Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L9: Colloidal Self Assembly and Interactions |
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Sponsoring Units: DFD Chair: Eric Dufresne Room: Morial Convention Center RO7 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L9.00001: Tunable Liquid Micromirror Based on Self-Assembly of ``Janus'' Particles Tom Krupenkin, Mike Bucaro, Paul Kolodner, J. Ashley Taylor In optofluidics, control over light propagation is primarily achieved by using the optical properties of liquid-gas and liquid-liquid interfaces. Currently, the vast majority of existing optofluidic systems are refractive optical devices. However, reflective optofluidic devices potentially have a number of important advantages over their refractive counterparts, since they are not constrained by the relatively low refractive index contrast commonly found in liquid-liquid and liquid-gas interfaces. In this work, we propose and experimentally demonstrate a novel approach that makes it possible to create tunable reflective liquid surfaces by combining the flexibility and tunability of liquid-liquid interfaces with the excellent reflective properties of solid metal surfaces. We employ self-assembly of reflective solid ``Janus'' particles at the interfaces between polar and non-polar liquids to create highly flexible, continuous, reflective ``carpets'' capable of acting as spherical micromirrors. We have successfully demonstrated electrowetting-based dynamic tuning of these micromirrors, including electrical control over mirror shape and focal distance. The mirror self-assembly process was studied as a function of the particle functionalization and of the chemical properties of the liquids involved. Potential applications of the proposed mirrors are also discussed. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L9.00002: Emerging Structures for Colloidal Brushes: from Dispersions and Agglomerates to Spherulites, Wires, and beyond Alberto Striolo A large variety of nanoparticles holds extraordinary promises for practical applications, e.g., in catalysis and materials science. For these and other applications it is necessary to assemble the nanoparticles to yield supra-molecular aggregates of desired morphology. We are interested in the self-assembly of spherical colloids (i.e., nanoparticles) induced by interactions that become anisotropic because of entropic effects. Thus short polymer brushes are grafted on restricted regions of the spherical nanoparticles considered (e.g., the equatorial plane). Monte Carlo simulations were conducted to assess the properties of the self-assembled nanostructures as a function of the length of the brushes and of the strength of the particle-particle attraction. Depending on the specific solution conditions (particle-particle dispersive attractions, as well as length and density of the grafted polymer brushes) it is possible to obtain uniform dispersions, irregular aggregates, spherulites, one-dimensional wires, and two-dimensional colloidal sheets. We will discuss whether or not the effective colloid-colloid pair interactions at infinite-dilute conditions (i.e., the potential of mean force) can be used to predict the emerging behavior of the colloidal nanoparticles at larger concentrations. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L9.00003: Roughness-controlled depletion interactions for controlling colloidal self-assembly Kun Zhao, Thomas G. Mason The surfaces of colloidal particles resulting from many new fabrication methods are not molecularly smooth, so understanding how the surface roughness affects the depletion attraction is very important. We show that the depletion attraction between custom-shaped microscale platelets can be suppressed when the nanoscale surface asperity heights become larger than the depletion agent. In the opposite limit, the attraction re-appears and columnar stacks of platelets are formed. Exploiting this, we selectively increase the site-specific roughness on only one side of the platelets to direct the mass-production of a single desired assembly: a pure dimer phase. Furthermore, we model the interaction between flat plates coated by hemispheres having controlled sizes and densities relative to those of a spherical depletion agent. Overall, these studies provide significant insight into attractive bonds between particles that retain lubrication, and they provide a basis through which more complex assemblies can be made. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L9.00004: Corralled Colloids in Four Dimensions Stephen Anthony, Minsu Kim, Steve Granick Three colloidal particles were placed in small corrals and the strong correlations between their translation and rotation were quantified using the optical anisotropy of MOON (Modulated Optical Nanoprobes) particles to simultaneously measure their translation and rotation in an optical microscope. This system represents the simplest system which can capture one of the relevant components of multi-body interactions, the fact that while two particles can freely rotate together (like gears), once a third particle (or gear) is added there is no universally favorable set of rotations. This simple multi-body system provides a paradigm of how rotation influences translation and vice-versa. