Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A15: Soft Matter, Fluid Structure and Properties |
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Sponsoring Units: DFD Chair: Wendy Zhang, University of Chicago Room: 316 |
Monday, March 16, 2009 8:00AM - 8:12AM |
A15.00001: Impact of a viscous drop Wendy W. Zhang, Robert D. Schroll, Christophe Josserand, Stephane Zaleski Recent experiments [1] reveal that reducing the ambient air pressure entirely suppresses the splash generated by the impact of an oil drop at several m/s onto a dry smooth wall. Motivated by these observations, we simulate two types of drop impact: impact onto a smooth, dry solid wall and head-on collision of two identical liquid drops. In both cases we make the additional simplification that impact simply arrests the downward fall and redirects the liquid radially outwards in a thin, expanding sheet. It does not break the drop surface. Since experiments suggest that splash is created by airflow deforming the thin sheet, we focus on the time-evolution of the thin liquid sheet but restrict ourselves to the simpler situation of negligible airflow effects. In this regime, we find that the ejected sheet is always characterized by two different lengthscales. Surface tension controls the rim size. The thickness over the rest of the sheet is controlled by a different mechanism. Impact onto a solid surface creates a pancake whose thickness is controlled by viscous dissipation. Head-on collision creates a sheet that thins continuously with distance from the collision center. Its thickness is controlled by the kinematics of impact.\\[0pt][1] Stevens, Keim, Zhang \& Nagel, FC03 APS DFD meeting (2007) [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A15.00002: Focused impact through layers of aqueous cornstarch solution Bin Liu, Jun Zhang, Michael Shelley A layer of aqueous cornstarch solution, when punched with a solid sphere, will create a thickened mass on the sphere that transmits the impact towards the bottom. As a consequence, the mass can leave an imprint on the bottom, if composed of a soft molding clay. The impact transmitted through the fluid layer is more localized for slower speeds of the sphere, giving rise to an imprint with sharper curvature. Our work shows that a layer of shear-thickening fluid may help to focus the impact rather than dissipate it when punched slowly enough. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A15.00003: Drop pinch-off of concentrated surfactant solutions in the lamellar phase Itai Cohen, Patrick Spicer, Marco Caggioni, John Savage Droplet pinch-off in air is a common phenomenon that occurs all around us. At the point of pinch-off, the drop radius shrinks to zero in a finite amount of time. The pressure exerted by the interface is inversely proportional to the minimum radius and becomes singular at Pinch-off. In Newtonian fluids, this finite time singularity gives rise to universal features in the pinch-off process that can be described by similarity solutions for the fluid air interface. In this talk I will address the question of how this process is altered when observed in concentrated surfactant solutions that are in the lamellar phase. Remarkably we find that pinch-off in these systems is a mix between universal and non-universal behavior. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A15.00004: Watching the Paint Dry: Dynamics of Drying in Porous Media Lei Xu, Simon Davies, Andrew Schofield, David Weitz What is the dynamics of drying in porous media? It has been difficult to visualize due to the non-transparency of the media. We study this phenomenon in an optical index matched colloidal system with confocal microscopy. We observe abrupt air invasions which result from the strong flow from menisci in large pores to menisci in small pores. The size and structure of the air invasions are in accord with 3D invasion percolation. By varying the particle size and contact angle we unambiguously demonstrate that capillary pressure dominates the drying process. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A15.00005: Monitoring Three-dimensional Fluid Configurations in Porous Media Amber Krummel, David Weitz The spatial and time resolution of confocal microscopy affords the ability to collect three-dimensional images during the course of two-phase flow experiments. We fully instrument the microscope with precise flow and pressure measurements, such that we can begin to understand the origins and consequences of the three-dimensional fluid configurations that evolve in the sample. The porous media used in this work is composed of slightly sintered, borosilicate glass beads that are 150 microns in diameter. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A15.00006: Shear banding fluids in microchannels: high shear rheology, slippage and Poiseuille flow instability Philippe Nghe, Guillaume Degre, Patrick Tabeling, Armand Ajdari We characterize by Particle Image Velocimetry the Poiseuille flow a semi-dilute solution of wormlike micelles (a CTAB and sodium nitrate aqueous solution) in pressure resistant microchannels. At low shear rates, we observe a parabolic profile. Increasing the pressure driving the flow, the fluid separates into two phases above a critical shear rate at the wall. This is the so called shear-banding regime. Deducing the non-linear rheology from the velocity profiles by a local calculation, we are able to measure the stress versus shear rate curve at least one order of magnitude above the dynamical range attainable in Couette geometries, independently from the slippage, revealing a strongly shear-thinning structure. In addition, by extrapolation of the velocity profiles to the wall position, we measure an absence of slippage at the wall. Looking into more details to the increase in velocity fluctuations in the downstream direction, we characterize a supercritical instability in this shear-banded Poiseuille flow, localized at the interface between the two phases with a wavelength comparable to the confining dimension. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A15.00007: Nonlinear Dynamics in Viscoelastic Jets Trushant Majmudar, Matthieu Varagnat, Gareth McKinley Instabilities in free surface continuous jets of non-Newtonian fluids, although relevant for many industrial processes, remain poorly understood in terms of fundamental fluid dynamics. Inviscid, and viscous Newtonian jets have been studied in considerable detail, both theoretically and experimentally. Instability in viscous jets leads to regular periodic coiling of the jet, which exhibits a non-trivial frequency dependence with the height of the fall. Here we present a systematic study of the effect of viscoelasticity on the dynamics of continuous jets of worm-like micellar surfactant solutions of varying viscosities and elasticities. We observe complex nonlinear spatio-temporal dynamics of the jet, and uncover a transition from periodic to quasi-periodic to a multi-frequency, broad-spectrum dynamics. Beyond this regime, the jet dynamics smoothly crosses over to exhibit the ``leaping shampoo'' or the Kaye effect. We examine different dynamical regimes in terms of scaling variables, which depend on the geometry (dimensionless height), kinematics (dimensionless flow rate), and the fluid properties (elasto-gravity number) and present a regime map of the dynamics of the jet in terms of these dimensionless variables. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A15.00008: Relating shear banding and orientational order in wormlike micellar solutions Matthew Helgeson, Matthew Reichert, Eric Kaler, Norman Wagner Shear banding has been observed in a variety of complex fluids, including polymer solutions, colloidal suspensions and, most prominently, wormlike micelles (WLMs). However, accurate modeling of shear banding fluids remains a challenge, due to the inability to identify the mechanism(s) leading to banding. Using a novel approach that combines measurements of phase behavior, rheology, and spatially-resolved microstructure on model WLMs, we present the first complete study of local rheology and microstructure through the shear banding transition for model WLMs in the vicinity of an equilibrium isotropic-nematic transition (I-N). The rheology of such fluids is well-described by the Giesekus constitutive equation with incorporated stress diffusion, which allows simultaneous description of rheology, flow kinematics, and spatially-resolved microstructure under shear. The results show that shear banding coincides with a first-order, shear-induced transition to a paranematic state at critical values of micellar orientation and alignment, which can be related directly to a non-monotonic constitutive relation. Furthermore, the model allows for the construction of non-equilibrium state diagrams that elucidate a number of experimental observations in shear banding fluids. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A15.00009: Structure and Phase Behavior of Ion--Dipole Mixtures Wonki Roh, Erik Luijten It is well established that Coulombic interactions induce a liquid--liquid transition in ionic solutions. By contrast, the occurrence of phase separation driven by anisotropic dipolar interactions is still a matter of debate, with our recent simulation results excluding this phase separation for a large region of the temperature--density plane. These observations naturally lead to the question whether phase separation takes place in mixtures that contain ions as well as dipolar particles. Employing large- scale grand-canonical Monte Carlo simulations, we investigate four prototypical ion--dipole mixtures: \emph{ion-dominated} systems in which the dipole moment is either strong or weak, and \emph{dipole-dominated} systems with strong or weak dipolar strength. We focus on the low-temperature regime and search for phase separation by varying the chemical potentials of the ions as well as the dipolar particles. Depending on temperature and on the magnitude of the dipole moment, remarkable liquid structures are found that may have implications not only for the behavior of ion--dipole mixtures, but also for self-assembly in suspensions containing charged and dipoloar colloids. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A15.00010: Surface Layering Near Room Temperature in a Nonmetallic Liquid Sudeshna Chattopadhyay, Benjamin Stripe, Patrick Shively, Geunnadi Evmenenko, Pulak Dutta, Steven Ehrlich, Haiding Mo Oscillatory density profiles (layers) have been observed at the free surfaces of many liquid metals at and above room temperature [1]. A surface-layered state has been previously reported only in one dielectric liquid, tetrakis(2-ethylhexoxy)silane (TEHOS), and only at lower temperatures [2]. We have used x-ray reflectivity to study a molecular liquid, pentaphenyl trimethyl trisiloxane. Below T$\sim $ 267K (well above the freezing point for this liquid), density oscillations appear at the surface. This liquid has a higher $T_{c}$ ($\sim $1200K) than TEHOS ($\sim $950K), so that layers appear at $T/T_{c} \quad \approx $ 0.2 in both cases. Our results indicate that surface order is a universal phenomenon in both metallic and dielectric liquids, and that the underlying physics is likely to be the same since layers always appear at T$<\sim $0$.$2$T_{c}$ as theoretically predicted [3] \\[3pt] REFERENCES: \\[0pt] [1]. e.g. O. M. Magnussen \textit{et al}., Phys. Rev. Lett. \textbf{74}, 4444 (1995) \\[0pt] [2]. H. Mo et al. \textit{Phys. Rev. Lett.