asymmetric particles whose inhomogeneities appear on length scales smaller than the wavelength of light. Here, we combine numerical and experimental studies to investigate effective-sphere characterization of symmetric dimers of micrometer-scale spheres and colloidal ellipsoids. Our studies demonstrate that the effective-sphere interpretation usefully identifies dimers in holographic characterization studies of monodisperse colloidal spheres. The effective-sphere estimate for a dimer's axial position closely follows the ground truth for its center of mass. Trends in the effective-sphere diameter and refractive index, furthermore, can be used to measure a particle's three-dimensional orientation. When applied to colloidal dimers and ellipsoids transported in a Poiseuille flow, the estimated orientation distribution is consistent with expectations for Brownian particles undergoing Jeffery orbits.    

Segregation of microspheres by applying a mechanical-driven force

Recently, experiments have been performed using agglomerated hydrophilic silica, or less agglomerated hydrophobic polystyrene microspheres with diameters between 3 to 10 μm to study their self-organization by either agitating or manually rubbing these spheres on (non-) coated silicon substrates. We serendipitously observe that these microspheres preferentially adhere to and form monolayers on the fluorocarbon-coated substrates. Using Kelvin probe force microscopy, we show that the tribocharging mechanism lies at the heart of these observations. The particles and substrates acquire opposite charges, inducing an electrostatic attraction. Consequently, the particles adhere to the fluorocarbon layers. Our results are further corroborated with colloidal probe (CP) measurements performed at different humidity levels, in which a long-range electrostatic attraction can be measured between the CP and substrate.

Altogether, the results highlight that the relative humidity and, surprisingly, even the substrate material carrying the fluorocarbon layer strongly affects the tribocharging process.  Soon, the same type of experiments will be performed to explore the self-organization of binary mixtures into monolayers on different polymer surfaces driven by the tribocharging mechanism.   

Spin vortices in Janus colloid monolayer

Colloid has been used for decades to imitate various aspects of molecular materials and has provided a foundation for confirming theoretical models of nucleation or melting. Here, we propose an experimental system where Janus spheres are on top of a triangular lattice, and interaction between them is orientationally dependent. In this model, we control orientational interaction between Janus spheres by tuning the applied electric field. By this model, we find a vortex pattern on Janus colloidal monolayer and validate that the z-component of Janus spheres' orientation is correlated with their movement in XY-plane. Our works will probably provide us with insights into correlations between orientational collective motions and topological defects.   

Sizing multimodal suspensions using differential dynamic microscopy

Particle sizing is vital in both fundamental and applied colloid science. Differential Dynamic Microscopy (DDM) combines real space imaging with analysis in Fourier space to extract the same quantity as dynamic light scattering (DLS), the intermediate scattering function, at considerably lower wavevectors. But, it is based on different physics. Fitting of the ISF yields diffusion coefficients and hence particle size.

However, the limits of DDM in sizing have not been investigated in depth. In particular most studies have focused on monodisperse systems. Using well-characterised multimodal suspensions we study the ability of DDM to size small particles in the presence of large ones. Under both brightfield and phase contrast conditions the DDM signal scales with the sixth power of particle radius (the same as DLS). Nevertheless, by selecting different regions of the real-space images to analyse with a DDM algorithm we were able to isolate the signal from small particles. This allows DDM to extract the size of small particles in the presence of a higher concentration of larger species than DLS.