Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session M9: Interfacial/Thin Film Instability VI |
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Chair: Mohamed Gad-el-Hak, Virginia Commonwealth University Room: 25B |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M9.00001: Electrohydrodynamic instabilities in thin viscoelastic films: AC and DC fields Leonardo Espin, Andrew Corbett, Satish Kumar Electrohydrodynamic instabilities in thin liquid films are a promising route for the self-assembly of well-defined topographical features on the surfaces of materials. Here, we study the effect of viscoelasticity on these instabilities under the influence of AC and DC electric fields. Viscoelasticity is incorporated via a Jeffreys model and both perfect and leaky dielectric materials are considered. In the case of DC fields, asymptotic methods are employed to shed light on the nature of a singularity that arises when solvent viscosity is neglected (i.e., the Maxwell-fluid limit). In the case of AC fields, we apply a numerical procedure based on Floquet theory to determine the maximum growth rate and corresponding wavenumber as a function of the oscillation amplitude and frequency. Elasticity is found to increase both the maximum growth rate and the corresponding wavenumber, with the effects being the most pronounced when the oscillation period is comparable to the fluid relaxation time. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M9.00002: Dynamics of a thin ferrofluid film subjected to a magnetic field Devin Conroy, Alex Wray, Demetrios Papageorgiou, Richard Craster, Omar Matar We consider a thin film flowing down a rigid, impermeable inclined plane subjected to a magnetic field. The film corresponds to a ferrofluid and is bounded from above by a hydrodynamically-passive gas. The ferrofluid is considered to be weakly-conducting, and its dynamics are governed by the steady Maxwell's equations, coupled to the Navier-Stokes, and continuity equations. The magnetisation of the ferrofluid is a function of the magnetic field, which can be represented by a nonlinear Langevin function; in this work, however, we take the limit of small Langevin parameters, in which this function becomes linear. We use the long-wave limit to expand the governing equations associated with the film; no such approximation is applied in the gas phase in which the full Laplacian for the potential is solved. A one-dimensional partial differential equation is then derived that governs the nonlinear evolution of the interface. This equation is solved numerically for a wide range of system parameters. The results of this parametric study will be presented. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M9.00003: Nonlinear traveling waves in confined ferrofluids S\'ergio Lira, Jos\'e Miranda We study the development of nonlinear traveling waves on the interface separating two viscous fluids flowing in parallel in a vertical Hele-Shaw cell. One of the fluids is a ferrofluid and a uniform magnetic field is applied in the plane of the cell, making an angle with the initially undisturbed interface. We employ a mode-coupling theory which predicts the possibility of controlling the speed of the waves by purely magnetic means. The influence of the tilted magnetic field on the waves shape profile, and the establishment of stationary traveling wave structures are investigated. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M9.00004: Sustainability of Superhydrophobicity Under Pressure Hooman Vahedi Tafreshi, Mohamed A. Samaha, Mohamed Gad-el-Hak Prior studies have demonstrated that superhydrophobicity of submerged surfaces is influenced by hydrostatic pressure and other environmental effects. Sustainability of a superhydrophobic surface could be characterized by both how long it maintains the trapped air in its surface pores, so-called ``longevity,'' and the pressure beyond which it undergoes a global wetting transition, so-called ``terminal pressure.'' In this work, we investigate the effects of pressure on the performance of electrospun polystyrene fibrous coatings. The time-dependent hydrophobicity of the submerged coating in a pressure vessel is optically measured under elevated pressures, up to 10 bar. Rheological studies are also performed to determine the effects of pressure on drag reduction and slip length. The measurements indicate that surface longevity exponentially decays with increasing pressure in perfect agreement with prior studies conducted at much lower pressures. It is found, however, that fibrous coatings could resist hydrostatic pressures significantly higher than those of previously reported surfaces. Our observations indicate that superhydrophobic fibrous coatings could potentially be used for underwater applications. