2006 APS March Meeting Baltimore, MD
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2006 APS March Meeting Baltimore, MD http://www.aps.org/meet/MAR06 i Monday, March 13, 2006 8:00AM - 11:00AM — Session A8 DFD GSNP: Pattern Formation and Nonlinear Dynamics Baltimore Convention Center 314 8:00AM A8.00001 The effects of initial seed size and transients on dendritic crystal growth ANDREW DOUGHERTY, THOMAS NUNNALLY, Dept. of Physics, Lafayette College — The transient behavior of growing dendritic crystals can be quite complex, as a growing tip interacts with a sidebranch structure set up under an earlier set of conditions. In this work, we report on two observations of transient growth of NH4Cl dendrites in aqueous solution. First, we study growth from initial nearly-spherical seeds. We have developed a technique to initiate growth from a well-characterized initial seed. We find that the approach to steady state is similar for both large and small seeds, in contrast to the simulation findings of Steinbach, Diepers, and Beckermann[1]. Second, we study the growth of a dendrite subject to rapid changes in temperature. We vary the dimensionless supersaturation ∆ and monitor the tip speed v and curvature ρ. During the transient, the tip shape is noticeably distorted from the steady-state shape, and there ∗ 2 is considerable uncertainty in the determination of the curvature of that distorted shape. Nevertheless, it appears that the “selection parameter” σ = 2d0D/vρ remains approximately constant throughout the transient. [1] I. Steinbach, H.-J. Diepers, and C. Beckermann, J. Cryst. Growth, 275, 624-638 (2005). 8:12AM A8.00002 Control of eutectic solidification microstructures through laser spot pertur- bations , SILVERE AKAMATSU, CNRS, KYUYONG LEE, Ames Laboratory, WOLFGANG LOSERT, UMD — We report on a new experimental technique for controlling lamellar eutectic microstructures and testing their stability in directional solidification (solidification at fixed rate V in a uniaxial temperature gradient) in thin sample of a model transparent alloy. A eutectic binary alloy solidifies into a mixture of two crystal phases. In stationary regimes, periodic front patterns made of an alternate stacking of lamellae of the two solid phases are observed. We observe the solidification front in real time by optical microscopy. We use micromanipulation with laser spot arrays for perturbing the solidification front on a scale ranging from one to ten times the average value of the lamellar spacing (spatial period), i.e., typically 10 to 100 microns. These perturbations arise from local heating due to the absorption of the laser light by the liquid slightly ahead of the front. We use the laser spot perturbation technique as a tool for mapping out the large range of accessible lamellar spacings at given V and for creating desired patterns (smooth spatial modulation, tilt domains). 8:24AM A8.00003 Pattern Formation in a NaCl Crystal undergoing Strain-enhanced Dissolu- tion , ZVI KARCZ, DENIZ ERTAS, RICHARD POLIZZOTTI, ExxonMobil Research and Engineering, EINAT AHARONOV, Weizmann Institute of Science, CHRIS SCHOLZ, Lamont Doherty Earth Observatory — Observations of an initially circular contact (∼300µm in diameter) between the [100] face of a single- crystal NaCl shaped as a truncated cone and a flat silicate plate immersed in saturated solution indicate that the crystal deforms in two sequential stages under constant normal load. The first is characterized by contact area reduction and slow convergence rates, and the second by fluctuations in contact area and fast and fluctuating convergence rates. Fluctuations are on a timescale of ∼14 hours. The transition between the stages occurs at the maximum contact stress, which shortly precedes the maximum convergence rate. Confocal images indicate that the crystal dissolves coaxially during the first stage, producing a decreasing static contact. During the second stage, the contact shape is highly irregular, with channels and ridges forming inside the contact. These observations reflect a system evolving towards a non-equilibrium steady state, controlled by the interaction between strain-energy driven undercutting dissolution and plastic flow. Undercutting dissolution reduces the area of the contact, and preferentially removes regions with high dislocation density, while plastic flow increases the contact area by mobilizing dislocations that strain harden the crystal. The feedback between these two mechanisms drives the system towards a dynamic steady state. 