Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J24: GSNP Student Presentation Session and Far From Equilibrium Systems |
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Sponsoring Units: GSNP Chair: Mark Robbins, Johns Hopkins University Room: LACC 411 |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J24.00001: Dynamical heterogeneities and long-lived stress: signatures of jamming in a dense granular flow Allison Ferguson, Bulbul Chakraborty Recent interest in understanding the dynamical arrest leading to a fluid $\rightarrow$ solid transition in both thermal and athermal systems has led to questions about the nature of these jamming transitions (PRL {\bf 86}, 111 (2001), Nature {\bf 411}, 772 (2001)). It is believed that these jamming transitions are dependent on the influence of extended structures on the dynamics of the system (Science {\bf 287}, 627 (2000)). Is it possible to construct a simple model of a driven, dissipative system in which these structures are seen to form? Simulations of steady-state gravity-driven flows of inelastically colliding hard disks show the formation of large-scale linear chains of particles with a high collision frequency even at flow velocities well above the jamming transition (EPL {\bf 66}, 277 (2004)). These chains can be shown to carry much of the collisional stress in the system due to a dynamical correlation that develops between the momentum transfer and time between collisions in these ``frequently-colliding'' particles. The lifetime of these chains is seen to grow as the flow velocity decreases towards jamming, leading to slowly decaying stress correlations reminiscent of the slow dynamics observed in supercooled liquids. Long-lived stress chains seem to be precursors to force chains in static granular piles and understanding the dynamical principles behind their formation and decay can lead to increased insight into the mechanism of dynamical arrest. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J24.00002: Ordered Phases of Diblock Copolymers in Selective Solvent: Micelle Interactions and Lattice Geometry Gregory Grason, Randall Kamien Diblock copolymers in selective solvents are known to aggregate into micelles for copolymer concentration above the critical micelle concentration (CMC). Well above the CMC (volume fractions greater than about 10\%) micelles begin to overlap appreciably and interbrush repulsion becomes relevant. To minimize the effect of these interactions, micelles assemble into ordered phases. Recently, thermoreverisible transitions between such cubic phases of copolymer micelles have been observed [1]. Here, the thermodynamics of micelle aggregation couples to the strength of the intermicelle repulsions, tuning the relative importance of the constrained micelle entropy and average micelle interactions. We propose a model that captures both these thermodynamics as well as the dependence of micelle repulsion and translational entropy on lattice geometry. This model relies on effective theories for the brush regions of the micelles as well as an Einstein crystal description of the lattice. The core region can be treated as a molten polymer brush while the outer brush must be treated as a semi-dilute polymer brush. This ``multi-scale" approach allows us to predict the phase behavior of ordered phases in copolymer solutions under a variety of experimentally realizable conditions. [1] T. P. Lodge, J. Bang, M. J. Park, K. Char, Phys. Rev. Lett. 92, 145501 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J24.00003: When does continuum theory describe mechanical contacts? Binquan Luan, Mark O. Robbins Continuum theories of contact assume that discrete atomic displacements can be described by continuously varying strain fields and also that surfaces are perfectly smooth at small scales. While the first assumption is know to fail as dimensions decrease to atomic scales, continuum results are routinely applied to atomic force microscope tips and other nanoscale contacts. We have used molecular simulations of contact between a rigid sphere or cylinder and a flat elastic half space to test the limits of continuum theory. The flat surface was a (100) or (111) surface of an fcc crystal. Tips were made by bending perfect crystals to a radius of curvature $R$, or cutting surfaces of mean radius $R$ from crystals or amorphous solids. The normal displacement vs. load curves for all tips are close to continuum predictions, as are extracted elastic moduli. Local quantities such as the width of the contact and local pressures can vary by more than a factor of two from continuum predictions. Friction forces and lateral contact stiffnesses vary by at least an order of magnitude with atomic scale geometry. Analysis of these results shows that the assumption of smooth surfaces is a greater source of error than use of continuous strain fields. