Session D12: Colloidal Crystals, Suspensions and Films

Glassy Dislocation Relaxation in Colloidal Peanut Crystals

Previous studies of dislocations in crystals of colloidal dimers have revealed unusual restrictions on dislocation glide. In the current study, we induce defect formation in such crystals using an optically manipulated spherical intruder particle dragged through an otherwise pure dimer crystal grain. We find that the relaxation response of the perturbed crystal changes as a function of the size of the grain. For small grains, the crystal relaxes via unrestricted dislocation glide, while in larger grains, other slower relaxation mechanisms are utilized. Furthermore, we have uncovered a two-stage defect relaxation process in crystals of dimers, reminiscent of relaxation in glassy systems, in which an initial fast glide response is followed by a slower relaxation process where dislocations hop between caged configurations. We find that the relaxation decay of dislocations is consistent with the combination of a fast exponential decay followed by a slow logarithmic decay characterized by a timescale 5 orders of magnitude longer than that of the exponential decay. Together these results reveal an interesting new class of materials possessing crystalline order but whose defects are characterized by glassy behavior.   

Elastically Disordered Perfect Colloidal Crystals

We use spherical microgel colloidal particles to study lattice dynamics in a three-dimensional crystal using optical microscopy. We find that the local bond length fluctuations vary by as much as 75{\%} from bond to bond despite less than 2{\%} fluctuations in the equilibrium bond lengths. We show how to calculate the low-energy eigenmodes and the density of states in the presence of the strong heterogeneity. We find that the lowest energy eigenmodes are dominated by a few long-wavelength planewaves, and the density of states shows Debye-like behavior at low energy. This work has been partially supported by the NSF through Grants DMR-0619424 and DMR-0645596, by ACS-PRF and Alfred P. Sloan foundation.   

Formation and Phase Transformations of Helical Structures of Colloidal Spheres

We experimentally explore the ordering of thermal quasi-one dimensional helical structures of monodisperse spheres.~ Helical packings of thermoresponsive colloid particles are formed in glass microcapillaries and display evidence of long-range orientational order at high volume fractions.~ As volume fraction is decreased, these ordered packings transition to structurally disordered states.~ Orientational order parameters and susceptibilities demonstrate the abrupt nature of this crossover.~ Coexistence of ordered and disordered states is also exhibited at lower volume fractions, as well as coexistence of ordered domains with different pitch and chirality.~ Such coexistence lends credence to the notion of discontinuous transitions in these structures.~ We also present preliminary experimental work on producing and controlling the formation of ordered helical structures of unconfined colloids by tuning both short-range attractive and dipolar interactions between particles.   

Melting of Colloidal-Crystal Films

We studied the melting of multilayer colloidal crystals composed of diameter tunable microgel spheres with short-ranged repulsive interactions confined between two glass walls. Samples are annealed into large crystalline domains so that the finite size effects are negligible. Different melting behaviors were observed in three thickness regimes: 1. Thick films ($>$ 4-layer) melt from grain boundaries in polycrystals and from surfaces in single crystals. The liquid-solid coexistence regime decreases with the thickness and vanishes at 4 layers. 2. Thin films (2 to 4-layer) melt homogenously from both grain boundaries and surfaces. One-step melting is observed in 2-, 3- and 4-layer triangular and square lattices. 3. Monolayers melt in two steps with a middle hexatic phase.   

Exploring the role of strain in colloidal thin film crystallization

We present results of experiments studying the effect of isotropic and directed strain on the dynamics of thin film crystallization in colloids with short-range attractive interaction. Our system consists of micron size colloidal particles and a tunable depletant allowing reversible control of the interaction with small temperature changes. We explore the role of strain on the dynamics of melting and freezing and the equilibrium structures formed under directed strain. We find that in comparison with previously performed experiments on flat unpatterned substrates, dynamics and equilibrium morphologies on such surfaces alter dramatically. For example, crystals formed on square lattices strained along one direction tend to become highly elongated along the other direction. We consider the competition of strain and surface tension during the nucleation process under these extreme conditions.   

Melting scenario for two-dimensional plasma crystals

The solid-liquid phase transition in two-dimensional (2D) systems is not completely understood. Two most important (and competing) models of 2D melting are the dislocation theory of melting - the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory and the theory of grain-boundary-induced melting. We performed experimental study of melting in 2D crystalline lattices using complex plasma as a model system. Complex (dusty) plasmas consist of fine solid particles suspended in a weakly ionized gas. At our experimental conditions, the suspension forms a highly ordered 2D triangular lattice, where all particles can be traced using video microscopy. This lattice is very soft and can be readily melted using e.g. the radiation of a focused laser beam. We found an Arrhenius dependence of the defect concentration on the kinetic temperature in steady-state experiments, and show the evidence of metastable quenching in unsteady experiments, where the defect concentration follows a power-law temperature scaling. In all experiments, independent indicators suggest a grain-boundary-induced melting scenario.   

Phase separation in binary complex plasmas

Complex plasmas are composed of a weakly ionized gas and charged microparticles and represent an ideal system to investigate multicomponent mixtures. Microparticles usually acquire high negative charges determined by the balance of absorption of the surrounding electrons and ions, and interact via the Yukawa potential. The effective screening length characterizing the interactions is typically two orders of magnitude larger than the particle size, and can be varied from a few tenths to a few interparticle distances. This allows us to span the interaction regimes from short-range to many-body. Recent experiments performed with binary complex plasma under microgravity conditions onboard the ISS revealed different regimes of the phase separation. The interparticle interactions in complex plasmas are characterized by a positive nonadditivity which always stimulates the phase separation. For typical experimental conditions the regime of the spinodal decomposition is easily achievable.   

