Session B13: Focus Session: Polymer Colloids: Structure, Function, and Dynamics II

Predicting long-time Brownian dynamics of ultrasoft colloid suspensions from thermodynamics

Suspensions of ultrasoft colloids, such as Gaussian-core particles and Hertzian spheres, have received significant research interest due to their reentrant melting behavior and dynamic anomalies. Many of the previous dynamic studies of these systems have focused on molecular dynamics simulations, which by their nature ignores the solvent medium. We have conducted Brownian dynamics simulations of these ultrasoft colloid suspensions to show their long-time dynamic behavior near the reentrant melting transition. In addition, we have developed a novel method for quantitatively and qualitatively predicting the long-time Brownian dynamics of ultrasoft colloidal suspensions from their thermodynamic properties.   

Development of surfaces repelling negatively buoyant solid particles

Using a hybrid computational method that integrates the lattice Boltzmann model for fluid dynamics and the lattice spring model for solids, we examine the motion of negatively buoyant solid microparticles in shear flow near a solid wall decorated with regularly distributed rigid posts. The posts are arranged in a square pattern and tilted relative to the flow direction. We show that when rigid posts are tilted against flow, secondary flows emerge that prevent the deposition of suspended particles on the solid surface. We probe the effect of post geometry on the development of secondary flows and identify the optimal post architecture in terms of the mass of levitated solid particles. Our results are useful for designing anti-fouling surfaces that repel colloidal particles carried by fluid.   

Microscopic structure of confined colloidal suspensions under shear

We report a study of driven colloidal suspensions by Stokesian dynamics simulation. The suspension is confined by two parallel plates, and is being driven far away from equilibrium by shearing induced by translation of the parallel plates. The separation of the plates is varied so the suspensions form either a single layer or two layers. Both the structure of the non-equilibrium steady state and the dynamics of the relaxation of the non-equilibrium state back to the equilibrium are examined, at a wide range of shearing strengths (the non-dimensional ratio quantifying the driven motion relative to the Brownian motion of the colloidal particles, the Peclet number is tuned from 0.1 to 100) and packing fractions. We observe string-like structures at low packing fractions and shear-induced crystallization at high fractions. A mechanism is proposed for how hydrodynamic interactions give rise to these structures.   

Evaporation of Lennard-Jones Fluids

Solvent evaporation is a process frequently used to disperse particles in a bulk material or at a substrate. The local order and packing of particles can be controlled by controlling the evaporation rate. The first step to fully understand this complicated process is to understand the evaporation process of pure liquid at the microscopic scale. We have carried out large scale molecular dynamics simulations to study the evaporation of Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers. For LJ monomers in contact with a vacuum, the evaporation rate is found to be very high with significant evaporative cooling and an accompanying density gradient in the liquid domain near the liquid/vapor interface. Increasing the chain length to just dimers significantly reduces the evaporation rate. The velocity distributions of evaporated monomers are measured and compared to a kinetic theory and their dependence on the evaporation conditions is discussed. For nanoparticle suspensions, the nanoparticles order at the surface, which causes the evaporation to significantly slow down.   

Rotational and Translational Diffusion of PMMA Colloidal Clusters

Colloidal clusters, 3-7 $\mu $m in size, are a good model system for various 2D and 3D structures depending on the aggregation number, $N$. We measure the translational and rotational diffusion of individual dyed PMMA clusters of dimers and trimers using high speed confocal scanning microscopy and particle tracking. We report measurements of the rotational and translational diffusion coefficients~(and their ratios) as a function of volume fraction.   

