Session L29: Colloids III

Clustering instabilities in lattice gas models with isotropic repulsive interactions

In previous work we have shown that liquid crystalline order arises in systems of particles with purely repulsive, spherically symmetric pair interactions. We have observed a variety of liquid crystalline phases, as well as rich crystalline and quasicrystalline polymorphism, in simulations of two and three dimensional systems of particles with isotropic pair potentials consisting of an impenetrable hard core plus an isotropic penetrable, repulsive soft shoulder. We have further explored the clustering instabilities in the model by using mean field theory and Monte Carlo simulations of lattice gas models with an isotropic soft-shoulder repulsion that extends out many lattice spacings. The lattice gas model maps exactly onto an Ising model with antiferromagnetic interactions. At low temperatures this repulsive soft shoulder leads to the development of structure on the length scale of the repulsion. Mean field theory predicts both layered and solid structures in the temperature/magnetic field (chemical potential) plane. Monte Carlo simulations display liquid phases with short range patterns, layered incommensurate phases with quasi-long-range order, long- range ordered layered solids, hexagonal micellar solids with quasi- long-range ordering of micelles and long-range ordered hexagonal phases.   

Structures Formed by Small Numbers of Colloidal Particles Bound to a Spherical Interface

We study the behavior of micron sized colloidal particles adsorbed on the interface of spherical droplets not much larger than the colloids. We compare the structures formed by interfacially-bound particles at low particle number to predicted geometries such as the proposed solutions to the Thomson and Tammes problems. The predicted geometries depend critically on the interactions between particles in this low particle number regime. Because all particles on a droplet must be tracked simultaneously, such colloidosome systems have not yet been explored experimentally due to the limited time and z-resolution of confocal and bright-field microscopy. To overcome such limits, we use digital holographic microscopy to locate all particles within a volume of roughly 100$\times $100$\times $50 $\mu $m$^{3}$ at speeds of up to 500 frames per second. The experimental setup and reconstruction algorithms will be discussed along with our results.   

Mechanisms of Size and Shape Selection and Control in Self-Assembly of Colloid Particles Synthesized from Nanosize Crystalline Precursors

The importance of well-defined dispersions of particles of different shapes, ranging in sizes from nanometer to colloidal, has been widely recognized in applications and in basic studies of advanced materials. Our program endeavors to advance understanding of formation of uniform particles of simple and composite structure, with focus on synthesis involving self-assembly of nanosize particles and their new unique properties for dimensions smaller than the typical submicron-size colloid scales. Presently, there is convincing experimental evidence that many monodispersed colloids of various shapes, obtained by precipitation in solutions, are formed by aggregation of such nanocrystalline subunits. Our group's theoretical explanation of this process expands the classical model of formation of uniform particles, by LaMer, and offers an interesting link between nanosize and micrometer size particles.   

Orientational Order of Chain Forming Ferroelectric Nano Particles in Heptane .

Previous computational work [1] has shown that under the appropriate conditions, dipolar spheres aggregate and form chains. In this report, we study nano-sized ferroelectric BaTiO$_{3 }$particles dispersed in heptane. We demonstrate dependence of the particles organization in the colloid \textit{vs.} particles size and concentration. When the particles are large ($>$40 nm) they sediment to the bottom of the solution; smaller particles ($\sim $10-15 nm) form gels or networks that do not sediment. Probing particle organization by means of freeze fracture electron microscopy reveals that at small sizes ferroelectric particles form a network of chains of particles that have local nematic like order. We compare our observations with the described in literature predictions. [1] J. Weis, D. Levesque Phys. Rev. Lett. 71, 2729 (1993).   

