Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session GF: Instabilities: Interfaces, Films and Droplets |
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Room: 003B |
Monday, November 24, 2008 8:00AM - 8:13AM |
GF.00001: Self-assembly of diblock copolymer blends on an air-water interface Hsiang-Wei Lu, Anette Hosoi, Shenda Baker Spreading diblock copolymers on an air-water interface produces nanoscale features via self assembly. As a predictive tool for the size and morphology of the observed features, we develop a phase field model which includes the effects of polymer vitrification and diffusion in the self-assembly of polymer blends. Stability analysis indicates that the characteristic feature dimensions can be tuned by adjusting the blend composition and the initial surface concentration. In addition, simulations suggest an internal structure within the regularly spaced nanoscale dots that can provide additional functionality. [Preview Abstract] |
Monday, November 24, 2008 8:13AM - 8:26AM |
GF.00002: Asymptotic Transitions and Interactions of Interfacial Instabilities in Shear Flows Svetlana Sushchikh, Theo Theofanous We consider global behaviors of instabilities in miscible and immiscible two-phase flows---pressure or shear driven superposed layers. We find Yih-like instabilities with diffuse layers occupying as much as 10{\%} of the flow even at rather low Reynolds numbers. These approach asymptotically the Yih instability as the diffuse layer thickness decreases, and we quantify this asymptotic transition in terms of the Peclet number. Then we find both Yih and Yih-like instabilities to transition to the inviscid mode (corresponding inviscid problem) at some value of the Reynolds number that depends on the viscosity ratio and the Peclet number (in Yih-like case), including stability if the inviscid mode is stable. Finally at high enough Reynolds number, we show that Yih and Yih-like instabilities can be in mutual influence with the T-S mode when there is some proximity in the respective critical layers. When this proximity is even closer, this influence becomes a strong interaction, and transformation of both into a new kind of instability. [Preview Abstract] |
Monday, November 24, 2008 8:26AM - 8:39AM |
GF.00003: Numerical simulation of Faraday waves Nicolas Perinet, Damir Juric, Laurette Tuckerman Faraday\footnote{M.~Faraday, {\it Phil. Trans. R. Soc. Lond.}~{\bf 52}, 319 (1831).} first described in 1831 the pattern of standing waves generated at the surface of a vertically oscillated fluid layer; the corresponding linear stability analysis was carried out in 1954 by Benjamin and Ursell\footnote{T.B.~Benjamin \& F.~Ursell, {\it Proc. R. Soc. Lond. A}~{\bf 225}, 505 (1954).} for inviscid fluids and in 1994 by Kumar and Tuckerman\footnote{K.~Kumar \& L.S.~Tuckerman, {\it J. Fluid Mech.}~{\bf 279}, 49 (1994).} for viscous fluids. Linear stability analysis, however, predicts only the critical wavenumber and oscillation amplitude, and not the variety of periodic lattice patterns manifested by Faraday waves which have long intrigued researchers. The experimental observation in 1992 of quasicrystalline patterns by Edwards and Fauve\footnote{W.S.~Edwards \& S.~ Fauve, {\it J. Fluid Mech.}~{\bf 278}, 123 (1994).} has inspired an abundance of experimental and theoretical research. However, this has not been accompanied by realistic numerical computations. Here, we report on fully three-dimensional and nonlinear Navier- Stokes simulations of Faraday waves using a front tracking method for the interface between two immiscible fluids.\footnote {S.~Shin \& D. Juric, {\it J. Comput. Phys.}~{\bf 180}, 427 (2002).} [Preview Abstract] |
Monday, November 24, 2008 8:39AM - 8:52AM |
GF.00004: On the evaporation/condensation dynamics of thin liquid films and sessile droplets Pierre Colinet, Alexey Rednikov, Severine Rossomme Some classical lubrication-type models are examined further and applied to the study of thin liquid film evaporation and condensation on heated or cooled horizontal and flat homogeneous substrates. The focus is on particular solutions in the form of sessile droplets surrounded by an adsorbed (or precursor) film. Using a combination of analytical results and numerical simulations, particular attention is devoted to the apparent contact angles and their evolution when evaporation or condensation takes place. Different forms of the disjoining pressure isotherm are considered, allowing both partial and complete wetting situations to be studied. The influence of an inert component in the gas phase, in addition to vapor, is considered through a slightly modified form of the mathematical expression for the phase change rate. The role of thermocapillary flows in the vicinity of the contact lines receives particular attention, while the vapor recoil is showed to be negligible in general. [Preview Abstract] |
Monday, November 24, 2008 8:52AM - 9:05AM |
GF.00005: Advancing contact line dynamics in the presence of surfactant above the critical micelle concentration David Beacham, Richard Craster, Omar Matar We examine the dynamics of a contact line in the presence of surfactant above the critical micelle concentration. We couple a lubrication model to advection-diffusion equations for surfactant transport allowing for micelle formation and break-up in the bulk and adsorptive fluxes at both the gas-liquid and liquid-solid interfaces. Equations of state are used to model variations in surface tension and wettability. We also account for the structural component of the disjoining pressure within our lubrication model, which is dependent on the concentration of the micelles. We discuss this effect on the spreading rate and on the shape of the interface in the vicinity of the contact line. [Preview Abstract] |