A diversity of binary colloidal crystals using DNA-directed interactions

DNA is the premier tool for directing the controlled self-assembly of nanoscopic and microscopic objects. The interactions between microspheres due to the hybridization of DNA strands grafted to their surface have been measured and can be modeled in detail, using well-known polymer physics and DNA thermodynamics. Knowledge of the potential, in turn, enables the exploration of the complex phase diagram and self-assembly kinetics in simulation. In experiment, at high densities of long grafted DNA strands, and temperatures where the binding is reversible, these system readily form colloidal crystals having a diverse range of symmetries. For interactions that favor alloying between two same-sized colloidal species, our experimental observations compare favorably to a simulation framework that predicts the equilibrium phase behavior, crystal growth kinetics and solid-solid transitions. We will discuss the crystallography of the novel alloy structures formed and address how particle size and heterogeneity affect nucleation and growth rates.   

Correlating Structural and Spectral Fluctuations in a Lasing Colloidal Suspension

When multiply scattering media with optical gain are optically pumped above a critical threshold, they emit coherent radiation in many spectral lines. This phenomenon is known as random lasing. The wavelengths of these spectral lines depend sensitively on the spatial distribution of scatterers, but this relationship has only just begun to be explored. We study the time and frequency domain statistics of random laser spectra emitted from dense colloidal suspensions doped with laser dye with an eye toward using this information as a probe of the underlying colloid dynamics.   

Smart colloidosomes with tunable permeability and a dissolution trigger

Self-assembly of colloidal particles in the liquid interface of double emulsion droplets can be used to fabricate ``colloidosome'' microcapsules, which have great potential as vehicles for the controlled delivery of drugs or other cargoes. Here we present a novel class of aqueous core colloidosomes that combine the benefit of low capsule permeability (good cargo retention) with the option of a stimulus-triggered fast release in a target environment. Complete or partial dissolution of the capsule walls in response to a mild pH change is achieved in each case through the use of responsive particles made from polymers with pH-switchable solubility. We demonstrate three methods of controlling the capsule permeability prior to release while maintaining the intended response to the release trigger.   

A theoretical study of colloidal forces near an amphiphilic polymer brush

Polymer-based ``non-stick'' coatings are promising as the next generation of effective, environmentally-friendly marine antifouling systems that minimize nonspecific adsorption of extracellular polymeric substances (EPS). However, design and development of such systems are impeded by the poor knowledge of polymer-mediated interactions of biomacromolecules with the protected substrate. In this work, a polymer density functional theory (DFT) is used to predict the potential of mean force between spherical biomacromolecules and amphiphilic copolymer brushes within a coarse-grained model that captures essential nonspecific interactions such as the molecular excluded volume effects and the hydrophobic energies. The relevance of theoretical results for practical control of the EPS adsorption is discussed in terms of the efficiency of different brush configurations to prevent biofouling. It is shown that the most effective antifouling surface may be accomplished by using amphiphilic brushes with a long hydrophilic backbone and a hydrophobic end at moderate grafting density.   

Phase transition of colloidal particles on curved surfaces

Defects and disclinations have to appear in crystalline domains on a curved surface with non-zero Gaussian curvature. These geometrical frustrations can qualitatively change the physics of phase transition. We encapsulate micron sized polystyrene (PS) colloidal particles within emulsion droplets and use nanometer sized polyNIPAM hydrogel particles to introduce depletion attraction between PS particle and interface, as well as between PS particles. We use this experimental model system and confocal microscopy to study phase transitions on curved surfaces. We will present both experimental phenomena and theoretical analysis.   

Dielectric effects in self-assembly of binary colloid mixtures

Colloidal self-assembly is often controlled by electrostatic interactions. The solvent and colloids typically have different dielectric constants, thereby inducing polarization charge at the colloid surfaces. A shortcoming of previous simulations of charged colloids with implicit solvent is the neglect of the effective many-body interactions resulting from such dielectric effects. We study colloidal self-assembly using a method that properly accounts for polarization charge. In simulations of weakly charged colloids with large size asymmetry, we find that dielectric effects modify the pair correlation function in a nontrivial way and at low temperatures alter the observed crystal phase.   