Specific and Reversible Assembly of DNA Coated Colloids

We aim to create a new class of materials that self-assemble and self-replicate. Biotin-terminated DNA strands are attached to neutravidin coated polystyrene particles via the well established avidin-biotin coupling mechanism. The DNA is composed of a 61-base strand and a 50-base complementary strand, leaving a single sticky end of 11 bases to interact with its complement attached to another particle. Complementary particles mixed together aggregate into fractal structures. Increasing the temperature leads to dehybridization of the DNA strands and disaggregation of the particles. A typical cycle of aggregation, disaggregation, and reaggregation, as investigated by videomicroscopy, takes $\sim $ 20 minutes and has been repeated more than a dozen times. Our melting curves are sharp and show a strong dependence with buffer concentration. In a highly ionic environment, the aggregation is well described by a diffusion limited process and slows down considerably as the aggregation temperature approaches the melting temperature. We show how these materials are promising for creating new self-replicating structures.   

Search for Optical Binding with Shape Phase Holographic Optical Trapping

Light scattered by an illuminated particle should repel that particle's neighbors through radiation pressure. Nearly two decades ago, Burns, Fournier and Golovchenko (BFG) proposed that the coherent superposition of scattered fields can lead to an attractive interparticle interaction, which they called optical binding. Their pioneering experimental observation has generated considerable interest, most of which has focused on developing the theory for the effect. Accurate measurements of the optical binding force in the BFG geometry have been lacking, however. The need to quantify optical binding forces is particularly acute for colloidal interaction measurements on linear optical traps. We present a new method to directly measure optical binding forces between colloidal spheres that exploits the ability of shape-phase holography to create linear optical traps with accurately specified intensity and phase profiles. Our ability to control the trap's phase profile makes possible precise discrimination between intensity- and field-dependent interactions, i.e. between radiation pressure and optical binding. The same novel technique that allows us to project holographic line traps also can be used to project two- and three-dimensionally structured ring traps, novel Bessel-beam traps, which we also will describe.   

Segregation of Defects at Grain Boundaries.

Interstitial impurity segregation at grain boundaries plays an important role in materials properties such as cohesion, grain growth kinetics, and transport. Unfortunately, direct measurement of grain boundary composition is difficult in bulk crystals and polycrystals. In this contribution we directly study impurity segregation at grain boundaries using a model colloidal crystal. The polycrystals are made of temperature-sensitive micron size NIPA microgel particles [1]. We add 100-200 nm fluorescent polystyrene particles to this system to model interstitial impurities. The impurities are then tracked using video microscopy close to and far from the grain boundaries. We find that impurities hop from one position to another and diffuse anisotropically when far from the grain boundaries, and they diffuse isotropically in the grain boundaries. Upon increasing the temperature, the packing volume fraction of NIPA particles decreases and grain boundaries start to melt. We also explored the effects of the segregated impurities on grain boundary melting. [1] A. M. Alsayed, M. F. Islam, J. Zhang, P. J. Collings, A. G. Yodh, Science 309, 1207 (2005). This work was supported by grants from NSF (DMR-0505048 and MRSEC DMR05-20020) and NASA (NAG8-2172).   

Realizing Colloidal Artificial Ice on Arrays of Optical Traps

We demonstrate how a colloidal version of artificial ice can be realized on optical trap lattices. Using numerical simulations, we show that this system obeys the ice rules and that for strong colloid-colloid interactions, an ordered ground state appears. We show that the ice rule ordering can occur for systems with as few as twenty-four traps and that the ordering transition can be observed at constant temperature by varying the barrier strength of the traps.   

Shear Events in Colloidal Glasses

We analyze shear events that occur in sheared amorphous colloidal suspensions. We use the three-dimensional particle positions determined by confocal microscopy to determine irreversible local rearrangements that give rise to high local strain. These shear regions show a long-range strain field characteristic of dipolar strain events. Large displacements of only one or a few particles in the shear event core are enough to stabilize the new configuration and lead to permanent deformation. We will elucidate the interplay between thermal fluctuations and local strain that drives the nucleation of these shear regions.   

Hydrodynamic Forces in the Lubrication Regime: A Molecular Dynamics Study

We report on classical molecular dynamics simulations of large spheres moving toward a flat substrate and large spheres moving toward each other. The simulations are designed to investigate hydrodynamics at the molecular scale. We show a new decomposition approach appropriate for force microscopy measurements, and extract the static and dynamic components of the total force from approaching- and receding-force curves that are obtained from simulations or experiments. The dynamic force is evaluated for a range of sphere sizes and approach velocities, with different fluids and as well as with different surface characteristics - smoothness, roughness, and compliance. A comparison with hydrodynamic predictions for the dynamic force is made for these various cases.   

