Observation of the Disorder-Induced Crystal-to-Glass Transition

The role of frustration and quenched disorder in driving the transformation of a crystal into a glass is investigated in quasi-two-dimensional binary colloidal suspensions. Frustration is induced by added smaller particles. The crystal-glass transition is measured to differ from the liquid-glass transition in quantitative and qualitative ways. The crystal-glass transition bears structural signatures similar to those of the crystal-fluid transition: at the transition point, the persistence of orientational order decreases sharply from quasilong range to short range, and the orientational order susceptibility exhibits a maximum. The crystal-glass transition also features a sharp variation in particle dynamics: at the transition point, dynamic heterogeneity grows rapidly, and a dynamic correlation length scale increases abruptly

Wetting and Contact Lines of Micrometer-Sized Ellipsoids

We experimentally and theoretically investigate the shapes of contact lines on the surfaces of micrometer-sized polystyrene ellipsoids at the water-air interface. By combining interferometry and optical trapping, we directly observe quadrupolar symmetry of the interface deformations around such particles. We then develop numerical solutions of the partial wetting problem for ellipsoids, and use these solutions to deduce the shapes of the corresponding contact lines and the values of the contact angles, Θc(k), as a function of the ellipsoid aspect ratio k. Surprisingly, Θc is found to decrease for increasing k suggesting that ellipsoid microscopic surface properties depend on ellipsoid aspect ratio

Quasi-Two-Dimensional Diffusion of Single Ellipsoids: Aspect Ratio and Confinement Effects

We report on video-microscopy measurements of the translational and rotational Brownian motions of isolated ellipsoidal particles in quasi-two-dimensional sample cells of increasing thickness. The long-time diffusion coefficients were measured along the long (Da) and short (Db) ellipsoid axes, respectively, and the ratio, Da /Db, was determined as a function of wall confinement and particle aspect ratio. In three dimensions (3D), this ratio (Da /Db) cannot be larger than 2, but in quasi-two dimensions, wall confinement was found to substantially alter diffusion anisotropy and substantially slow particle diffusion along the short axis compared to 3D

Phonon Spectra, Nearest Neighbors, and Mechanical Stability of Disordered Colloidal Clusters with Attractive Interactions

We investigate the influence of morphology and size on the vibrational properties of disordered clusters of colloidal particles with attractive interactions. From measurements of displacement correlations between particles in each cluster, we extract vibrational properties of the corresponding "shadow" glassy cluster, with the same geometric configuration and interactions as the "source" cluster but without damping. Spectral features of the vibrational modes are found to depend strongly on the average number of nearest neighbors, $\bar{NN}$, but only weakly on the number of particles in each glassy cluster. In particular, the median phonon frequency, $\omega_{med}$, is essentially constant for $\bar{NN}$ $<2$ and then grows linearly with $\bar{NN}$ for $\bar{NN}$ $>2$. This behavior parallels concurrent observations about local isostatic structures, which are absent in clusters with $\bar{NN}$ $2$. Thus, cluster vibrational properties appear to be strongly connected to cluster mechanical stability (i.e., fraction of locally isostatic regions), and the scaling of $\omega_{med}$ with $\bar{NN}$ is reminiscent of the behavior of packings of spheres with repulsive interactions at the jamming transition. Simulations of random networks of springs corroborate observations and suggest that connections between phonon spectra and nearest neighbor number are generic to disordered networks.Comment: 5 pages, 3 figure

Coffee Rings and Coffee Disks: Physics on the Edge

As many a coffee drinker knows, a drying drop of coffee typically leaves behind a ring-shaped stain of small grounds. Though the phenomenon is common, the mechanisms that drive it are rich with physics. As first elucidated by Robert Deegan and colleagues in 1997, the coffee ring results from radially outward fluid flows induced by so-called contact line pinning: The outer edge of a spilled coffee droplet grabs onto rough spots on the solid surface and becomes pinned in place. The evaporating drop thus retains its pinned diameter and flattens while it dries. That flattening, in turn, is accompanied by fluid flowing from the middle of the drop toward its edge to replenish evaporating water. Suspended particles—the coffee grounds—are carried to the edge of the drop by that flow. Once there, they pile up, one at a time, into a tightly jammed packing and produce the coffee ring. Deegan and company studied the ring growth empirically by following the individual frames in a video of plastic colloidal spheres suspended in an evaporating droplet

