Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A13: Convection and Crystal Growth 
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Sponsoring Units: DFD GSNP Chair: Joseph Niemela, Abdus Salam International Center for Theoretical Physics Room: B112 
Monday, March 15, 2010 8:00AM  8:12AM 
A13.00001: Rotating turbulent convection at high Rayleigh and Taylor numbers Joseph Niemela, Simone Babuin, Katepalli Sreenivasan We report heat transport measurements in a cylindrical convection apparatus rotating about the vertical axis. The aspect ratio was 1/2. The working fluid was cryogenic helium gas and the following parameter ranges applied: The Rayleigh number, $Ra$, varied in the range $10^{11} < Ra < 4.3 \times 10^ {15}$, the Taylor number, $Ta$, in the range $10^{11} < Ta < 3 \times 10^{15}$, the convective Rossby number, $Ro$, in the range $0.4 < Ro < 1.6$, and the Prandtl number, $Pr$, in the range $0.7 < Pr < 5.9$. Boussinesq conditions applied quite closely. The heat transport for steady rotation, under all conditions of the present experiments, was smaller than that for the nonrotating case. When the rotation rate varied periodically in time a sharp transition to a state of significantly enhanced heat transport was observed for modulation Taylor numbers $Ta^* \ga 10^{14}$, where $Ta^*$ is based on the maximum of the modulation angular velocity. [Preview Abstract] 
Monday, March 15, 2010 8:12AM  8:24AM 
A13.00002: Concentrationdependent Onset of Natural Convection in Magnetic Fluids Yi Liu, Jun Huang, Zhenyu Zhou, Weili Luo The convective heat transfer in magnetic fluids was studied as a function of particle concentrations in a quasione dimensional cell with externally applied temperature difference across the sample. The local temperature distribution measured by eight thermal sensors indicates that the onset of the convection depends monotonically on the concentration of particles, suggesting the resistance to the fluid motion from the particles. From the timedependent temperature profile we obtained the speed of the flow front to be in the order of 10$^{4}$ m/s. This work renders the possibility of studying the effect of applied fields to the convective flow. [Preview Abstract] 

A13.00003: ABSTRACT WITHDRAWN 
Monday, March 15, 2010 8:36AM  8:48AM 
A13.00004: The fielddependent flowfront speed of natural convection in magnetic fluids Jun Huang, Yi Liu, Zhenyu Zhou, Weili Luo The flow front of natural convection in a magnetic fluid was studied in applied field with two configurations: one with temperature gradient, $\nabla T$, parallel to the field gradient, $\nabla B$, and the other with $\nabla T$ antiparallel to $\nabla B$. The temperature profiles inside the two quasi onedimensional cells were used to analyze the speeds of flow fronts. We found that when $\nabla B$ is antiparallel to $\nabla T$, the flow speed is slower than that in zero field; while when $\nabla B$ is parallel with $\nabla T$, the flow speed is faster than that in zero field. These results confirmed our earlier work that in the parallel configuration the field enhances, while in the antiparallel configuration the field suppresses the convection. [Preview Abstract] 
Monday, March 15, 2010 8:48AM  9:00AM 
A13.00005: Probing Instability using Pattern Control in RayleighB\'{e}nard Convection Adam Perkins, Roman Grigoriev, Michael Schatz Identifying and characterizing the mechanisms of instability in spatiotemporally complex systems is of extreme interest, both fundamentally and for realworld applications such as forecasting. We report on a new experimental approach to study instability in a paradigm of such pattern forming systems, RayleighB\'{e}nard convection. The convective fluid absorbs incident infrared laser light, thereby altering the fluid flow. Rapid scanning of the light allows nearly simultaneous actuation at many spatial locations of the pattern. This approach is used to impose reproducibly a given convection pattern. Control is demonstrated by preparing repeatedly a pattern near a straight roll instability. Selected perturbations are applied to this ensemble and decay lifetimes are measured as the system relaxes to the base state. We find that decay lifetimes give a quantitative measure of distance from instability and observe expected critical slowing down as the instability boundary is approached. We also extract the spatial structure of the modes governing the instability and the corresponding growth rates. [Preview Abstract] 