Monday, November 24, 2008 9:05AM - 9:18AM |
GF.00006: Simulation of evaporation of a sessile drop using a diffuse interface model Khellil Sefiane, Hang Ding, Kirti Sahu, Omar Matar We consider here the evaporation dynamics of a Newtonian liquid sessile drop using an improved diffuse interface model. The governing equations for the drop and surrounding vapour are both solved, and separated by the order parameter (i.e. volume fraction), based on the previous work of Ding et al. JCP 2007. The diffuse interface model has been shown to be successful in modelling the moving contact line problems (Jacqmin 2000; Ding and Spelt 2007, 2008). Here, a pinned contact line of the drop is assumed. The evaporative mass flux at the liquid-vapour interface is a function of local temperature constitutively and treated as a source term in the interface evolution equation, i.e. Cahn-Hilliard equation. The model is validated by comparing its predictions with data available in the literature. The evaporative dynamics are illustrated in terms of drop snapshots, and a quantitative comparison with the results using a free surface model are made. [Preview Abstract] |
Monday, November 24, 2008 9:18AM - 9:31AM |
GF.00007: Pinning, retraction and terracing of evaporating droplets containing nanoparticles Omar Matar, Richard Craster, Khellil Sefiane We consider the dynamics of a slender, evaporating droplet containing nanoparticles. We use lubrication theory to derive a coupled system of equations that govern the film thickness and the concentration of nanoparticles. These equations account for capillarity, Marangoni stresses, evaporation and disjoining pressure; the nanoparticles-induced structural component of the disjoining pressure is also considered. Contact line singularities are avoided through the adsorption of ultra-thin films wherein evaporation is suppressed by the disjoining pressure; a similar approach has recently been used by Ajaev, who has built on the previous work of Moosman and Homsy. The results of our numerical simulations indicate that, depending on the value of system parameters, the droplet exhibits a variety of different behaviours, which include spreading, evaporation- driven retraction, contact line pinning, and ``terrace''- formation. [Preview Abstract] |
Monday, November 24, 2008 9:31AM - 9:44AM |
GF.00008: Laminar Flow Deformation of a Droplet Adhering to a Wall in a Channel Gitanjali Seevaratnam, Olivier Michel, Jerry Heng, Hang Ding, Peter Spelt, Omar Matar We study the deformation of a droplet adhering to a solid, rigid wall in a rectangular channel induced by a pressure-driven flow of another fluid. Our experimental investigation involves the systematic study of the effects of initial droplet volume, flow rate, viscosity ratio and substrate wettability on the various modes of droplet motion and deformation. Our results probe the critical conditions at which detachment of the droplet from the surface, sliding, entrainment and deformation occur. Comparisons of our experimental results with numerical predictions will be made. [Preview Abstract] |
Monday, November 24, 2008 9:44AM - 9:57AM |
GF.00009: Experiments of Interfacial Instability on a Ferrofluid Droplet Ching-Yao Chen, Y.-Z. Cheng, W.-K. Tsai, Jose A. Miranda The interfacial morphologies of an extremely thin layer of ferrofluid droplet under a constant perpendicular magnetic field are investigated. Striking patterns consisting of numerous sub-scale droplets that develop from Rosensweig instability are observed. For a dry plate the breaking pattern of sub-scale droplets can be characterized by a dimensionless magnetic Bond number, $Bo_m$. In general, a more pronounced instability, which is evident by a greater number of breaking sub-scale droplets $N$, arises with a higher $Bo_m$. For a magnetic Bond number that is larger than a critical value, the central droplet is torn apart. For a prewetted plate, a nearly flat fluid surface is achieved due to a smaller contact angle, which then leads to virtually evenly distributed sub-scale droplets. A global size for all breaking sub-scale droplets is observed regardless of their initial diameters. On the other hand, when a ferrofluid droplet is immersed in a thin layer of a nonmagnetic fluid, a formation of intriguing interfacial structures is observed, and the development of a hybrid-type ferrohydrodynamic instability is verified, where peak and labyrinthine ferrofluid patterns coexist and share a coupled dynamic evolution. [Preview Abstract] |
Monday, November 24, 2008 9:57AM - 10:10AM |
GF.00010: Initial transients and normal-mode analysis of capillary jets F. Javier Garc\'Ia, Heliodoro Gonz\'alez In many numerical and experimental works on the evolution of axisymmetric capillary jets, it is evident the existence of an initial transient with non-exponential growth of the initially induced perturbation, previous to the classical exponential-growth phase. Contrary to previously thought, it is proven here that a careful normal-mode analysis accounts for those transients for any possible initial conditions. We deduce simple formulae for the transient duration, useful for measuring the growth rate of perturbations accurately. Analytic predictions are contrasted against previous computations of the initial-value problem, experiments and our own numerical simulations with a one-dimensional model. [Preview Abstract] |
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