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L9.00005: Interactions among microdroplets at the water-air interface Chuan Zeng, Anthony D. Dinsmore There has been a great mystery concerning the origin of measured long-range attraction among microparticles at fluid interfaces. Recent theoretical work$^{1}$ showed that electrostatic interactions should not lead to long-range attraction, but the possibility remains that attraction arises from an irregular contact line on the particles' surfaces. Replacing the solid particles with liquid droplets eliminated surface roughness and thus reduced the complexity of the system. We captured micron-sized oil droplets at water-air interface and measured the interaction between them. The dynamics of droplets at interface were imaged using optical microscopy, from which the droplets' motions were tracked and analyzed. The interaction between two isolated droplets was calculated from their trajectories through the Markovian dynamics extrapolation method developed by J. C. Crocker and D. G. Grier$^{2}$. We acknowledge support from NASA through the Fluid Physics program (NRA 02-OBPR-03-C) and from the NSF-supported MRSEC on Polymers (DME-0213695).\\ $^{1}$ See, for example, M. Oettel, A. Dominguez, and S. Dietrich, \textit{Phys. Rev. E} \textbf{71}, 051401 (2005).\\ $^{2}$ J. C. Crocker and D. G. Grier, \textit{Phys. Rev. Lett.} \textbf{73}, 352 (1994). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L9.00006: Geometrical frustration in colloidal ``antiferromanget'' Yilong Han, Yair Shokef, Ahmed Alsayed, Peter Yunker, Tom Lubensky, Arjun Yodh We report experiments about a self-organized colloidal system that exhibits geometrical frustration similar to that of antiferromagnetic Ising spins on a triangular lattice. Novel thermally sensitive microgel NIPA (N-isopropyl acrylamide) spheres are close packed between two parallel flat walls with a vertical separation of about 1.5-particle diameters. The particles form an approximate in-plane triangular lattice. Neighboring particles tend to push each other toward opposite walls leading to out-of-plane local up and down buckling. We tune the strength of such effective antiferromagnetic interactions by varying temperature-tunable diameter of spheres. ``Spin'' flipping was directly visualized with video microscopy. We investigated the static structures, the dynamics of particles with different degrees of frustration and the degenerated ground state. This experiment is the first dynamic measurement in a geometrical frustrated system at single-particle resolution. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L9.00007: Correlated Motion of Ellipsoids Diffusing in 3D Kenneth Desmond, Eric R. Weeks Currently the hydrodynamic interaction between two ellipsoids in a fluid is not well understood. By observing the Brownian motion of micron sized ellipsoids suspended in a fluid using confocal microscopy, we directly measure these interactions. The ellipsoids exhibit both translational and rotational diffusion. The motion of an ellipsoid induces a flow field, which couples the motion of other ellipsoids with the first one. In our experiments we measure the translational and rotational diffusion of polystyrene ellipsoids suspended in a water glycerol mixture in three dimensions, and examine the spatial correlations between the rotational and translational motion of pairs of ellipsoids. Rotational motions set up a dipolar flow field, and thus the resulting correlations decay quicker than the correlations caused by translations. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L9.00008: Simulating Collective Dynamics of Confined Colloids Jonathan K. Whitmer, Erik Luijten We investigate the dynamical behavior of colloidal particles under confinement, by means of computer simulations that explicitly account for hydrodynamic interactions. Even under dilute conditions, long-range solvent-mediated coupling of the translational and rotational degrees of freedom influences the relative motion of colloidal particles. These effects on the collective dynamics are often ignored in simulations. Our calculations utilize the hybrid Stochastic Rotation Dynamics/Molecular Dynamics method [A. Malevanets and R. Kapral, J. Chem.\ Phys.\ \textbf{112}, 7260 (2000)] to incorporate both hydrodynamic and Brownian forces exerted on colloids by the solvent. The computational results are compared to recent experiments on few-body colloidal systems where the particle number is limited through confinement in a cylindrical trap. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L9.00009: Stripes and their zigzagging in buckled hard spheres Yair Shokef, Yilong Han, Ahmed Alsayed, Peter Yunker, Tom Lubensky, Arjun Yodh We use a hard sphere model to describe recent experiments on buckled colloidal monolayers. Our detailed Monte Carlo simulations exhibit the behavior, observed experimentally, of antiferromagnetic order and the formation of stripes that randomly zigzag around the system. Using free volume calculations, we deduce the strength of the effective antiferromagnetic interactions between neighboring particles. We furthermore explain how the geometrical frustration is partially removed by collective effects arising from sphere packing. We show how lattice distortions enable striped configurations to pack better than disordered ground states of the simple antiferromagnetic Ising model and that zigzagging of these stripes does not affect the free volume of the system. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L9.00010: Breakdown of Pairwise Additivity in Colloidal Electrostatics Sunil Sainis, Eric Dufresne Predictions of the structure and stability of charged colloidal suspensions typically assume pairwise additive forces. We directly measure electrostatic forces in small clusters of two to seven particles in a nonpolar solvent. We find that electrostatic interactions are not pairwise additive when the particle separations are much smaller than the screening length. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L9.00011: Mystery on Charge Asymmetry: Anionic Macroions in Periodic Lattices Held by Hydrated Cations and Not vice versa William Kung, Monica Olvera de la Cruz We propose a mean-field analytical model to account for the observed asymmetry in the ability to form long-range attraction by the negatively charged colloidal particles and not their equivalently charged positive counterpart. We conjecture that this asymmetry is due to solvation effects, and we phenomenologically capture its physics by considering the relative strength of this water-induced short-range repulsion between the different charge species. We then apply our model to the colloidal system of negatively charged disks that are neutralized by a sea of counterions and strongly absorbed to an interface in a compressible binary system. We demonstrate the resulting coexistence between a dilute isotropic ionic phase and a condensed hexagonal lattice phase as a function of density and interaction strength. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L9.00012: Charged colloids in low polar solvents Andrew Hollingsworth, Mirjam Leunissen, William Irvine, Paul Chaikin, Alfons van Blaaderen In a low polar environment, sterically-stabilized poly(methyl methacrylate) spheres become positively charged and exhibit extraordinary long-range repulsive interactions. Confocal microscopy shows that they can form low density, body centered cubic crystals with lattice constants up to 40 microns. We attribute this behavior to the cyclohexyl bromide (CHB) in which the colloidal particles are suspended. CHB is a desirable solvent due to its density matching capability; however, it is difficult to purify. Trace amounts of the hydrogen halide resulting from the hydrolysis of CHB apparently interact with the stabilizer layer, imparting charge to the colloids. Surprisingly, water can also be used to deionize the organic solvent, depending on the relative amounts of the two fluids. The addition of quaternary ammonium salts was used to screen charge (reducing long range particle interaction). Ionic strengths were computed using ionic association theory; in turn, particle charge and surface potentials were estimated from electrokinetic measurements. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L9.00013: Direct measurements of the pair potentials of colloids with light scattering and optical traps Kisun Yoon, Vinothan Manoharan We present a methodology of directly measuring the pair potentials of colloids. We take snapshots of the thermal fluctuation of a pair of colloidal particles in equilibrium. The probability distribution of the separation distance obtained from the snapshots should follow the Boltzmann distribution because the separation distance of the particle pair is the only independent variable necessary to describe the effective free energy of a macrostate of the colloidal particle pair in equilibrium. The measurement of the pair potentials can be achieved by appropriately subtracting the unwanted potentials due to optical traps and optically induced interactions from the effective free energy. Accurate measurement of the separation distance between colloidal particles has critical importance in measuring colloidal interactions. Conventional Video Microscopy used for separation distance measurement is significantly restricted due to the two-dimensional nature of the measurement. Furthermore, the measurement is seriously distorted when the two particles are nearly in contact because of the diffraction of light and multiple scattering effect. We introduce a new technique to accurately measure the separation distances using light scattering. This light scattering technique can measure the separation distance in 3D and appropriately considers the multiple scattering effect. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L9.00014: Concentration Gradients in Mixed Magnetic and Nonmagnetic Colloidal Suspensions Randall Erb, Benjamin Yellen The ability to form concentration gradients in mixed magnetic/nonmagnetic colloidal suspensions using magnetic field gradients has many practical applications in the fields of biosensors and life science diagnostics. Previously, we developed and experimentally confirmed a self-consistent model describing the local distribution of magnetic nanoparticles exposed to a magnetic field gradient. Here, we have derived an analytic expression to describe the local concentration of nonmagnetic colloids which are also affected by field gradients when inside magnetic colloidal suspensions. The model calculates the force on particles as a function of local magnetic particle concentration, and solves for the equilibrium distribution of particles through the drift-diffusion equations. We investigate the ability to concentrate and deplete nonmagnetic particles from specific regions of a substrate, such as nearby patterned micro-magnets on a substrate. Also, we have qualitative experimental results to support our expression. Our results show that nonmagnetic particles which are 5-10 times larger than the magnetic nanoparticles can be effectively concentrated or depleted at specified regions of the substrate. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L9.00015: Interactions and self assembly of two heterogeneously charged surfaces Robert Brewster, Philip Pincus, Samuel Safran Recent experiments$^{1,2}$ have measured attractive interactions between two surfaces that each bear two molecular species with opposite charge. Theoretical considerations predict equilibrium finite-sized domains of each species, consistent with experiment. These domains, whose observed sizes are typically tens of nanometers, are the result of a balance between the line tension, which prefers macroscopic separation, and the electrostatics, which prefers mixing. Additionally, two such surfaces show a long range attraction. We present a theoretical model that predicts the domain size, phase behavior and forces for two such interacting surfaces. \\* \\* (1) E. E. Meyer, Q. Lin, T. Hassenkam, E. Oroudjev, J. N. Israelachvili PNAS {\bf 102}, 6839 (2005). \\* (2) S. Perkin, N. Kampf, J. Klein, Phys. Rev. Lett. {\bf 96}, 038301 (2006). [Preview Abstract] |
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