Fabrication of polymer-bridged monolayer of colloidal crystal at water surface

We have developed a new method to prepare a 2D colloidal crystal at the water/air interace, transferred the crystal onto a substrate, and stabilized the crystal structure of domain size 200x200 $\mu $m$^{2}$ by polymer bridging effect. We analyzed the interparticle spacing from the diffraction patterns and found that even at very high area fraction there was tiny separation between particles at water/air interface due to Coulomb repulsive force and Brownian fluctuation. After adding polyethylene oxide (PEO) into the solution, the interparticle separation is further reduced. PEO is known to adsorbed onto particle surface and provide bridging between particles. During transferring the particles onto a substrate, this adsorbed polymer layer provides a repulsive barrier to prevent the pulling from the capillary force which causes cracks in the original crystal structure. The resulting large domain of single 2D crystals can be used as a mask for fabricating periodic nanostructures.   

Nanoparticles in Aqueous Media: Crystallization and Solvation Charge Asymmetry

We examine the issue of whether dispersion forces can lead to crystallization in a system of charged nanoparticles in aqueous solution with NaCl salt. To this end, we determine the effective pair potential (EPP) among the nanoparticles, starting from a model system that explicitly includes the salt ions and the water molecules. In particular, we used the well-tested simple point charge extended (SPC/E) model for the water molecules and the reference interaction site model (RISM) equation complemented with the hypernetted-chain (HNC) closure to compute the pairwise correlations among the components. As such, we derive the phase diagram for our system using a mean-field approach based upon the computed EPP, for a range of (finite) nanoparticle densities and salt concentrations, and demonstrate crystallization. Findings from our model also suggest strong trends of charge asymmetry due to solvation effects.   

Glassy dynamics of geometrically frustrated colloidal system

Geometric frustration arises when lattice structure prevents simultaneous minimization of local interaction energies. It leads to highly degenerate ground states and, subsequently, to complex phases of matter. Recently, a simple geometrically frustrated system composed of closely packed colloidal spheres confined between parallel walls was studied. Diameter-tunable microgel spheres are self-assembled into a buckled triangular lattice with either up or down displacements, analogous to an antiferromagnetic Ising model on a triangular lattice. This tunable soft-matter system provides a means to directly visualize the dynamics of frustration. In the present study, we probe spin dynamics on the single particle level by quenching our system to large packing fractions at different rates. Spin dynamics are found to exhibit behaviors characteristic of glasses.   

Capillary interactions between silica-particles in organic solvents

Small-angle neutron scattering (SANS) is used to study the interactions of silica nano-particles with an average diameter of 10 nm in methanol and methanol/toluene mixtures at 25 $^{\circ}$C. SANS intensities are analyzed as a product of a form factor and a structure factor. Methanol is a polar solvent with a dielectric constant of e = 32 at ambient temperatures the interaction of silica in methanol is considered to be through electrostatic repulsion. The presence of toluene reduces the polarity of the solvent since toluene is a non-polar liquid with e = 2. At fractions of toluene less than 44 {\%}, the dispersion of silica particles is stable and non-viscous. The analysis of the structure factor shows that the silica particles reduce their charge with increasing fraction of toluene. At intermediate fractions of toluene, between 44 and 65 {\%}, the viscosity increases by two orders of magnitude which suggests formation of two dimensional network of silica particles. Computer simulations of a pearl necklace-like chain of spheres is conducted to explain the structure factor at these intermediate fraction of toluene.   

Capillary interactions in nano-particles suspensions

We have investigated the structures formed by colloidal particles suspended in solvents at volume fractions below 10{\%} and interacting through capillary bridges. Such systems resemble colloidal gas of sticky nano-spheres that form pearl-necklace like chains that, in turn, induce strong viscoleasticity due to the formation of 3-D fractal network. The capillary force dominates the electrostatic and Van der Waals forces in solutions and can bridge multiple particles depending of the volume of the capillary bridge. Small-angle neutron scattering (SANS) is used to study nanoparticles with an average diameter of 10 nm in polar and non-polar organic solvents at ambient temperatures. Computer simulations of a pearl necklace-like chain of spheres is conducted to explain the structure factor when capillary bridges are present. We have also studied the properties of the capillary bridge between a pair of particles. The significance of this study is to explore the possibility of using capillary force as a tool to engineer new colloidal structures and materials in solutions and to optimize their viscoelastic properties.   

Structure of Viral Aggregates

The aggregation of virus particles is a particular form of colloidal self-assembly, since viruses of a give type are monodisperse and have identical, anisotropic surface charge distributions. In small-angle X-ray scattering experiments, the Qbeta virus was found to organize in different crystal structures in the presence of divalent salt and non-adsorbing polymer. Since a simple isotropic potential cannot explain the occurrence of all observed phases, we employ computer simulations to investigate how the surface charge distribution affects the virus interactions. Using a detailed model of the virus particle, we find an asymmetric ion distribution around the virus which gives rise to the different phases observed.