Session Y20: Invited Session: New Anisotropy-Driven Phenomena in Colloidal Suspensions

Entropically patchy particles: Shape-driven self assembly of hard colloids

Although the structural diversity of colloidal fluids and crystals has grown substantially in recent years, it still aspires to that of atomic and molecular systems. Ionic colloidal crystals and binary nanoparticle superlattices, by exploiting electrostatic interactions in mixtures of particles of opposite charge, have substantially broadened the diversity of structures beyond those obtainable in traditional hard sphere systems, but rely on energetic interactions as well as entropy for their stability. Likewise, ``traditional'' patchy particles with sticky interactions exploit explicit attractive interactions for assembly. Here we explore the role of shape and entropy in phase transitions of hard particle fluids, in the absence of all other interactions. Using computer simulations, we show that particle shape alone can suffice to produce a rich diversity of colloidal crystals, quasicrystals, glasses and mesophases through thermodynamic self-assembly whose complexity rivals that of atomic analogues. We compare self-assembled phases of hard polyhedra with their dense packings obtained from small unit cell compressions, and show the packings tend to be less structurally complex than the assemblies. Based on our findings, we present design rules for anisotropic hard, facetted colloids as ``entropically patchy particles'' for self assembly.   

Lock-and-Key Colloids

We have developed a new kind of colloidal particle that is spherical but with one or more spherical dimples in the particle surface. These dimples serve as docking points for colloidal spheres whose radius matches the radius of dimples. The attractive docking force is provided by the depletion interaction and can be controlled by changing the depletant concentration or, in some cases, the temperature. The docking is completely reversible and mimics the classic lock-and-key interaction often used to describe protein binding. The lock-and-key binding is size specific and can be used to assemble a number of interesting colloidal superstructures, including flexible jointed chains, dumbbells, trimers, tetramers, and other assemblies. A new synthetic method for making the dimpled particles can be generalized to make a number of other new structured colloidal particles, with different functionalities analogous to block copolymers.   

Glass transitions in quasi-two-dimensional suspensions of colloidal ellipsoids

Colloidal glasses constitute of anisotropic particles were mainly studied by simulations in three dimensions with incomplete phase diagrams. Here we report the experiment about glass transitions in a colloidal suspension of micro-ellipsoids at the single-particle level. Video microscopy revealed a two-step glass transition corresponding to inter-domain freezing and inner-domain freezing respectively. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an ``orientational glass'' in the area fraction range 0.72 $<$\textit{ $\phi $} $<$ 0.79 for aspect ratio $p$ = 6 ellipsoids and 0.60 $<$\textit{ $\phi $} $<$ 0.72 for $p$ = 9. Such orientational glass is expected to be replaced by the rotator phase at small aspect ratios and the nematic phase at large aspect ratios. The observed decoupling between diffusion and relaxation for both of translational and rotational motions reflects the dynamic heterogeneity. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as approaching the glass transitions. The translational and rotational fastest- and slowest-moving ellipsoids are all spatially anticorrelated: most translational fast-moving ellipsoids and rotational slow-moving ellipsoids formed at different areas within pseudonematic domains, while most rotational fast-moving ellipsoids and translational slow-moving ellipsoids formed at different areas around the domain boundaries.   

Glassy dislocation dynamics in colloidal dimer crystals

Dislocation mobility is central to both the mechanical response and the relaxation mechanisms of crystalline materials. Recent experiments have explored the role of novel particle anisotropies in affecting the rules of defect motion in crystals. ``Peanut-shaped'' colloidal dimer particles consisting of two connected spherical lobes form densely packed crystals in 2D. In these ``degenerate crystals,'' the particle lobes occupy triangular lattice sites while the particle axes are randomly oriented among the three crystalline directions. One consequence of the random orientations of the dimers is that dislocation glide is severely limited by certain particle arrangements in the degenerate crystals. Using optical tweezers to manipulate single lobe-sized spherical intruder particles, we locally deform the crystal, creating defects. During subsequent relaxation, the dislocations formed during the deformation leave the crystal grain, either via annihilation with other dislocations or by moving to a grain boundary. Interestingly, in large crystalline grains this dislocation relaxation occurs through a two-stage process reminiscent of slow relaxations in glassy systems, suggesting the novel concept that glassy phenomena may be introduced to certain kinds of colloidal crystals via simple anisotropic constituents.