Colloid-Polymer Demixing in the Protein Limit: A Simulation Study

Mixtures of hard colloidal particles and nonadsorbing polymers can exhibit entropy-driven demixing into colloid-rich and colloid-poor phases. The classic Asakura-Oosawa-Vrij (AOV) model idealizes the polymers as effective spheres that are mutually noninteracting but impenetrable to the colloids. Here the AOV model is adapted to the protein limit by assuming the polymers to be (1) penetrable to the smaller colloids (or nanoparticles) and (2) polydisperse in size (radius of gyration). Using Gibbs ensemble Monte Carlo simulation, we explore the influence of the colloid-polymer penetration energy profile on the demixing instability and polymer size distribution. Structural and thermodynamic properties (radial distribution functions, osmotic pressures, and demixing phase diagrams) are computed and compared with predictions of density-functional theory.\footnote{M. Schmidt and M. Fuchs, {\it J. Chem. Phys.} {\bf 117}, 6308 (2002).}   

Colloidal Electrostatic Interactions Measured on Holographic Line Traps

We measure the electrostatic colloidal interaction between two colloidal particles diffusing in water along a quasi-1D potential that we generated by shape-phase holography. Interparticle potential measurements are affected in principle by light-induced contributions generated by the confining potential. We present both a measurement of such effect and a method to correct for it without the need for an independent measurement. Fast and accurate measurements on a line tweezer have the potential to become a standard method for assessing locally both equilibrium and out-of-equilibrium processes.   

Phase Behavior of Charged Colloids: Closed versus Donnan Equilibrium$^1$

The influence of chemical boundary conditions on thermodynamic properties of deionized charge-stabilized colloidal supensions is analyzed. Effective electrostatic interactions and phase behavior are shown to depend fundamentally on whether a suspension is confined to a closed (electroneutral) cell or is in Donnan equilibrium with a microion reservoir, {\it e.g.}, electrolyte solution. Linear-response theory$^2$ predicts that at low ionic strength closed suspensions of highly charged macroions and monovalent microions can phase separate, while microion exchange with a reservoir stabilizes the fluid phase. \\[1ex] $^1$~Supported by National Science Foundation grant DMR-0204020. \\[0.5ex] $^2$~A.R.~Denton, {\it Phys. Rev.} E {\bf 73}, 41407 (2006).   

Epitaxial Growth of Thin Colloidal Films in the Presence of a Depletant

We describe the epitaxial growth of thin films comprised of hard-sphere colloidal particles sedimenting in the presence of a depletant polymer. The depletant polymer induces an effective attraction between microspheres, causing them to nucleate islands that grow and coalesce with one another. In addition, we use photolithography to control the morphology of the substrate. This allows us to investigate the effects of the underlying substrate structure on the epitaxial growth process. Using confocal microscopy, we image and track the colloidal particles as they diffuse, aggregate and rearrange their configurations during deposition. Island density and degree of layer-by-layer growth are determined as functions of the deposition rate and depletant concentration. The ease with which we are able to image deposition in real time and the similarity of our results to those obtained in atomic deposition experiments suggest that our system will allow us to model various processes that occur in atomic thin film epitaxial growth.   

Patterning Colloidal Films via Evaporative Lithography

We investigate evaporative lithography as a route for patterning colloidal films during drying. Specifically, films composed of mixtures of silica microspheres and polystyrene nanoparticles are patterned by placing a mask above the film surface to induce periodic variations between regions of free and hindered evaporation. Fluorescence and confocal microscopy, coupled with surface profilometry measurements, reveal that particles segregate laterally within the drying film, as fluid and entrained particles migrate towards regions of higher evaporative flux. The colloidal films exhibit remarkable pattern formation that can be regulated by carefully tuning the initial suspension composition, separation distance between the mask and underlying film, and mask geometry.