Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U29: Suspensions and Fluid Dynamics |
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Sponsoring Units: DFD Chair: Jerzy Blawzdziewicz, Yale University Room: Colorado Convention Center 303 |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U29.00001: Wall-induced Particle migration in Dilute Suspensions of Spheres in Creeping Flow Jerzy Blawzdziewicz, Mauricio Zurita-Gotor, Eligiusz Wajnryb The effects of confinement on the dynamics of binary encounters between spherical particles in shear flow are studied for a system bounded by a single planar wall or two parallel planar walls under creeping flow conditions. We show that wall proximity gives rise to a new class of binary trajectories resulting in cross-streamline migration of particles. In contrast, in unbounded space spherical particles on open trajectories return to their original streamlines after a binary encounter is completed (with no non-hydrodynamic forces present). The physical origin of the new trajectories is explained in terms of counter-rotation of particle pairs that is driven by the dynamic pressure distribution. The new type of trajectories constitutes the dominant cross-streamline migration mechanism in dilute wall-bounded suspensions. We show that this mechanism is responsible for the unusually large self-diffusivity observed in experiments by Zarraga and Leighton (2002). The effect of the new migration behavior in dilute suspensions is illustrated using a Boltzmann--Monte Carlo simulation technique. We show that apart from the enhanced self-diffusivity, the walls may also cause formation of a layered suspension microstructure in the low-concentration regime. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U29.00002: Effect of surface roughness on rate-dependent slip in simple fluids. Nikolai Priezjev The influence of molecular-scale surface roughness on the slip length in a flow of simple fluids is investigated using molecular dynamics simulations. The parabolic fit of the steady state velocity profiles induced by a constant force is used to define the value of interfacial shear rate. At weak wall-fluid interactions, the slip length increases non-linearly with the shear rate provided that the liquid/solid interface forms incommensurable structures. A gradual transition to the linear rate-dependence is observed upon increasing the wall-fluid interaction. Thermal surface roughness is found to affect the slip behavior significantly: for soft walls the slip length weakly depends on the shear rate. With increasing elastic stiffness of the wall, the linear rate-dependence of the slip length is restored again. Periodically and randomly corrugated surfaces strongly suppress both the magnitude and slope of the rate-dependence of the slip length even for weak wall-fluid interactions. A relation to recent slip flow experiments is discussed. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U29.00003: Rheology of Deformable Particle Suspensions by Dissipative Particle Dynamics Anuj Chaudhri, Jennifer R. Lukes Understanding the behavior of colloidal suspensions, emulsions, and other complex fluids under shear flow is important in liquid crystal switching, lab-on-chip processing of biological fluids, self-assembly of polymer structures, and other areas of soft matter physics. Various analytical and computational approaches, including Brownian dynamics, dissipative particle dynamics, and Stokesian dynamics, have been applied to study the rheology of \textit{rigid} particle suspensions. Still lacking are methods capable of treating suspensions containing \textit{deformable} particles such as blood cells or macromolecules. Here we present a new, dissipative particle dynamics-based computational method with this capability. This method is used to calculate the shear rate dependence of viscosity for suspensions of deformable particles with varying stiffnesses. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U29.00004: Reducing Viscosity of Liquid Suspensions by pulsed electric or magnetic field R. Tao Viscosity of liquid suspensions is of great importance. Controlling the viscosity is vital in science and engineering. In electrorheological (ER) or magentorheological (MR) fluids, electric or magnetic field is used to increase the viscosity. However, in most cases we need to lower the viscosity. For example, reducing blood's viscosity improves circulation and prevents cardiovascular events. Lowering the viscosity of crude oil is the key to transporting offshore oil via undersea pipelines. Unfortunately, to date there are no effective methods for reducing the viscosity except by changing the temperature. In case that changing temperature is not an option, such as in the above examples, reducing the viscosity becomes formidable. Here we present a theory and experimental results showing that application of a suitable electric or magnetic field pulse can significantly reduce the viscosity of liquid suspensions for several hours with no change of temperature. The field induces dipolar interactions between the suspended particles and forces them to aggregate into large particles. The aggregation changes the rheological properties of the fluids and reduces the effective viscosity. Positive experimental results with MR fluids and crude oil indicate that this method, developed from the basic mechanism of viscosity, is universal and powerful for all liquid suspensions with broad applications. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U29.00005: Microbubbling viscous liquids and suspensions Ketan Pancholi, Mohan Edirisinghe Using a T-junction together with a cross flow technique, we have carried out a detailed study on the formation of near-monodisperse microbubbles in liquids with viscosities in the range of 5-950 mPa s. The data collected were analysed in the context of the classical momentum equation for viscous liquid flow to propose an analytical equation correlating dimensionless viscosity ratio ($\mu _{l}$/$\mu _{g})$ to the ratio of liquid pressure to gas pressure (P$_{l}$/P$_{g})$ required to generate bubbles. This equation is useful in predicting P$_{l}$/P$_{g}$ for microbubbling a liquid having a known viscosity. Our experimental results show that in the liquids investigated, the ratio of P$_{l}$/P$_{g}$, which is a function of dynamic equilibrium of pressure of liquid and gas at the T-junction, is decreasing proportional to dimensionless viscosity ratio. We calculated radial pressure for a given liquid pressure (P$_{l})$ to establish that for liquid viscosities $\ge 48.5$mPa s the radial velocity of liquid, which is responsible for imposing radial pressure on the gas-jet, dominates the mechanism of microbubble pinch-off. In contrast, in the low viscosity regime ($\le 48.5$mPa s), deceleration of the gas stream from the initial velocity is largely the cause of pinch-off of microbubbles. We made ceramic liquid foams using the technique. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U29.00006: Non-Newtonian Impact Denis Bartolo, Gregoire Narcy, Daniel Bonn Spray deposition is widely used in industry (spray painting, pesticide spraying...), but is often inefficient due to an unfavourable wetting interaction of the liquid with the surface. Non-Newtonian polymer effects have been suggested to improve the deposition efficiency, but so far the mechanism has remained elusive and controversial. Here we provide the detailed and quantitative mechanism of the action of the polymers, opening the way to use the non-Newtonian properties to control deposition. We study the impact and subsequent retraction of aqueous drops onto a hydrophobic surface for which rebound of the droplets limits deposition. Adding very small amounts of large molecular weight, flexible polymers dramatically slows down the retraction, inhibiting rebound. We show that the polymers generate strong normal stress effects near the moving contact line of the drop; these can be measured in conventional rheology and can be used to quantitatively account for the slowing down of the retraction. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U29.00007: Spectra of single bubble sonoluminescence from noble gas mixtures Mogens Levinsen In single bubble sonoluminescence a gas bubble trapped by a resonant sound-field emits pulses of light in synchrony with the exciting field. The exact nature of the light emitting processes is, however, not known, and the extent to which internal compressional waves or even shock-waves in the gas affect these processes is still an open question. Simulations suggest that most likely such waves would lead to segregation of species which presumably would have consequences for the intensity and spectrum of the light emitted. We have measured the spectra from single sonoluminescing bubbles seeded with various mixtures of noble gasses. The results are discussed in the light of theoretical expectations. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U29.00008: Steady-state structure formation of two-phase flow in porous media. Thomas Ramstad, Alex Hansen Transport of fluids in porous media is highly complex and creates remarkable patterns. We study these structures and the physics behind them in numerical models based on real porous sediments. These are embedded in a steady-state environment so that they represent selections of a larger, global system. As the saturation of the phases are changed within our models , we see a process towards creation of fluid clusters that eventually span the whole system and have a distribution that approaches a power law behavior. The critical saturation where this phase transition takes place, is dependent of the ratio between viscous and capillary forces inside the pores. We study these scaling properties and the physics that leads to the cluster behavior. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U29.00009: Numerical simulation of flow past an oscillating cylinder beneath a free surface Serpil Kocabiyik, Oleg Gubanov, Larisa Mironova A computational study of laminar flow of a viscous incompresible fluid past an oscillating cylinder close to a free surface is performed. The integral form of unsteady two dimensional Navier- Stokes equations is only discretized in the fluid flow region using fixed Eulerian staggered grid. Well-posed boundary conditions are used at the inflow and outflow boundaries. The no- slip boundary conditions are prescribed at the solid boundary. At the free surface boundary conditions are described by neglecting the motion of ambient air. The volume of fluid method is used to track a moving free surface interface. A piecewise- linear interface reconstruction algorithm is used at each time step for determining the position of both the free surface and fluid-body interfaces. The reconstructed free surface is then advected using computed local velocity field based on a geometrical area-preserving volume of fluid advection algorithm. The numerical simulations are conducted at a fixed Reynolds number, $R=200$, and at displacement amplitude-to-cylinder diameter ratios of $A=0.25$ and $A=0.5$ when submergence depth- to-cylinder diameter ratio is $1.25$. Previously computed and observed flow fields around submerged cylinders are compared to current numerical results and good agreement is found. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U29.00010: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U29.00011: Non-Newtonian behavior of complex plasma fluids Alexei Ivlev, Victor Steinberg, Roman Kompaneets, Gregor Morfill One of the remarkable aspects of complex plasmas is that although they are intrinsically multiphase systems, the rate of momentum exchange through collisions between the microparticles (grains) can exceed the coupling to the background neutral gas significantly. Therefore complex plasma fluids can act as an essentially single-fluid system. Numerical simulations predict that the shear viscosity of complex plasmas should have strong non-monotonous dependence on the kinetic temperature of grains. We proposed a self-consistent model which allows us to obtain explicit dependence of the viscosity on the velocity shear rate, with well-pronounced shear-thinning and thickening effects. Under certain condition, the stress vs. strain rate dependence becomes N-shaped, suggesting formation of shear bands. We performed a series of experiments in a planar or cylindrical shear flow geometry, similar to the Couette and Poiseuille flows. This allowed us to retrieve the viscosity of complex plasmas, which turned out to be in fairly good agreement with the theory. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U29.00012: Magnetic-Force Enhanced Temperature Gradient Jonathan Fraine, Weili Luo The temperature gradient was established in a quasi-one dimensional magnetic fluid by controlling the initial heating and cooling rates. Measurements were done to monitor temperature gradient verses time before and after the cooling and heating were stopped in both zero and applied magnetic field. We found that the magnetic field can enhance the temperature gradient across the sample. The theoretical calculation shows that the effect of field on the temperature gradient is attributed to the magnetic body force that depends on the gradient of the susceptibility. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U29.00013: Dynamics of the Shock Waves Generated by High-Speed Liquid Jets Kyoung-su Im, Seong-Kyun Cheong, Jin Wang, Ming-Chia Lai Ultra fast x-radiography and a multiphase numerical simulation were used to reveal complete dynamical characteristics of the shock waves generated by supersonic liquid jets. Unlike the conventional shock waves by a rigid body compression, this shock waves generated by highly transient liquid jets are characterized by an immediately expansion after short compression caused by the liquid deformation due to aerodynamic drag on the jet front. A transition mechanism from the transonic to the supersonic has been clearly analyzed. With the quantitative analysis and the numerical simulation, the dynamic behavior induced by the compression and decompression in ambient gas in the vicinity of the shock front has been examined, and also we demonstrated the dependence of the shock characteristics on spray angles. Under specific injection condition, we provided the detailed internal structures and interacting mechanisms between the ambient gas and liquid spray jet by simultaneously simulating the fluid parameters such as gas velocities, density contours, and liquid sprays. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U29.00014: Optical tweezer based study of the motion of a sphere in an oscillatory boundary layer Shankar Ghosh, Prerna Sharma, Shobo Bhattacharya Drag forces on a single polystyrene sphere in the vicinity of an oscillatory plate have been measured using an optical tweezer. The phase of the sphere is found to be a sensitive probe of the dynamics. The evolution of the phase from an inertia-coupled regime to a velocity-coupled regime is explored. The frequency dependence of the response is found to be characteristic of a damped oscillator with an effective inertia which is orders of magnitude greater than that of the bare sphere. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U29.00015: Dynamics of electrorhoelogical(ER) fluids Jianwei Zhang, Chun Liu, Ping Sheng Electrorhoelogical (ER) fluids are a class of colloids whose rheological characteristics can be controlled by applying an external electric field. Most applications of ER fluid are determined by its dynamic properties, reflecting the competition between the kinetic energy and internal energies. The relevant physics of dynamic processes is very different from that in static situations. We derive the fully coupled hydrodynamic system modeling the ER fluid dynamics using the energetic variational approach. The interaction between particles is treated as dipole-dipole in character, with a repulsive core. The solid particles and carrier fluid are treated as a two-component incompressible material. The induced electrical polarization and local fields are obtained self- consistently. The forces on the particles and the fluid are derived from the coupling between the transport of the particles and the induced stress. The total force on the moving boundary in stationary state is calculated via total dissipation inside the ER fluid. [Preview Abstract] |
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