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J24.00004: Understanding ecosystems using statistical physics Igor Volkov I will show, based on analytic theory and computer simulations, that ecosystems are organized in the vicinity of a new type of phase transition quite akin to Bose-Einstein condensation but occurring in a living system without quantum features. A special case of our model is akin to neutral theory, which postulates that an ecosystem can be characterized by random birth and death processes influenced by immigration and speciation with the further simplifying assumption that all species behave similarly in terms of their birth and death rates. I will present a theoretical framework for the neutral theory of biodiversity and an analytical solution for the distribution of the species composition both for a large metacommunity and for a semi-isolated local community. I will demonstrate that the analytical solution provides an excellent fit to field data. I. Volkov, J. R. Banavar, S. P. Hubbell and A. Maritan, Neutral theory and relative species abundance in ecology, Nature 424, 1035, (2003). I. Volkov, J. R. Banavar and A. Maritan, Organization of ecosystems in the vicinity of a novel phase transition, Phys. Rev. Lett. 92, 218703, (2004). [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J24.00005: The effects of gas pressure on liquid splashing Lei Xu, Wendy Zhang, Sidney Nagel The corona splash due to the impact of a liquid drop onto a dry smooth glass substrate is investigated with high speed photography. We find a striking phenomenon that the splashing vanishes when the surrounding gas pressure is lowered. The relationship of threshold gas pressure (where the splashing ceases to occur) to the impact velocity is measured. Four different gases (Air, He, Kr, SF6) as well as three different liquids (Methanol, Ethanol and 2-Propanol) are used in the experiment. We find a scaling relationship of the threshold pressure in terms of the gas molecular weight and liquid viscosity. A model considering compressibility of the gas is proposed to explain these observations. These experiments shed new light on the phenomenon of how a splash is generated when a liquid hits a smooth substrate. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J24.00006: Critical Behavior of the Banded-Unbanded Spherulite Transition in a Mixture of Ethylene Carbonate with Polyacrylonitrile John Bechhoefer, Bram Sadlik, Laurent Talon, S\'ebastien Kawka, Russell Woods Banded spherulites appear generically when materials with viscous melts are frozen at high undercoolings. The characteristic striped pattern observed in thin samples is believed to reflect a rotation of crystalline axes that occurs as the front propagates radially away from a nucleation site. Common features include an onset of banding at finite undercooling and a divergence of the wavelength near this critical undercooling. Here, by carefully considering systematic errors, we show that the band spacing diverges with a power-law form showing scaling over nearly two decades. We also observe that the bands disorder as the transition point is approached. The critical exponent is non-classical. One possible explanation is that the transition is actually weakly first order. An analogous situation exists for cholesteric liquid crystals in the vicinity of a cholesteric--smectic-A transition. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J24.00007: Persistence in Conserved Order Parameter Coarsening Philip Marquis, Benjamin Vollmayr-Lee Persistence in conserved order parameter coarsening is studied via computer simulation of the Cahn-Hilliard equation. Persistence $P(t_1, t_2)$ is defined as the fraction of the system that has not been traversed by a domain wall between times $t_1$ and $t_2$. We measure persistence as a function of volume fraction and establish that it decays according to a power law $P \sim t_2^{-\theta}$ for all volume fractions studied. We find that the persistence exponent $\theta$ depends on the volume fraction. Our results are then compared with an exact calculation applicable in the dilute limit. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J24.00008: Anisotropic Lifshitz-Slyozov Theory Melinda Gildner, Benjamin Vollmayr-Lee, Fawntia Fowler We study Lifshitz-Slyozov theory for the dilute limit of conserved order parameter coarsening with the addition of an anisotropic surface tension. We calculate the drop shapes and drop size distribution perturbatively in anisotropy strength. We find the $L \sim t^{1/3}$ growth law unchanged with drop shapes that depend only on the scaled drop size. The drop shapes are nonspherical and do not have the equilibrium Wulff shape. The drop size distribution is modified from the isotropic Lifshitz-Slyozov result. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J24.00009: Cahn-Hilliard Simulation of Anisotropic Coarsening Jaime Wallace, Benjamin Vollmayr-Lee The influence of surface tension anisotropy on the dynamics of coarsening is studied via computer simulations. The Cahn-Hilliard equation in dimension $d=2$ is modified to include an arbitrary surface tension anisotropy. For all cases studied, we find asymptotic late-stage scaling with the growth law $L \sim t^{1/3}$ unchanged. The structure factor $S({\bf k},t)$ is binned into angular wedges, and is found to exhibit scaling collapse distinct for each wedge, indicating that the asymptotic domain structure is indeed anisotropic. The Porod tail is found to be a sensitive diagnostic, allowing for quantitative measurement of the degree of anisotropy in the domain structure. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J24.00010: Test of the steady-state fluctuation theorem in turbulent Rayleigh-B{\'e}nard convection Penger Tong, Xiaodong Shang, Keqing Xia Local convective heat flux in turbulent thermal convection is obtained from simultaneous velocity and temperature measurements in an aspect-ratio-one cell filled with water. It is found that large positive fluctuations of the vertical heat flux occurs more often in the plume-dominated sidewall region and their histograms are highly asymmetric. The statistical properties of the time-averaged local flux fluctuations are analyzed and the results are compared with the predictions of the steady state fluctuation theorem of Gallavotti and Cohen. Work supported by the Research Grants Council of Hong Kong SAR under Grant Nos. HKUST603003 (P.T.) and CUHK403003 (K.Q.X.). [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J24.00011: Experimental measurement of power input fluctuation in a turbulent flow Daniel Lathrop, Barbara Brawn, Nicolas Mujica We study the power input fluctuations in a turbulent flow driven by body forces. The local velocity is measured in a system driven by a known pattern of Lorentz forces. The local power input is computed $P = \vec{F} \cdot \vec{v}$ and studied in the context of the Fluctuation-Dissipation theorem. This liquid sodium flow has a Reynolds number $R \sim 10^4$ leading to turbulent fluctuations in the local power inpug. The probability distribution of the power input is consistent with predictions from the Fluctuation-Dissipation theory, even though that should only apply to the spatially averaged input power. These results can be interpreted as suggesting a generalization to the theoretical ideas for far from equilibrium systems. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J24.00012: The noise thermal impedance of a diffusive wire Bertrand Reulet, Daniel Prober The current noise density $S_2$ of a conductor at equilibrium is determined by its temperature $T$: $S_2=4k_BTG$ with $G$ the conductance (Johnson noise). The noise temperature $T_N=S_2/ (4k_BG)$ generalizes $T$ for a system even out of equilibrium. We introduce the noise thermal impedance of a sample as the amplitude of the oscillation of $T_N$ when heated by an ac power. It is the usual thermal impedance for a macroscopic sample. We show for a diffusive wire, how this (complex) frequency-dependent quantity gives access to the electron-phonon interaction time in a long wire and to the diffusion time in a shorter one, and how its real part may also give access to the electron-electron interaction time. We will also present experimental results in various limits. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J24.00013: Nonequilibrium Statistical Mechanics Using Maximum Entropy Methods Mandar Inamdar, Effrosyni Seitaridou, Rob Phillips, Kingshuk Ghosh, Ken Dill Phenomena like Fick's Law of diffusion, and chemical decay processes belong to the domain of nonequilibrium thermodynamics. We believe that the principle of maximum caliber, formulated by E.T. Jaynes, can provide the necessary framework to explain such processes. In this work we formulate simple models for dynamical processes like particle diffusion, heat diffusion, and chemical kinetics. Following the maximum caliber principle, we identify the phase trajectories in each case and write down the corresponding entropy. We then maximize this entropy, subject to the physical/chemical constraints involved in the process, to obtain the probability distribution for its trajectories. From this probability distribution we can get the mean value and fluctuations for the variables of interest. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J24.00014: Mathematical origin of time arrow Yury Shimansky Laws describing the main types of physical interactions are symmetrical with respect to the direction of time flow. At the same time, many virtually irreversible processes are observed. This ``time arrow'' paradox usually is associated with the law of entropy increase. The fact that physical systems obey this law regardless of their physical nature suggests that it may be based on a certain, yet unknown, mathematical principle. Here it is demonstrated that, if, on a time \textit{micro} scale, the intensity of fluctuations of a certain parameter depends on the parameter's value, it would appear to an external observer on a time \textit{macro} scale that the parameter tends to be modified in the direction of fluctuation intensity decrease. It is shown that the law of entropy increase is a consequence of this principle, if it is applied to entropy as a state variable of a thermodynamic system. The fundamental nature of this principle suggests that it must operate on virtually every level of physical reality. The principle is of great potential value for understanding mechanisms of self-organization, learning, adaptation, and evolution. [Preview Abstract] |
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