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M9.00005: Effects of Hydrostatic Pressure on the Drag Reduction of Submerged Aerogel-Particle Coatings Mohamed Gad-el-Hak, Hooman Vahedi Tafreshi, Mohamed A. Samaha Hydrophobic aerogel particles with different average diameters are randomly deposited onto metallic substrates with a thin adhesive coating to achieve a combination of hydrophobicity and surface roughness. The resulting surfaces show different degrees of superhydrophobicity and are used to study the effects of elevated pressure on the drag reduction and the degree of hydrophobicity (sustainability) of such surfaces when used for underwater applications. We also developed an image-thresholding technique to estimate the gas area fraction of the coating. The results indicate that there exists a new parameter, the terminal pressure, beyond which the surface undergoes a global transition from the Cassie state to the Wenzel state, and therefore can no longer generate drag reduction. This terminal pressure differs from the previously identified critical pressure. The latter is the pressure above which the surface starts the transition process at some location, but not necessarily at other spots due to the heterogeneity of the surface. For the particle coatings used herein, the terminal pressures are measured to range from 100 to 600 kPa, indicating that such coatings could potentially be used for deep underwater applications. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M9.00006: Dynamics of an inclined film in the presence of soluble surfactants Anna Georgantaki, George Karapetsas, Vasilis Bontozoglou We investigate the dynamics of a thin film flowing down an inclined solid surface in the presence of soluble surfactants. Lubrication theory for the fluid motion, and advection-diffusion equations as well as chemical kinetic fluxes for the surfactant transport, lead to coupled evolution equations for the film thickness, interfacial concentrations of surfactant monomers and bulk concentrations of monomers. We solve numerically the evolution equations using the finite element method and we perform a full parametric study. The results of our simulations show that surfactants have a strong stabilizing effect on the flow due to the presence of Marangoni stresses. The wave patterns that arise differ significantly from the case of clean fluids. It will be shown that the dominant structures, even at high Re numbers, are sinusoidal traveling waves in direct agreement with experimental observations. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M9.00007: Thin soap films are quasi-2D fluids and thick soap films are not Skanda Vivek, Eric R. Weeks We use microrheology to measure the 2D (interfacial) viscosity of soap films. Microrheology uses the diffusive motion of tracer particles suspended in the soap film to infer the viscosity. Our particles are colloids of diameter $d = 0.5$ $\mu$m. We measure the interfacial viscosity of soap films ranging in thickness from $h = 0.5$ $\mu$m to 2.0 $\mu$m. The thickness of these films is measured using the infrared absorbance of the water based soap films, based on a previous setup [X. L. Wu, R. Levine, M. A. Rutgers, H. Kellay, W.I. Goldburg, {\it Rev. Sci. Inst.} {\bf 72}, 2467 (2001)]. From the knowledge of the film thickness and the viscosity of the fluid used to make the film, we can infer the interfacial viscosity due to the surfactant layers at the film/air interfaces. Consistent results are found for thin films ($h/d < 3$) whereas for thicker films inconsistent and unphysical results are found indicating 3D effects begin to play a role. The transition from 2D to 3D properties as a function of $h/d$ is sharp. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M9.00008: Thin film dynamics of viscoelastic fluids Luc Lebon, Laurent Limat We present here viscoelastic fluids in thin film flows, such as liquid bells or liquid curtains. The viscoelastic property of the liquids exhibits specific dynamics in such flows. In the case of bells, the elastic strength tends to extend the bell size for example. In the case of curtain flows, original behaviour of holes are observed with specific growth mechanism for bubbles trapped in the flow. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M9.00009: Fluid displacement under elastic membranes: Dynamics and interfacial instabilities Talal Al-Housseiny, Ivan Christov, Anne Juel, Howard Stone The spreading of fluids under a flexible membrane is a feature of many systems such as the lateral intrusion of magma under a terrestrial crust, or when blood spreads underneath the skin giving the signature color of bruises. In this work, we investigate the displacement of a viscous fluid by a gas underneath an elastic membrane. We consider a radial Hele-Shaw cell where the upper plate is an elastic sheet. The dynamics of the interface between the injected gas and the displaced fluid are fundamentally modified by the presence of an elastic boundary, which leads to the suppression of viscous fingering below a critical flow rate. We demonstrate theoretically the mechanism of suppression and find the corresponding critical flow rate. In addition, we study the dynamics of a stable (circular) interface propagating underneath an elastic membrane and derive the scaling laws for both the position of the interface and the shape of the elastic membrane. Our theoretical findings agree very well with the experimental results of D. Pihler-Puzovic et al. (PRL 2012). [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M9.00010: Elastically-driven surface plumes in rimming flow of a non-Newtonian fluid Gabriel Seiden, Victor Steinberg A polymer solution partially filling a rotating horizontal drum undergoes an elastically driven instability at low Reynolds numbers. This instability manifests itself through localized plumelike bursts, perturbing the free liquid surface. We present experimental results on the dynamics of individual plumes and the statistics pertaining to the complex collective interaction between plumes, which leads to plume coagulation. [Preview Abstract] |
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