Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W34: Fluid Structure & Properties |
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Sponsoring Units: DFD Chair: Steve Granick, University of Illinois Room: Baltimore Convention Center 337 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W34.00001: Scaling fields and the nature of liquid-gas asymmetry in fluids Jingtao Wang, Claudio Cerdeiri\~{n}a, Mikhail Anisimov, Jan Sengers Fisher and coworkers [Phys. Rev. Lett. \textbf{85}, 696 (2000); Phys. Rev. E \textbf{67}, 061506 (2003).] recently suggested that in fluids the two theoretical scaling fields, commonly known as ``ordering'' and ``thermal'', are mixtures of three physical fields, namely, chemical potential, temperature, and pressure. We have examined experimental consequences of this formulation (``complete scaling'') with regard to the asymmetry of vapor-liquid coexistence in real fluids. By analyzing the coexisting curves of various fluids, we have shown that the vapor-liquid asymmetry originates from two different sources: one from mixing of chemical potential and pressure into the thermal field and another one from mixing of pressure into the ordering field. The first source is attributed to a correlation between entropy and density, whereas the second source is associated with the excluded volume. Real fluids can be mapped into the symmetric lattice-gas (Ising-like) model by a redefinition of the order parameter that can be now expressed through a combination of density, entropy, and molar volume. We have also demonstrated which molecular parameters of fluids control these two sources of vapor-liquid asymmetry. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W34.00002: NMR characterization of complex fluids by diffusion -- relaxation distribution functions Martin Hurlimann, Albina Mutina Many natural fluids are complex mixtures of different types of molecules. As an example, the molecular composition of crude oils typically consists of molecules with a number of carbon atoms that range between one to over 100. In addition to the diverse size, the constituent molecules can be classified into different chemical classes, such as saturates, aromatics, resins and asphaltenes. It is well known that measurements of diffusion and NMR relaxation times can give information on molecular size. We demonstrate that two-dimensional diffusion -- relaxation time distribution functions, f(D,T2), can provide a more unique fingerprint of complex fluids with information on both chain length distribution and chemical composition. The new approach is illustrated with results for different crude oils. The experiments were conducted at a Larmor frequency of 5 MHz and temperatures between 10 C and 58 C. The measurements show a strong correlation between the distributions of diffusion coefficients and relaxation times that are sample specific. The diffusion - relaxation correlation function provides information on the correlation between the rotational and the translational diffusion coefficients of each component of the fluid. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W34.00003: Structural Change of the Mixtures of Ionic Liquid and Water Studied by Infrared Absorption Spectroscopy Doseok Kim, Yoonnam Jeon, Jaeho Sung, Yukio Ouchi Infrared absorption spectra of the mixtures of ionic liquid and water (1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM]BF$_{4})$ with varying concentrations were obtained by Attenuated Total Reflection (ATR) method. Investigation of the spectra in the OH-stretch vibration range indicated the structural change of the water with the change in the concentration. At very low concentration of water, two peaks around 3600cm$^{-1}$ were assigned to the monomeric form of water molecules weakly hydrogen bonded to the BF$_{4}^{-}$ anions. With the increase in the water concentration, the broad feature at $\sim $3460cm$^{-1}$ corresponding to the bulk water took over the above monomeric peaks, which gradually redshifted with the increased water concentration. In the range from 2800 to 3200cm$^{-1}$ for the various CH-stretch vibration modes in the cation, the peaks in this ranged blueshifted with the increase in the water concentration. This blueshift was as much as $\sim $7cm$^{-1}$ for the CH$_{3}$ vibration modes of butyl chain while it hardly changed for the modes for the CH attached to the imidazolium core, suggesting varying degree of interactions between the carbon-bonded hydrogen and the water molecules. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W34.00004: Weighted density functional theory for water Sahak Petrosyan, David Roundy, Jean-Francois Briere, Tom\'{a}s Arias We report a weighted density functional theory for water that correctly describes bulk properties of water as well as perturbations at large and small length scales. Calculation of the free energy of solvation for hard sphere solutes of different sizes verifies that this functional gives a simple description of the hydrophobic effects in water. Use of this functional within a joint-density functional theory framework allows a rigorous replacement of molecular water with a continuum in Kohn-Sham calculations of systems in equilibrium with a solution. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W34.00005: Small-angle neutron scattering study of pH dependence of the liquid structure factor of concentrated solutions of eye lens gamma-B crystallin Kenneth Desmond, George Thurston, Anna Stradner, Peter Schurtenberger We are evaluating the pH dependence of the liquid structure of aqueous solutions of the eye lens protein, gammaB crystallin, near its critical point for liquid-liquid phase separation, to help evaluate the influence of protein charge on the phase separation. We have obtained small-angle neutron scattering data from gammaB crystallin solutions at pH 6.4, 7.1 and 7.4 in a 0.1 M sodium phosphate buffer, and at pH 4.5 in a 0.020M sodium acetate buffer, all in D2O. Protein concentrations ranged from 6 to 260 mg protein/ml solution and the scattering vector magnitude (q) ranged from 0.004 to 0.45 inverse Angstroms. At pH 6.4 to 7.4 liquid structure factors vs. concentration and temperature near the cloud point for liquid-liquid phase separation are well represented, in general, by the Baxter sticky sphere model. In contrast, at pH 4.5, concentrated gammaB shows a very different liquid structure indicating highly repulsive interprotein interactions, consistent with both high net protein charge and reduced screening. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W34.00006: Phase behavior and mixing-demixing transitions in binary liquid mixtures with spherical and non-spherical interactions Enrique Diaz-Herrera, Guillermo Ramirez-Santiago, J. Antonio Moreno-Razo We have carried out extensive equilibrium molecular dynamics simulations to study the temperature versus density phase diagrams and the mixing-demixing transition line in fluid equimolar binary mixtures modeled by: (i) Lennard-Jones, (ii) Stock-Mayer, and (iii) Gay- Berne molecular interactions. These studies are performed as function of miscibility parameter, $\alpha = \epsilon_{AB}/ \epsilon_{AA}$, where $\epsilon_{AA} = \epsilon_{BB}$ and $\epsilon_{AB}$ stand for the parameters related to the attractive part of the intermolecular interactions for similar and dissimilar particles, respectively. When the miscibility of the Lennard-Jones mixture varies in the range $0 < \alpha < 1$, a continuous critical line of consolute points $T_{\rm cons}(\rho)$, appears. This line intersects the liquid-vapor coexistence curve at different positions depending on the values of $\alpha$, yielding mainly three different topologies for the phase diagrams. These results are in qualitative agreement to those found previously for square well and hard-core Yukawa binary mixtures. We also carry out a detailed study of the liquid-liquid interfacial and liquid-vapor surface tensions, as function of temperature and miscibility as well as its relationship to the topologies of the phase diagrams. Similar studies and analysis are also performed for Stock-Mayer and Gay-Berne binary mixtures. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W34.00007: Confined Fluids: the Time Variable in the Force-Distance Profile Janet Wong, Sung-Chul Bae, Steve Granick Hitherto-overlooked time dependence is known to play a prominent role in determining the friction of confined fluids. In this study, for the first time we introduce the time variable into measuring force-distance profiles of several simple alkane fluids. The existence of near-surface layered structures in confined fluids is generally manifested as oscillatory forces in force-distance profiles obtained using surface forces apparatus (SFA) and atomic force miscroscopy (AFM) experiments. While it is generally agreed that the rate of the experiment should be slow enough to achieve a quasi-static state, it is less clear what the appropriate rate should be. In this study, while maintaining the experimental time scale uniformly slow enough to avoid trivial hydrodyanamically-induced surface deformations, we demonstrate time dependence in the measured force-distance profile. The role of time scale on the actual structure of the confined fluid will be discussed. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W34.00008: Spectroscopic Observation of Fluid Molecular Alignment in a Molecularly-Thin Confined Geometry Minsu Kim, Shan Jiang, Sung Chul Bae, Steve Granick For the first time, we present data of molecular alignment of a linear chain (1,3-dicyanopropane) under confinement. Confinement was produced between two mica surfaces within a surface forces apparatus (SFA) and measurements employed confocal Raman spectroscopy. We focused on the CH$_{2}$ symmetric stretch vibrations and CN triple bond stretch vibrations. A polar plot of Raman band intensity as a function of incident light polarization allows us to determine the orientation and order parameter of alignment. It is confirmed that alignment can be achieved in molecularly-thin films. The decrease of alignment as the film thickness increases will be mentioned. Also, the effect of shear on molecular alignment will be discussed. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W34.00009: Active Microrheology of Dense Colloidal Suspensions Alexander Meyer, Eric M. Furst We investigate the active microrheology of a colloidal suspension using laser tweezers. The experimental system described here is composed of a hard sphere suspension of fluorescent, index-matched poly(methyl methacrylate) particles seeded with a low concentration of index-mismatched melamine probes. The probe particles are held in an optical trap and subjected to a uniform flow, enabling measurements of the suspension microrheology. Additionally, confocal microscopy is used to obtain non-equilibrium microstructural information. An anisotropic pair distribution function, with a dense region at the leading surface of the probe and a wake trailing it, is observed as the P\'{e}clet number increases to much greater than unity. This structural transition gives rise to a shear thinning regime in the measured microviscosity. The results are in qualitative agreement with recent simulation [I. C. Carpen and J. F. Brady, J. Rheol. 49, 1483-1502 (2005)], and demonstrate the non-linear microrheology of colloidal suspensions. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W34.00010: Nonlinear microrheology of wormlike micelle solutions using magnetic nanowire probes Nathan Cappallo, Clayton Lapointe, Robert L. Leheny, Daniel H. Reich Using ferromagnetic Ni nanowires we investigate the local mechanical properties of wormlike micelle solutions composed of equimolar concentrations of the surfactant cetylpyridinium chloride (CPCl) and sodium salicylate (NaSal). Rotating the nanowires with external magnetic fields, we access both linear and nonlinear regimes of the fluid's rheology. The linear viscosity at low rotation rates is strongly temperature dependent as expected from mechanical rheometry experiments. At high rotation rates the viscosity exhibits pronounced shear thinning that is independent of temperature. The onset of the nonlinear response is characterized by a hysteretic shear thickening that is strongly dependent on temperature, but has no counterpart in the macroscopic rheometry. Further, the nonlinear regime coincides with a transient, anisotropic shear-induced state in the fluid that generates a torque on the wire, causing it to tip out of the plane of rotation when the field is removed. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W34.00011: Myelin figures: an Elastic Instability? Ling-Nan Zou, Sidney R. Nagel Myelin figures form when certain lamellar phase surfactants swell upon exposure to water. The formation of these myelins, which are tubular structures composed of multiple bilayers of surfactant, is puzzling because it represents the formation of a higher bending-energy configuration from a lower bending-energy initial state. We show that single myelins can be produced in isolation and require a driving force to form and grow; they retract into their parent structure when the driving is removed. We present a model, consistent with our experimental observations, where the formation of myelins is due to an elastic instability of the lamellar phase under internal stress. We propose an experiment to test of this model in comparison to other models, such as that of Huang et al.[1] \newline \newline [1]J.-R. Huang, L.-N. Zou, and T. A. Witten, Eur. Phys. J. E (2005). DOI: 10.1140/epje/e2005-00035-8. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W34.00012: Charge fluctuations and correlations in finite electrolytes Young C. Kim, Michael E. Fisher Charge fluctuations, $\langle Q^{2}_{\Lambda}\rangle$, for the 1:1 equisize hard-sphere electrolyte with the diameter $a$ are computed via grand canonical Monte Carlo simulations, where $Q_{\Lambda}$ is the total charge inside a subvolume $\Lambda$ contained in a simulation box of dimensions $L\times L\times L$ with periodic boundary conditions. The charge fluctuations increase like the surface area $|\partial\Lambda|$ as $\Lambda$ increases, even for small system sizes $L\leq 12a$. For slabs of dimensions $L\times L\times \lambda L$ with $0 < \lambda < 1$, the scaled charge fluctuations, $\langle Q^{2}_{\Lambda}\rangle/|\partial\Lambda|$, approach the thermodynamic limits exponentially fast. The extrapolations to $L\rightarrow\infty$ then yield the Lebowitz length, $\xi_{\mbox{\scriptsize L}}(T,\rho)$, where densities $\rho\alt 3\rho_c$ and temperatures $T\agt T_c$ have been studied. An exact asymptotic expression is obtained for $\langle Q^2_\Lambda \rangle$. This enables one to compute the charge correlation length $\xi_{Z}(T,\rho)$ precisely. The results for $\xi_Z(T,\rho)$ agree with Debye-H\"{u}ckel-type theories at low densities, but show deviations as the density increases. Charge oscillations at higher densities are also observed, as anticipated theoretically. \newline \noindent [1] Y. C. Kim, E. Luijten, and M. E. Fisher, Phys. Rev. Lett. {\bf 95}, 145701 (2005). [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W34.00013: Solid or Liquid ? -- Kinetically induced solidification in a simple nanoconfined liquid Peter Hoffmann, George Matei, Shivprasad Patil, Ahmet Oral For many years there has been a controversy regarding the supposed solidification of simple liquids when they are confined to a few nanometer film thickness. By using a novel, ultra-small amplitude Atomic Force Microscopy (AFM) technique, we have found that solidification in these systems seems to be due to a kinetic effect and does not occur in thermodynamic equilibrium. In particular, we studied OMCTS confined between a flat silicon surface and a silicon tip and found that at very low approach speeds ($<$= 0.3 {\AA}/sec) the confined fluid remains liquid-like with no change in mechanical relaxation time from the bulk, although ordering is observed in the stiffness and damping of the film. However, when approaching the tip slightly faster at or above 6 {\AA}/sec, the liquid suddenly changes properties dramatically. In the ordered regime, damping is greatly reduced and the mechanical relaxation times show large peaks, indicating an elastic, solid-like response. This result suggests that the observed solidification is a non-equilibrium effect induced at very long time scales. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W34.00014: Investigation of Liquid Transport/Diffusion through a Nanopore Driven by a Constant Pressure/Chemical Potential Difference. Cunkui Huang, Kumar Nandakumar, Phillip Choi, Larry Kostiuk Fluid transport/diffusion through a nanopore in a membrane was investigated by using a novel molecular dynamics approach proposed in this study. The advantages of this method, relative to dual-control-volume grand-canonical molecular dynamics (DCV-GCMD), are that it eliminates disruptions to the system dynamics that normally created by inserting or deleting particles from control volumes, and that it functions well for dense systems as the number of particles in the studied system remain fixed. Using this method, we examined liquid argon transport/diffusion through a nanopore by performing non-equilibrium molecular dynamics (NEMD) simulations under different back-pressures/chemical potentials. The MD code was validated firstly by comparison with published experimental data, and NEMD results of the present method show that constant pressure/chemical potential difference across the membrane was readily achieved. The soundness of classical Navier-Stokes (NS) solutions for these nanochannel flows was also checked by direct comparison between the NS predictions and results from the proposed NEMD method. The density distributions along the nanopore for both methods were found to be significantly different, but the velocity profile had a similar pattern, although some difference between them exists. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W34.00015: Phase behavior in binary fluid mixtures with spherical and non-spherical interactions Enrique Diaz-Herrera, Guillermo Ramirez-Santiago, J. Antonio Moreno-Razo We have carried out extensive MD simulations to study the T vs. $\rho$ phase diagram and the mix-demix transition in fluid binary mixtures with (1) Lennard-Jones, (2) Stock-Mayer and (3) Gay-Berne molecular interactions. This analysis is performed in terms of the miscibility parameter, $\alpha=\epsilon_{AB}/\epsilon_{AA}$, with $\epsilon_{AA}=\epsilon_{BB}$. When the miscibility of the mixture is in the range $0<\alpha<1$, a continuous critical line of consolute points appears. This line interscts the LV coexistence curve at different positions depending on the value of $\alpha$, yielding mainly three different topologies for the phase diagrams. We also carried out a detailed study of the interfacial properties as function of $T$ and $\alpha$. [Preview Abstract] |
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