Monday, March 15, 2010 9:00AM  9:12AM 
A13.00006: Convection Cells driven by Spontaneous Symmetry Breaking Michel Pleimling, Beate Schmittmann, R.K.P. Zia A clear signature of farfromequilibrium systems, convection cells are ubiquitous in nature. Typically, they are driven by external forces, like gravity (in combination with temperature gradients) or shear. Here, we show the existence of such cells in a simple (possibly the simplest) system involving only a temperature gradient. In particular, we study a twodimensional Ising lattice gas in contact with two thermal reservoirs, one at infinite temperature and another at a finite T. When T drops below the critical temperature, phase separation emerges and creates convection cells. [Preview Abstract] 

A13.00007: ABSTRACT WITHDRAWN 
Monday, March 15, 2010 9:24AM  9:36AM 
A13.00008: Phase separation in fluids with chaotic advection Daniel Beller, Ben VollmayrLee, Sohei Yasuda When immiscible fluids are advected by an externally applied chaotic flow field, a nonequilibrium steady state arises from the competition between coarsening and the chaotic rippingapart of domains. We simulate a twodimensional binary fluid system advected by two different flow fields: a periodic alternating vortex flow and a periodic alternating sine flow. For each case, we examine the local degree of chaos in the flow field by computing finitebackwardtime Lyapunov exponent values at each point in the system. We find that this Lyapunov exponent field is correlated with the advected fluids' local free energy density, which is inversely related to the local timeaveraged size of phaseseparated domains in the steady state. This raises the possibility of making universal predictions of steadystate characteristics based on Lyapunov analysis of the flow field. [Preview Abstract] 
Monday, March 15, 2010 9:36AM  9:48AM 
A13.00009: Passivescalar separation using chaotic advection Andrew Duggleby, Pradeep Rao, Mark Stremler Separation of two substances with slightly different diffusivities using chaotic advection is explored for finite Reynolds numbers (up to Re$\sim10$) and high average Schmidt numbers ( $\overline{\mathrm{Sc}}=(\mathrm{Sc}_1 + \mathrm{Sc}_2)/2 = 10^6$)) for a modified liddriven cavity. In this approach, exponential stretching of material interfaces enhances diffusion and accelerates separation of concentrated molecules having slightly different diffusivities. At low Re the flow can be reversed and the separated molecules extracted. Using the exponential convergence afforded by the use of a 2D FourierChebyshev spectral algorithm for streamfunctionvorticity formulation with passive scalar transport enables accurate tracking of exponential stretching of material lines in the flow and capturing of the sharp concentration gradients associated with large $\overline{\mathrm{Sc}}$. The two substances separate significantly faster than for simple diffusion. Application to real separation systems will be discussed. [Preview Abstract] 
Monday, March 15, 2010 9:48AM  10:00AM 
A13.00010: A (1 + 1)dimensional model to study the kinetic roughening transition in molecular beam epitaxial growth Crist\'{o}v\~{a}o Dias, Nuno Ara{\'u}jo, Ant\'{o}nio Cadilhe We present a novel model to study the molecular beam epitaxial growth which belongs to different universality classes depending on the values of the flux and temperature. In the present work, we take that thermally activated processes evolve by bond counting. The model exhibits different regimes that from the ballistic deposition limit (at particle low mobility) to layerbylayer growth (at high particle mobility). Finally, we provide a detailed analysis of the properties of the model at the roughening transition. [Preview Abstract] 
Monday, March 15, 2010 10:00AM  10:12AM 