Helical Packings and Phase Transformations of Soft Spheres in Cylinders

The phase behavior of helical packings of thermoresponsive microspheres inside glass capillaries is studied as a function of volume fraction. Stable packings with long-range orientational order appear to evolve abruptly to disordered states as particle volume fraction is reduced, consistent with recent hard sphere simulations. We quantify this transition using correlations and susceptibilities of the orientational order parameter psi_6. The emergence of coexisting metastable packings, as well as coexisting ordered and disordered states, is also observed. These findings support the notion of phase transition-like behavior in quasi-1D systems.Comment: 5 pages, with additional 4 pages of supplemental material, accepted to Physical Review E: Rapid Communication

Irreversible Rearrangements, Correlated Domains, and Local Structure in Aging Glasses

Bidisperse colloidal suspensions of temperature-sensitive microgel spheres were quenched from liquid to glass states by a rapid temperature drop, and then the glass was permitted to age. Irreversible rearrangements, events that dramatically change a particle’s local environment, were observed to be closely related to dynamic heterogeneity. The rate of these irreversible events decreased during aging and the the number of particles required to move as part of these irreversible rearrangements increased. Thus, the slowing dynamics of aging were governed by growing, correlated domains of particles. Additionally, short-range order developed, and a spatial decay length scale associated with orientational order was found to grow during aging

Two-dimensional freezing criteria for crystallizing colloidal monolayers

Video microscopy was employed to explore crystallization of colloidal monolayers composed of diameter-tunable microgel spheres. Two-dimensional (2D) colloidal liquids were frozen homogenously into polycrystalline solids, and four 2D criteria for freezing were experimentally tested in thermal systems for the first time: the Hansen–Verlet freezing rule, the Löwen–Palberg– Simon dynamical freezing criterion, and two other rules based, respectively, on the split shoulder of the radial distribution function and on the distribution of the shape factor of Voronoi polygons. Importantly, these freezing criteria, usually applied in the context of single crystals, were demonstrated to apply to the formation of polycrystalline solids. At the freezing point, we also observed a peak in the fluctuations of the orientational order parameter and a percolation transition associated with caged particles. Speculation about these percolated clusters of caged particles casts light on solidification mechanisms and dynamic heterogeneity in freezing

Melting of Multilayer Colloidal Crystals Confined Between Two Walls

Video microscopy is employed to study the melting behaviors of multilayer colloidal crystals composed of diameter-tunable microgel spheres confined between two walls.We systematically explore film thickness effects on the melting process and on the phase behaviors of single crystal and polycrystalline films. Thick films (\u3e4 layers) are observed to melt heterogeneously, while thin films ( ≤ 4 layers) melt homogeneously, even for polycrystalline films. Grain-boundary melting dominates other types of melting processes in polycrystalline films thicker than 12 layers. The heterogeneous melting from dislocations is found to coexist with grain-boundary melting in films between 5- and 12-layers. In dislocation melting, liquid nucleates at dislocations and forms lakelike domains embedded in the larger crystalline matrix; the “lakes” are observed to diffuse, interact, merge with each other, and eventually merge with large strips of liquid melted from grain boundaries. Thin film melting is qualitatively different: thin films homogeneously melt by generating many small defects which need not nucleate at grain boundaries or dislocations. For three- and four-layer thin films, different layers are observed to have the same melting point, but surface layers melt faster than bulk layers. Within our resolution, two- to four-layer films appear to melt in one step, while monolayers melt in two steps with an intermediate hexatic phase