Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B36: Colloids I |
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Sponsoring Units: DFD Chair: David Grier, New York University Room: LACC 510 |
Monday, March 21, 2005 11:15AM - 11:27AM |
B36.00001: Complex phases from spherically-symmetric repulsive pair-potentials Matthew A. Glaser, Steven A. Kadlec, Julia M. Santos, Paul D. Beale, Noel A. Clark We report computer simulation studies and zero temperature analyses of two and three-dimensional systems of particles interacting via spherically symmetric, monotonically-repulsive pair-potentials. We have examined both bounded potentials and potentials with hard cores. We find that a very large class of potentials of this type display extraordinarily rich phase behavior. We observe a variety of complex modulated crystalline phases, commensurate and incommensurate phases solid phases (both achiral and chiral), structured isotropic liquids, micellar liquids and solids, and dodecagonal quasicrystals. These phases display a variety of phase transitions, including reentrant melting and freezing transitions, and commensurate-incommensurate phase transitions. The complexity arises because of multiple length scales due to the hard core (if present), the range of the potential, and spinodal length scales (Likos, et. al.$^1$) in the Fourier tranform of the pair potential. This rich polymorphism dramatically expands the range of possible models of colloidal self-assembly, and raises the interesting prospect of control of colloid self-assembly by rational design of pair potentials using grafted polymers or complex solvents. $^1$C.N. Likos, et. al, Phys Rev E 63, 031206 (1998). [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B36.00002: Phase Separation in Charge-Stabilized Colloids: Influence of Nonlinear Screening Alan R. Denton A range of experiments provide evidence that suspensions of like-charged colloids can separate into macroion-rich and -poor phases at low salt concentrations. An apparently related phenomenon is predicted by theories that map the mixture of macroions and microions (counterions and salt ions) onto an effective one-component system and include a one-body volume energy in the effective Hamiltonian\footnote{R. van Roij, M. Dijkstra, J.-P. Hansen, Phys. Rev. E {\bf 59}, 2010 (1999).}$^,$\footnote{P. B. Warren, J. Chem. Phys. {\bf 112}, 4683 (2000).}. Theoretical studies of charged colloids commonly assume that electrostatic interactions between macroions are linearly screened by microions. However, as recently shown within the framework of response theory\footnote{A. R. Denton, Phys. Rev. E {\bf 70}, 031404 (2004).}, nonlinear microion screening induces triplet attractions, weakens pair repulsions, and modifies the volume energy. Taking these nonlinear interactions as input to a thermodynamic perturbation theory, equilibrium phase diagrams for aqueous suspensions of highly charged colloids are computed. The main conclusion: nonlinear effects do not suppress, and can even promote, the predicted phase separation of deionized suspensions. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B36.00003: Phase Separation in Charge-Stabilized Colloidal Suspensions: A Simulation Study Ben Lu, Alan R. Denton A variety of experiments\footnote{J. C. Crocker and D. G. Grier, Phys. Rev. Lett. {\bf 77}, 1897 (1996).}$^,$\footnote{C. P. Royall, M. E. Leunissen, and A. van Blaaderen, J. Phys.: Condens. Matter {\bf 15}, S3581 (2003).} suggest that deionized suspensions of highly charged colloids may exhibit an unusual fluid phase separation. Here we report results of Gibbs ensemble Monte Carlo simulations of a model system consisting of charged colloidal macroions and microions (dissociated counterions and salt ions) dispersed in water. As input to the simulations, we use effective electrostatic interactions -- a screened-Coulomb macroion-macroion pair potential and a one-body volume energy -- predicted by mean-field linear response theory\footnote{A. R. Denton, Phys. Rev. E {\bf 62}, 3855 (2000).}. The volume energy -- a natural byproduct of integrating out from the partition function the microion degrees of freedom -- plays a crucial role in the acceptance probabilities for trial volume changes and particle transfers in the Gibbs ensemble. We present the bulk fluid phase diagram, which exhibits vapor-liquid coexistence at low salt concentrations ($c_s<1$ mM), and compare with theoretical predictions. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B36.00004: The Role of Shear Bands in the Yielding of a Dense Colloidal Suspension Itai Cohen, David Weitz While much has been done to characterize phases of colloidal suspensions which are either equilibrated or driven out of equilibrium by an applied shear, very little is known about hybrid structures such as shear bands formed at the transition between these regimes. We have built a shear cell which can be loaded onto a confocal microscope thus allowing us to image the micro-structure of a dense colloidal suspension when it is subjected to an imposed oscillatory strain. In particular, we are able to characterize flows which are inhomogeneous both vertically, across the gap, and laterally, throughout the shear zone. In this talk I will address the role played by shear bands when a suspension undergoes the yielding transition from a solid-like material which can support a load to a liquid-like material which flows under an applied stress. I will show that the role played by shear bands varies dramatically with the gap between the shearing plates. In bulk, shear bands facilitate a continuous transition whereas under confinement, the transition becomes discontinuous and hysteretic. Moreover, we find that in such dense suspensions the flow profiles couple laterally so that a locally disordered or jammed region can produce shear bands throughout the shear zone even in regions which are perfectly ordered. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B36.00005: The effect of shearing on colloidal gels using small-angle light scattering Tahereh Mokhtari, Christopher Sorensen, Amit Chakrabarti We investigated the effect of shear on the structure and aggregation kinetics of polystyrene colloidal gels. We used 20 nm polystyrene latexes and $MgCl_2 $ to induce aggregation. We rotated the sample for 30 seconds at different times after the onset of aggregation. We used static light scattering to observe the structure of the gel. Rotation rates varied between 0.81 rpm and 27 rpm (Proportional to shear rate). When shearing the sample soon after mixing polystyrene and $MgCl_2 $, the aggregation followed DLCA kinetics, which yields a fractal dimension of 1.8. The gel time also remained nearly the same as the no shear situation. However, shearing at high rates during the later stages of gelation shortens the gel time, and causes a crossover between two different fractal dimensions due to shear-induced aggregation. At intermediate shear rates, there is a crossover in the structure after the shearing stops, but eventually Brownian aggregation overcomes the shear-induced double structure, so that the fractal dimension regains $1.8\pm 0.1$. At low shear rates, there was no crossover and the fractal dimension remained at 1.8. However, at all shear rates, there is a sudden change in light scattering when shearing is done at the later stages of gelation implying shear induced aggregation. The shear rate needed to change the gel structure depends on the stage of gelation. The later the stage of gelation, the lower the shear rate needed to overcome the Brownian aggregation. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B36.00006: Dynamics in Polymer-Clay Gels as studied by Rheology and Scattering Gudrun Schmidt, Elena Loizou, Lionel Porcar, Paul Butler We discuss the shear orientation and relaxation of network like polymer-clay gels and solutions using rheology and small angle scattering. The orientation of synthetic Laponite clay particles under shear is compared to the shear orientation of natural Montmorillonite particles under similar conditions. After cessation of shear, the relaxation of Laponite particles within a polymer clay network is found to occur within few seconds while large montmorillonite particles relax within hours. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B36.00007: Hydrodynamic Coupling Among Colloidal Spheres in a Holographic Optical Trap Array Marco Polin, Kosta Ladavac, Stephen Quake, Alan Sokal, David Grier Hydrodynamic couplings play a key role in colloidal suspensions, but have proven a very challenging ground for both theoretical and experimental study.In recent years, manipulation of mesoscopic objects using optical tweezers proved to be an invaluable tool for isolated study of hydrodynamic interactions. Previous works showed results compatible with standard approximations, but being limited to systems of two beads were unable to address superposition effects. We report direct measurements of many-body hydrodynamic interactions among colloidal spheres localized in arrays of optical traps with varying lattice constants.These measurements are made possible by the introduction of statistically optimal methods for calibrating the optical trap arrays while simultaneously measuring the trapped spheres' hydrodynamic radii. Analyzing the dispersion of the arrays' normal modes provides direct insights into nature of many-body hydrodynamic coupling, and particularly into the validity of the widely exploited approximations for assessing this coupling. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B36.00008: Use of Generalized Helical Modes of Light in Optical Trapping Steven Sundbeck, Ilya Gruzberg, David Grier Modes of light possessing topological defects may be focused to optical traps which have intriguing and potentially useful properties. Certain helical modes of light carry an orbital angular momentum. These modes of light may be focused to form circular traps known as optical vortices, which impart this angular momentum to trapped dielectric particles. Recently, experimental observations of the size dependence of optical vortices on the topological charge $\ell$ of the beam have led to the creation of more generalized optical traps from helical beams. These exotic topological traps can be designed in a variety of geometries and may be used to exert torques and forces noninvasively on colloidal and other microscopic systems. Examples of these exotic traps and a further theoretical understanding of the structure of these helical modes are presented. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B36.00009: Microoptomechanical Pumps Assembled and Driven by Holographic Optical Vortex Arrays Kosta Ladavac, David Grier Beams of light with helical wavefronts can be focused into ring-like optical traps known as optical vortices. The orbital angular momentum carried by photons in helical modes can be transferred to trapped mesoscopic objects and thereby coupled to a surrounding fluid. We demonstrate that arrays of optical vortices created with the holographic optical tweezer technique can assemble colloidal spheres into dynamically reconfigurable microoptomechanical pumps assembled by optical gradient forces and actuated by photon orbital angular momentum. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B36.00010: Observation of Flux Reversal in a Symmetric Optical Thermal Ratchet Sang-Hyuk Lee, Kosta Ladavac, Marco Polin, David Grier We demonstrate that a cycle of three holographic optical trapping patterns can implement a thermal ratchet for diffusing colloidal spheres, and that the ratchet-driven transport displays flux reversal as a function of the cycle frequency and the inter-trap separation. Unlike previously described ratchet models, the approach we describe involves three equivalent states, each of which is locally and globally spatially symmetric, with spatiotemporal symmetry being broken by the sequence of states. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B36.00011: Self-diffusion of Colloidal Spheres and Ellipsoids in Nematic Phases of fd-virus J. Zhang, M.F. Islam, A.M. Alsayed, A.G. Yodh We report on the self-diffusion of micron sized colloidal spheres and ellipsoids in nematic phases of fd-virus. The mean square displacements (MSDs) of the spheres are diffusive along the nematic director, with the diffusion coefficients displaying an inverse dependence on sphere radius. Displacements perpendicular to the nematic director, however, are markedly slower and are sub-diffusive. The colloidal ellipsoids align with their long axis along the nematic director and also exhibit diffusive motion along the nematic director and a sub-diffusive motion perpendicular to the nematic director. The rotational motions of the ellipsoids are sub-diffusive at short time, and exhibit caged behavior at long times. We examine these motions as a function of nematic order parameter, and compare our results to theories of hindered tracer dynamics. This work has been partially supported by the NSF through MRSEC Grants DMR 00-79909 and DMR-0203378, and by NASA Grant NAG8-2172. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B36.00012: Nematic and Smectic phases of hard rods in 2 dimensions Paul Chaikin, Kun Zhao, Christopher Harrison, Matthew Sullivan, Thomas Mason, William Russel Although lots of work has been done on anisotropic colloidal systems in 3D, little work has been reported on 2D, especially experimental. In this talk, we present a study of high-density phases of an anisotropic colloidal suspension. This suspension consists of PMMA 4.5x0.7 micron circular disks that are standing up in an AC electric field. The disks have been prepared using photolithography. Their gravitational height is $\sim $0.1 micron so they form a single monolayer, especially when standing on edge. Concentrating the disks by slightly tilting the sample cell, we find several phases. Dilute regions are isotropic while more concentrated regions are nematic or quasi-smectic. The quasi-smectic consists of a monolayer of aligned columns of disks. We present measurements of the order parameter and the correlation functions and compare to simulations. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B36.00013: Orientational Anisotropy and Effective Electrostatic Interactions in Polyelectrolyte Star Solutions Hao Wang, Alan R. Denton In solutions of polyelectrolyte (PE) star macroions, electrostatic interactions between charged arms of approaching macroions can induce anisotropy in the arm orientational distributions. Within a rigid arm model, we explore the influence of arm orientational anisotropy on the effective pair interactions between stars. For strongly charged arms, where electrostatic energies much exceed thermal energies, a torque balance calculation predicts significant arm anisotropy and a corresponding change in the effective pair interaction between stars. For weakly charged arms where electrostatic and thermal energies are comparable, two independent methods -- one based on classical density-functional theory and the other on the Smoluchowski equation -- yield identical expressions for the equilibrium arm orientational distribution and numerical results suggest relatively weak orientational anisotropy. The resulting effective pair interactions between charged stars are fit with simple analytical functions. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B36.00014: Effect of Polymer Compression on Demixing of Colloid-Polymer Mixtures: Gibbs Ensemble Monte Carlo Simulation Shrikant Shenoy, Alan R. Denton Phase separation of a stable colloidal dispersion can be induced by adding nonadsorbing polymers. Depletion of polymers around the colloidal particles induces an effective attraction, leading to demixing at sufficient polymer concentration. The most widely studied model of colloid-polymer mixtures is the Asakura-Oosawa (AO) model\footnote{S. Asakura and F. Oosawa, J. Chem. Phys. {\bf 22}, 1255 (1954).}, which treats the colloids as hard spheres and the polymers as effective spheres, of fixed size, that have hard interactions with the colloids but are mutually noninteracting. To study the influence of colloid-induced polymer compression on equilibrium demixing, we incorporate size variation of polymers into the AO model. Within this extended model, we perform Gibbs ensemble Monte Carlo simulations, including size variation of polymers as a new Monte Carlo move. We compare our results with recent predictions of density-functional theory\footnote{A. R. Denton and M. Schmidt, J. Phys.: Condens. Matter {\bf 14}, 12051 (2002).}. [Preview Abstract] |
Monday, March 21, 2005 2:03PM - 2:15PM |
B36.00015: Dynamic electrorheological effects due to rotational diffusion of microparticles Hing-wa Tsang, Hiu-ching Lee, Kin-wah Yu Electrorheological (ER) fluids possess a variety of technological applications. The strength of the ER effect is originated from the interaction between the polarized dielectric particles in ER fluids. Existing theories assume the particles are at rest, which fail to agree with experimental results. We suggested the rotational motion of suspended particles be taken into account to close up the discrepancy between theory and experiment [1]. In this work, we have examined the motion of rotational polarized dielectric particles subject to a random torque. The rotational motion of the particles leads to a redistribution of the polarization charge on the surface of the particles [1]. We show that both the ensembled averaged dipole moment and the mean-square value of the angular velocity depend on the correlation function of the angular velocity and the dipole moment in the transverse direction of the applied flied. Implications of the results on dynamic ER effects will be reported. [1] J. T. K. Wan, K. W. Yu, and G. Q. Gu, Phys. Rev. E 64, 061501 (2001). [Preview Abstract] |
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