} \textbf{96}, 096107 (2006); \textit{Phys. Rev. B} 76, 024206 (2007) \\[0pt] [3]. e.g. E. Chac\'{o}n et al., Phys. Rev. Lett. \textbf{87}, 166101 (2001) [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A15.00011: Structural effect of sugars on water Simcha Srebnik, Ravit Matza, Iliya Kusner, Yoav D. Livney The modulation of the structure of liquid water by solutes has tremendous consequences in numerous fields, particularly on the stability of proteins. However, the reasons for the differences in effects of similar solutes are still unclear. Recently, Livney and coworkers [1] found a strong relationship between the hydration layer of sugars and its effect on the phase transition of a model polymer, which may be explained by the strong interaction between water and sugars leading either to cooperative structuring of the water and thus to large hydration numbers, or disrupting water structure near the sugar, resulting in lower hydration. Using atomistic Monte Carlo simulation, we studied the compatibility of various sugars with an ideal tetrahedral water structure, as embodied in hexagonal ice. Our simulations suggest the following order of compatibility with ideal water structure: galactose $>$ glucose $>$ mannose. In agreement, experimental measurements of isentropic compressibility show the same order of hydration numbers and kosmotropic effect. A simple physical model of the binary system is used to shed further insight on the structuring effect of sugars on water. 1. Shpigelman, A.; Portnaya, I.; Ramon, O.; Livney, Y. D. J Polym Sci Part B: Polym Phys 2008, 46, 2307-2318. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A15.00012: Reconstructing the dynamical solvent structure around a model `hydrated electron' using inelastic x-ray scattering R. Coridan, G.H. Lai, N. Schmidt, P. Abbamonte, G.C.L. Wong, R. Godewat, S. Garde, M. Krisch, A.Q.R. Baron The structure and dynamics of water on femtosecond timescales is relevant to many topics in physical chemistry such as electron solvation. We computationally reconstruct the {\AA}-scale spatial and fs-scale temporal evolution of density fluctuations in water using high-resolution inelastic x-ray scattering (IXS). The imaginary part of density propagator $\chi $(q,$\omega )$ is directly extracted from the IXS data, and the real part recovered using Kramers-Kronig relations. The resultant complex-valued $\chi $(q,$\omega )$ is the Fourier transform of the real-space density-density response function $\chi $(r,t) which measures the dynamical density fluctuations of water due to a point-like instantaneous pulse. We use this density propagator and linear-response theory to reconstruct a model of the hydrated electron. The water density fluctuations as the electron `diffuses' through bulk water can be observed. Moreover, preliminary data on the solvent response to changes in the electronic wave function will be presented. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A15.00013: Direct measurement of negative square gradient coefficients for density fluctuations in all-atom simulations of common liquids Colin Denniston, Lingti Kong, Dan Vriesinga We perform all-atom simulations of common liquids such as water (TIP3P) and organic liquids such as short-chain olefins. We show that square gradient coefficients for the mass density can be measured directly in a linear response measurement to sinusoidal forces at several different wavelengths. Surprisingly, in all fluids measured, the square gradient coefficient is negative implying that density gradients lower the free energy of the system. However, stability is maintained at any wavelength greater than the separation between molecules due to the global mass conservation constraint. We suggest that this provides a mechanism for the molecular scale cut-off of pressure singularities that arise in situations such as droplet pinch-off. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A15.00014: Chiral Structures of Thermoresponsive Soft Spheres in Hollow Cylinders Matthew A. Lohr, Ahmed Alsayed, Zexin Zhang, Arjun G. Yodh We experimentally observe the formation of closely packed crystalline structures in hollow cylinders. ~The structures have varying degrees of chiral order. The systems are created from aqueous suspensions of thermoresponsive N-isopropylacrylamide (NIPA) microgel particles packed in micron-diameter glass capillaries. We categorize these structures according to classifications used by Erickson for tubular packings of hard spheres [1]. By varying the temperature-tunable diameter of these particles, the system's volume fraction is changed, permitting observations of the resilience of these structures and their melting transitions. Melting of these thermal crystalline structures is observed. [1] R. O. Erickson, Science 181 (1973) 705-716. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A15.00015: Nonlinear Transverse Wave Excitations in Fluid Flows Dillon Scofield, Pablo Huq The interplay of inertia and dissipation in flows with nonlinear transverse wave excitations is described by including a vortex field into the stress-energy balance equation. The theory uses an acoustic spacetime which allows limiting the speed of propagation of fluid transverse waves to a maximum speed. In the low speed limit, the theory reduces to the Navier-Stokes equations. By examining other limiting cases we show that the Navier-Stokes theory neglects terms involved with the transport of vorticity and the dissipation of energy due to the vortex field. Comparison of the theory to experiment, relative to the Navier-Stokes theory, shows that the presence of the vortex field accounts for the observed relative increase in energy- dissipation, extended lifetime of vortex structures, and excitation structure of the transverse wave field. [Preview Abstract] |
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