8:36AM A8.00004 Controlled Irradiative Formation of Penitentes , VANCE BERGERON, Ecole Normale Su- perieure, Lyon, CHARLES BERGER, Ecole Normale Superieure, Paris, M. D. BETTERTON, University of Colorado — Spike-shaped structures are produced by light-driven ablation in very different contexts. Penitentes 1-4 m high are common on Andean glaciers, where their formation changes glacier dynamics and hydrology. Laser ablation can produce cones 10-100 µm high with a variety of proposed applications in materials science. We report the first laboratory gener- ation of centimeter-scale snow and ice penitentes. Systematically varying conditions allows identification of the essential parameters controlling the formation of ablation structures. We demonstrate that penitente initiation and coarsening requires cold temperatures, so that ablation leads to sublimation rather than melting. Once penitentes have formed, further growth of height can occur by melting. The penitentes intially appear as small structures (3 mm high) and grow by coarsening to 1-5 cm high. Our results are an important step towards understanding and controlling ablation morphologies. 8:48AM A8.00005 Transient growth and controlled side branching of xenon dendrites , MARCO FELL, J. H. BILGRAM, ETH Zurich, Switzerland — In our experiments we study the influence of transient growth conditions on the growth of xenon dendrites from undercooled melt. Here we report on the response of crystal growth on heating the melt. We start heating at a given temperature and steady-state growth. The dendrite tip reacts on this change by slowing down growth rate v and increasing tip radius R. We observe that side branches emerge from an unstable surface. As we continue heating up to slightly above melting temperature, the tip radius continuously decreases to a new value. The reverse temperature change unveils a hysteretic behavior: As soon as we cool down the melt from a temperature tight above melting temperature, v and R both increase. The curvature of the tip becomes too small to be stable at the given undercooling and an instability leads to a new, thin tip growing out of the oversized sphere-like tip. The value R2v shows a sharp peak and then settles to a constant value in only about 20 seconds. The same instability also gives rise to side branches whose formation can be controlled by a repetitive application of the described mechanisms. Highly symmetric xenon crystals can be grown by this technique. 9:00AM A8.00006 Late time growth dynamics in the Cahn-Hilliard equation1 , TMOTHY S. SULLIVAN, Department of Physics, Kenyon College, P. PALFFY-MUHORAY, Liquid Crystal Institute, Kent State University — Numerical simulations were carried out in 2D of the scaled Cahn-Hilliard equation ∂ψ/∂t = (1/2)∇2(−ψ + ψ3 − ∇2ψ) starting from Gaussian distributed, random initial conditions on a 540x540 square grid. Simulations were run for a dimensionless time of 200,000, a factor of ten beyond previously reported results. The simulations also covered a broad range of values of the mean composition, including several at values that had not previously been reported. For each composition and for time intervals of no longer than 5000 in dimensionless time, the structure factor was calculated for sixty separate runs and averaged. The pair correlation function was then calculated from the average structure factor and its first zero crossing,RG(t), taken as a measure of the average domain size, was determined. An equation of b the form RG(t) = at + c was then fit to our data over the dimensionless time range from 5000 to 200,000. In contrast to previous work, we find that the scaling exponent b varies with mean coomposition and does not appear to be consistent with the Lifshitz-Slyozov result b = 1/3. The largest deviation occurs at a mean composition of 0.2, where b = 0.244 ± 0.003. We discuss the possible effects of morphology on both the scaling law and the time it takes to reach the scaling regime. 1Work supported by Kenyon College, the Liquid Crystal Institute, and The Ohio Supercomputer Center 9:12AM A8.00007 Domain Growth in 2D Hexagonal Patterns with Diffuse Interfaces , DANIEL A. VEGA, LEOPOLDO R. GOMEZ,´ Department of Physics - Universidad Nacional del Sur - CONICET. (8000) - Bah´ıa Blanca - Argentina, RICARDO J. PIGNOL, Department of Mathematics - Universidad Nacional del Sur. (8000) - Bah´ıaBlanca - Argentina — The coarsening process in planar patterns has been extensively studied during the last two decades. Although progress has been made in this area, there are still many open questions concerning the basic mechanisms leading the system towards equilibrium. Some of these mechanisms (including curvature driven growth, grain rotation and defect annihilation) have mostly been addressed in systems displaying sharp interfaces. In this work we traced the dynamics of phase separation in hexagonal patterns with diffuse interfaces through the Cahn-Hilliard model. By studying orientational and translational order and densities of topological defects we were able to identify a mechanism of coarsening simultaneously involving curvature driven