Formation of three-dimensional colloidal nanoparticle supercrystals and probing the formation kinetics

A multiple solvent system consisting of colloidal nanoparticles in several solvents of gradually decreasing vapor pressures was investigated in the self assembly of hundred-layer thick colloidal nanoparticle superlattices in lithographically defined capillaries. Such a solvent system allows a very slow and tunable drying rate of solvents, which, together with the microfluidic flow into the capillaries, leads to the controllable formation of large, single crystalline 3D nanoparticle supercrystals. The underlying mechanism of superlattice formation was investigated via the drying rates for nanoparticle assembly for solvent systems of specific compositions. This technique generates single-crystalline 3D supercrystals of $\sim $micrometer size at spatially controlled locations, and large chunks (up to 40 $\mu $m by 40 $\mu $m by 5 $\mu $m) of single crystalline supercrystals on a flat Si substrate. The ordered nature of the structures formed was probed by high-resolution SEM and small angle x-ray scattering. In-situ x-ray scattering reveals the formation kinetics of the transition of nanoparticle assemblies from amorphous to ordered. This technique is versatile and has been applied to various types and sizes of colloidal nanocrystals, including those composed of CdSe, Au, PbS and Fe$_{3}$O$_{4}$.   

Nanoparticle-induced self-assembly of functionalized tetrapods

Recent advances in synthesis have made it possible to create monodisperse particles with well-defined shapes. In particular, tetrapods have been fabricated in a wide range of well-controlled dimensions and have been functionalized in several different ways. We present Monte Carlo simulations of the self-assembly of functionalized tetrapods. We consider how the addition of charged spherical nanoparticles provides another means to control the self-assembled structure. In addition, we report the results of simulations of planar tripods confined to two dimensions and demonstrate that highly regular structures can be achieved without functionalization, through nanoparticle-mediated depletion interactions.   

A ``diffusing diffusivity'' model of ``anomalous yet Brownian'' diffusion of colloidal particles

``Anomalous yet Brownian'' diffusion of colloidal beads, with a mean-square displacement (MSD) exactly linear in time (as in simple Fickian diffusion) but an exponential (rather than Gaussian) displacement distribution (DD) at short times for large displacements, has been reported recently by Granick's group [1] in several systems. We argue that a strictly linear MSD with a non-Gaussian DD is a universal feature of systems with ``diffusivity memory'' (a particle diffusing faster is likely to keep diffusing faster for some time), but without ``direction memory'' (a jump in a particular direction does not change the probability of subsequent jumps in that direction). We consider a series of toy models reproducing this behavior in which a particle undergoes regular diffusion, but its diffusivity itself performs a (perhaps biased) random walk. The DD is strictly exponential at short times when the diffusivity distribution itself is exponential, but an exponential remains a good fit for a variety of diffusivity distributions. \\[4pt] [1] Wang et al., PNAS 106 (2009) 15160.   

Reentrant and Isostructural Transitions in the Cluster-Crystal Forming GEM-4

Systems governed by soft, bounded, purely repulsive interactions show two possible equilibrium behaviors under compression: reentrant melting, as in the Gaussian core model (GCM), or clustering, as in the penetrable sphere model (PSM). The generalized exponential model of power 4 (GEM-4), which is the intermedia of the GCM and PSM with a simple isotropic pair interaction $u(r)\sim e^{-r^4}$, is thought to belong to the second family and was indeed found to form clusters at sufficiently high densities at high temperatures. Here, we present the low-temperature behavior of GEM-4 through Monte Carlo simulations using a specially developed free energy integration scheme. We find the phase behavior to be hybrid between the GCM and the PSM limits, showing a surprisingly rich phase behavior in spite of the simplicity of the interaction form. For instance, S- shaped doubly reentrant phase sequences and evidence of a cascade of critical isostructural transitions between crystals of different average lattice site occupancy are observed. The possible annihilation of lattice sites and accompanying clustering moreover leads to an unusual softening upon compression, which suggest that these materials may have interesting mechanical properties. We discuss possible experimental realizations and challenges of this class of materials.