A13.00011: Sidebranching in the Dendritic Crystal Growth of Ammonium Chloride Andrew Dougherty, Franklin Stinner We report new measurements of the dendritic crystal growth of NH$_4$Cl from supersaturated aqueous solution. We report the first measurement of the capillary length $d_0$ to be approximately $2 \times 10^{4} \mu$m. For growth at small dimensionless supersaturations $\Delta$ on the order of 0.005, we have estimated the stability constant $\sigma^*$ to be approximately 0.008. The origin of the sidebranches in dendritic growth is not fully understood, but one model is that they result from the selective amplification of microscopic noise. We will compare measurements of the sidebranch envelope with predictions of the noiseinduced sidebranching model of Gonz\'alezCinca, Ram\'irezPiscina, Casademunt, and Hern\'andezMachado [Phys Rev. E, 63, 051602 (2001)]. A second model is that sidebranches result from small oscillations of the tip. We have observed no such oscillations, but very small ones can not be ruled out. Given the finite experimental resolution, no measurement of the tip region can be completely free of contamination from early sidebranches. We will discuss this and other experimental challenges that need to be overcome before we can understand the origin of sidebranches. [Preview Abstract] 
Monday, March 15, 2010 10:12AM  10:24AM 
A13.00012: Growth and Scaling Dynamics of Condensed Water Drops around a NaCl crystal nucleus Wenceslao Gonz\'alezVi\~nas, Ramchandra D. Narhe, Jos\'e M. Guadarrama, Daniel Beysens We report experimental results on the evolution of condensed water drops in presence of a NaCl crystal nucleus on an ITO substrate. Initially, a drop of radius R starts to grow on the nucleus. At the same time, at distance r from the nucleus center, a condensation pattern is also growing. A region of inhibited condensation is present between the central drop and the pattern. The width of this region $\delta$ asymptotically decreases as $t^{1/6}$. The mean size of drops in the condensation patterns follows a power law $r^{\gamma}$, where $\gamma$ evolves in time and has an average value of 0.16$\pm$0.07. The role of surface diffusion on this system behavior is discussed. [Preview Abstract] 
Monday, March 15, 2010 10:24AM  10:36AM 
A13.00013: Scaling of walls in crystals: Deformation, grain boundaries, and dislocation structures Yong Chen, Woosong Choi, Stefanos Papanikolaou, James Sethna Some experiments of dislocation cell wall structures evolving in deformed metals have observed fractal structures; others have been analyzed in terms of distributions of cell sizes and misorientations that appear nonfractal, but scale with increasing deformation. We analyze a continuum simulation of geometrically necessary dislocations, relaxing in time. In the absence of climb, we observe selfsimilar (fractal) cellwall structures, which we exhibit via realspace renormalization group and analyze in terms of critical exponents for correlation functions of dislocation density, orientation, and plastic distortion. For the same simulation, we analyze the distribution of cell sizes and cell wall misorientations, compare to the corresponding experiments, and discuss how our conclusions depend on the application of external loading. In the presence of climb (roughly simulating grain boundary polygonization) we observe nonfractal scaling and polycrystalline behavior. [Preview Abstract] 
Monday, March 15, 2010 10:36AM  10:48AM 
A13.00014: Domain Structure Universality in the Asymmetric CahnHilliard Equation Benjamin VollmayrLee, Andrew Rutenberg, Sohei Yasuda The CahnHilliard equation, which describes phase separation dynamics with a locally conserved order parameter, is symmetric under interchange of the two equilibrium phases. We consider variations of the CahnHilliard equation in which this symmetry is broken, either by introducing a concentrationdependent asymmetric mobility, or by modifying the doublewell potential. We then simulate these modified systems to determine the influence of asymmetry on the domain structure. This study is motivated by our conjecture that the asymptotic, late time domain structure is determined by the asymptotic dynamics of domain walls. Analysis of the domain wall dynamics, in turn, predicts that mobility asymmetry should affect the domain structure and correlations but that the potential well asymmetry should not. A comparison to the simulation results will be presented. [Preview Abstract] 
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