Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K21: Colloids V |
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Sponsoring Units: DFD Chair: D. Grier, New York University Room: Baltimore Convention Center 318 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K21.00001: Giant Enhancement of Colloidal diffusion in a Corrugated Optical Vortex Sang-Hyuk Lee, David G. Grier We experimentally study thermally driven velocity fluctuations of a Brownian particle in a tilted washboard potential. Our system consists of a single fluid-borne colloidal sphere driven by a holographically projected superposition of optical vortices. A single optical vortex is a ring-like optical trap created by focusing a helical mode of laser light. Torque exerted by an optical vortex's orbital angular momentum flux drives a trapped colloidal particle around its circumference. Superposing two optical vortices with opposite helicities and different amplitudes creates a corrugated optical vortex with sinusoidal intensity variations around its circumference. The resulting tilted washboard potential admits both static trapped states and dynamic running states. Digital video microscopy measurements of the resulting particle trajectories reveal a hundred-fold enhancement of the effective self-diffusion coefficient near the static-to-running transition. [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K21.00002: Diffusion Limited Branched Polymers Carlos Mendoza, Guillermo Ramirez-Santiago We introduce an algorithm to construct polymers with defined branching structure and whose morphology is determined by diffusion. We apply this procedure for the case of star-branched polymers and calculate their fractal dimension. We also carried out a finite size scaling analysis and determine the scaling properties of the radius of gyration. This procedure may be useful to construct large branched polymers near their relaxed configurations which in turn may help to determine equilibrium configurations of dilute solutions made of these polymers. [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K21.00003: A Constant Force Dielectrophoretic Cell Maria Kilfoil, Vincent Pelletier, Andrew Scott, Allan Haldane We fabricated an especially designed cell to study colloidal suspension under an isomotive dielectrophoresis force. Under such conditions, the effects of a uniaxial force on gel structure and crystallization can be studied with an absolute control on its magnitude. Since the force is horizontal by design, we can take advantage of the higher resolution on the horizontal axis to obtain more precise particle locations. Preliminary results are shown. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K21.00004: Electric Double Layer Structures near Rough Surfaces: Molecular Dynamics Simulation Daejoong Kim, Eric Darve S. S. Dukhin in Surface and Colloid Science (1974) mentioned both the possibility of increase in zeta potential due to surface roughness and the possibility of decrease, depending on Debye length relative to surface roughness. In this work we report our results of molecular dynamic (MD) simulations on electric double layer structures near solid surfaces having roughness with the order of magnitude of Debye length. For computational simplicity only counter-ions are present. We computed static and dynamics properties including density profiles of water and ions, electrostatic potential distributions due to ions, polarization density profiles and self-diffusivities of water and ions. We also performed nonequilibrium MD to simulate electroosmotic flows. From electrostatic potential distributions and slip plane locations, we computed zeta potential and found that it decreases with surface roughness. It also showed a dependency on the spatial frequency of surface roughness. For comparison we used the Helmholtz-Smoluchowski relation and found the same trend. Currently we are studying pressure-driven flows, a computational counterpart to streaming current experiments. One of the purposes is to find more exact locations of slip planes by fitting to Poiseuille flow solutions. We are also simulating model systems with co-ions to investigate the possibility of charge inversion and other effects. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K21.00005: Heterogeneities in Two-Dimensional Pinned Liquids Jing-Xian Lin, Charles Reichhardt, Zohar Nussinov, Leonid P. Pryadko, Cynthia J. Olson Reichhardt We introduce a model system in which the amount of heterogeneous motion in a liquid phase just above melting can be controlled directly. Using numerical simulations, we place a two-dimensional assembly of repulsively interacting colloids on a commensurate triangular substrate of pinning sites, and then randomly deactivate a fraction $n_p$ of the pinning sites. Heterogeneous motion is induced when the unpinned colloids melt at lower temperatures than the pinned colloids, and this heterogeneity can be controlled by changing the fraction of active pinning sites. The melting transition occurs in a single step for $n_p=0$ or 1, and is considerably broadened for partially pinned samples. We measure the noise fluctuations of the dislocation density as a function of time and find a maximum noise power when $n_p=0.5$. Signatures of a two step melting process appear for up to 3/4 of the pinning sites removed. We also correlate the regions of high mobility with regions of high dislocation density, and analyze the heterogeneity using the Van Hove correlation function. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K21.00006: Computation of Super-Hydrophobic States and Stability Yongkang Chen, Danny Bolleddula, Ryan Jenson, Mark Weislogel Super-hydrophobic fluid phenomena have been the focus of an increasing number of research investigations over the past decade. Perhaps the greatest achievements recently have come by way of the highly controlled surfaces that can be produced using any one of a number of rapidly expanding surface microfabrication techniques. In this work we present a numerical approach to systematically probe both the states and stability of certain (super- or ultra-) hydrophobic surfaces as they depend on surface porosity, specific surface feature size and geometry, and fluid properties such as `equilibrium' contact angle and surface tension. Drop stability in terms of critical roll-off angles, advancing and receding contact angles (hysteresis), Bond number, and effective contact angle is computed and used as a measure of `super-hydrophobicity.' Both Wenzel and Cassie hydrophobic states are analyzed by the numerical method. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K21.00007: Design of Porous Wick Structures: Steady Flows Mark Weislogel, Ryan Jenson, Yongkang Chen, Lawrence Melvin III Methods of analysis developed for capillary flows in large complex containers for spacecraft are applied to microscale networks of interconnected repeat units to develop design methodologies to compute optimal geometries for high performance microporous materials and structures on Earth. The fundamental transport mechanism in the media is the interior corner geometry. The specific objectives of the research focus on the optimization of high performance wick structures employed in advanced two-phase passive cooling systems for microelectronic thermal control. The analysis employs the governing transport equations in a cell-by-cell approach to compute optimal pore structures in the low saturation limit where the media may be effectively modeled as a nodal network of interconnected interior corners and solved by matrix methods. The `all analytic' method under development does not employ empirically determined constants or highly varying numerical coefficients as other pore-scale investigations. Example pore geometries solved to date for steady flows clearly identify the origins of the manifold improvements possible in select porous structures. Applications for such methods are also helpful to improving transport processes in porous media such as fabrics and membranes. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K21.00008: Capillary waves at the water liquid-vapor interface Ahmed E. Ismail, Gary S. Grest, Mark J. Stevens Evidence for capillary waves at the liquid-vapor interface of water is presented from molecular dynamics simulations. The total interfacial width includes a correction term which depends logarithmically on the length $L_\parallel$ of the simulation cell parallel to the interface, and which is inversely proportional to the surface tension $\gamma_{cw}$. Comparison of $\gamma_{cw}$ for system sizes up to $10^5$ molecules to $\gamma_{p}$, obtained from the difference between the pressure parallel and the pressure perpendicular to the interface, yields adequate agreement only if one fits the interfacial profile to an error function and not to a hyperbolic tangent, as often assumed. Results for $\gamma$ for a number of atomistic three-site (SPC/E, TIP3P, TIP3P-CHARMM, and TIP3P-Ew) and four-site (TIP4P and TIP4P-Ew) non-polarizable water models are compared to experiment for temperatures from 300 K to 500 K, and for a variety of interaction cutoffs and reciprocal-space mesh refinements. Our results show that the SPC/E model is more accurate than the other available three-site models, while the original TIP3P model is closer to experimental data than its more recent parameterizations. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K21.00009: Self-assembly of Colloid-Polymer Mixtures Confined by Soft Walls Yu-qiang Ma We discuss how to control self-assembled ordering structures in colloid-polymer systems confined by soft walls, and find that with varying the colloidal concentration, the colloidal self- assembly undergoes a series of symmetry-changing transitions, due to the competition between the elastic entropy effect of soft walls and steric packing effect of colloids. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K21.00010: Rheological response of emulsions of drops immersed in electric fields. Arturo Fernandez Direct numerical simulation is used to examine the temporal response of an emulsion of drops immersed in an electric field. When a drop is immersed in a suspending fluid of different electrical properties, and an electric field is applied, surface electric charges accumulate on the boundary between drop and suspending fluid. These charges, coupled with an electric field, lead to the appearance of electric stresses at the interface between the fluids. When an emulsion is immersed between two plates moving at different velocities, the microstructure and rheological properties depend on the competition between electric and hydrodynamical forces. We present a study of these phenomena for DC and AC electric fields. The effect on microstructure is quantified by the PDF, and on rheological properties by effective viscosity and normal stress difference. The numerical simulations show that fluid shear, electrical properties of both fluids, frequency for an AC electric field, and intensity are the main parameters governing the response of the system. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K21.00011: Applications of Quartz Tuning Forks to Liquid Property Measurements James Bennett, Leonid Matsiev, Oleg Kolosov Sensor technology for fluid samples often measures a single physical parameter per sensor and for many applications this is sufficient. In some cases however, characterization of the state or condition of a fluid can be improved through the simultaneous measurement of multiple physical parameters. We will discuss our work in the application of flexural mechanical resonators such as quartz tuning forks to the characterization of fluids. We have previously shown that if a tuning fork resonator is immersed in a fluid, the changes in resonator response can be used to determine physical properties of the fluid. We will describe the development of a measurement system based on a tuning fork resonator, the associated electronics and analysis software. This system can provide simultaneous measurement of liquid properties such as viscosity, density, dielectric constant and ac conductivity. We have applied the tuning fork sensor to the characterization of fluids over a range of temperatures and conditions. Simultaneous measurement of viscosity, density and dielectric constant can provide information about the state or condition of the fluid. For example, for motor oils, clear differences in the measured parameters are observed between fresh and used oils and may be correlated to the oil condition. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K21.00012: Phase Segregation and Patterning in Two Dimensional Systems: Competition Between Van der Waals and Electrostatic interactions Sharon Loverde, Francisco Solis, Monica Olvera de la Cruz The formation of heterogeneities on surfaces, such as microdomains found in synthetic and natural lipid membranes, has not yet been yet fully explored from a theoretical point of view. It has been proposed, for example, that lipid rafts arise in membranes due to lipid-lipid and/or lipid-protein interactions. We consider a coarse-grained model of a mixture of charged lipids interacting within a monolayer. We analyze this model both analytically and with molecular dynamics simulations, and find a rich phase diagram. The complex phase behavior is generated by the competition between the short range van der Waals interactions and long range electrostatic interactions. In particular, we observe phase coexistence between an ``ionic gas'' phase with a dense patterned solid phase. We have examined the phase diagram of the system as a function of net charge density and charge asymmetry. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K21.00013: Binary Lennard-Jones Fluids: A Look Through Time Series Analysis Theodoros Karakasidis, Athanasios Fragkou, Antonios Liakopoulos In this paper we discuss the dynamical behavior of a binary Lennard-Jones fluid simulated at the atomic scale using Molecular Dynamics. The system was simulated at several fluid states as a function of system density and temperature and its instantaneous temperature was recorded. We report preliminary results on time-series analyses of the instantaneous system temperature as well as of the temperature of its two constituents. In this course we employ both linear (autocorrelation function and power spectrum) and non-linear tools (average mutual information and correlation dimension). It turns out that the time series present a complex 1/f$^{a}$ behavior. The dependence of the regimes on the physical state of the system is discussed. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K21.00014: 2D Lennard-Jones as complex liquid Alexander Patashinski, Rafal Orlik, Antoni Mitus, Mark Ratner The high viscosity, large relaxation times, and other signature features of complex liquids are associated with substantial local order and long-living significant liquid structures in these liquids. In 3D Lennard-Jones liquids, representing simple liquids, no detectible structural local order in 13-atom clusters was found at the melting line. In contrast to that, the 2D Lennard-Jones liquid appears locally ordered: about half of the atoms are, at any time, in recognizable hexagonal cages. We present results of a study of the statistics and dynamics of local order in 7-atom clusters in 2D Lennard-Jones-based system. The study includes states along a supercritical isotherm in the range of densities where the system changes from disordered liquid to ordered solid state. [Preview Abstract] |
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