Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G20: Interfacial/Thin Film Instability III |
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Chair: Omar Matar, Imperial College London Room: 323 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G20.00001: On the Resistance of Nanofibrous Superhydrophobic Coatings to Hydrostatic Pressures T.M. Bucher, B. Emami, H. Vahedi Tafreshi, M. Gad-el-Hak, G.C. Tepper We present a numerical study aimed at investigating the influence of microstructural parameters on the resistance of submerged fibrous superhydrophobic coatings to elevated hydrostatic pressures. In particular, we generate 3-D virtual geometries comprised of randomly or orthogonally oriented horizontal fibers with bimodal diameter distributions resembling the microstructure of coatings produced via DC and AC electrospinning, respectively. These virtual geometries are then used as the computational domain for performing Full Morphology (FM) simulations to establish a relationship between the coatings' critical pressure---pressure beyond which the surface departs from the Cassie state---and their microstructures. Our numerical simulations are aimed at providing guidelines for the design and optimization of the coatings' microstructures. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G20.00002: Comparison of Navier-Stokes solution and lubrication approximation: dewetting of nanoscale liquid rings Shahriar Afkhami, Lou Kondic, Javier A. Diez We present a combined computational and asymptotic approach to study evolution of liquid rings on partially wetting solids. The work is motivated by recent experiments involving the self-assembly of nanolithographically patterned metal rings into ordered nanoparticle arrays via nanosecond pulsed laser heating above the melt threshold (Langmuir 2010, 26, 11972). Three-dimensional numerical simulations of Navies-Stokes equations are compared with a hydrodynamic model based on lubrication approximation for the dynamics and shape of liquid rings. Good agreement is found for contact angles smaller than approximately 35 degrees. The results are encouraging from two viewpoints: a simple hydrodynamic model can capture the main features of the processes under small slope conditions, and offer the possibilities of further developments of computational approaches for modeling fluid dynamics on nanoscale. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G20.00003: Weighted integral boundary layer method for thin films along parallel undulating structures Mohammed Rizwan Sadiq Iqbal A first order weighted integral boundary layer method is derived for a falling film along parallel undulated structures under isothermal conditions. The linear stability of the equilibrium flow, which is derived under asymptotic approach based on small amplitude undulations of the system, is assessed theoretically under certain limiting cases. The complex eigenvalue problem for the linear stability is studied numerically on a periodic domain. For the flow on a planar geometry, the linear stability results of the present model are found to be in good agreement with the experimental data available in literature and predicts the results at the Hopf bifurcation threshold accurately. For an undulating geometry, the amplitude of the undulations play a crucial role in stabilizing the film flow. The evolution triggered by small disturbances to the equilibrium profile is studied numerically by integrating the system using a fast Fourier transform to evaluate the spatial derivatives and a fourth order Runge-Kutta scheme to march in the temporal direction. The maximum film thickness profile computed numerically confirms the stabilizing nature of undulations of the structure as shown by the linear stability results. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G20.00004: Numerical simulations of miscible channel flow with chemical reactions J.N. Kusuma, Kirti Sahu, Omar Matar We investigate the pressure-driven miscible displacement of one fluid by another in a horizontal channel in the presence of an exothermic chemical raction. We solve the continuity, Navier- Stokes, and energy conservation equations coupled to convective- diffusion equations of the reactant and product. The viscosity is assumed to depend on the volume fraction of the reactant and product as well as the temperature. The effects of relavent paramaters such as, Reynolds number, Peclet number, Damk$\ddot{o} $hler number and the viscosity ratio of the reactant and product will be presented at the meeting. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G20.00005: Surface-tension-driven convection in pure liquid layers evaporating into ambient air: influence of liquid volatility Fabien Chauvet, Sam Dehaeck, Pierre Colinet The spontaneous surface-tension-driven convective patterns induced by evaporation of a pure liquid layer are studied experimentally. A volatile liquid layer placed in a cylindrical container is left free to evaporate into air at rest under ambient conditions. The thermal dynamics of the evaporating liquid layer is visualized using an infrared camera. Evaporation rate and liquid thickness are measured by weighting. We focus on the transition between the convective state and the conductive state appearing at a certain instant during the drying of the liquid layer. The critical Marangoni number $Ma_c$ associated to this transition is estimated from evaporation rate and layer thickness measurements at this instant. The effect of the evaporation rate on $Ma_c$ and $k_c$ (the critical wavenumber) has been investigated by changing the container height and, separately, the effect of the liquid volatility has been studied by using different liquids. Interestingly, it appears that $Ma_c$ does not depend on the evaporation rate while it depends strongly on the liquid volatility. Given the typical uncertainties associated with liquid properties, a quite reasonable agreement is found with a ``one-sided'' linear stability analysis of this problem. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G20.00006: Dripping and jetting regimes in co-flowing capillary jets: unforced measurements and response to driving Charles Baroud, Maria-Luisa Cordero, Francois Gallaire We study the breakup of drops in a co-flowing jet, within the confinement of a microfluidic channel. The breakup can occur right after the nozzle (dripping) or through the generation of a liquid jet that breaks up a long distance from the nozzle (jetting). Traditionally, these two regimes have been considered to reflect an absolutely unstable jet or a convectively unstable jet, respectively. We first provide measurements of the frequency of oscillation and breakup of the liquid jet; the dispersion relation thus obtained compares well with existing theories for convective instabilities in the case of the jetting regime. However, the theories in the absolutely unstable mode fail to predict the evolution of the frequency and drop size in the dripping regime. We also test the jet response to an external forcing, using a focused laser to locally heat the jet. The dripping regime is found to be insensitive to the perturbation and the frequency of drop formation remains unaltered. In contrast, the jetting regime locks to the external frequency, which translates into a modification of the drop size in agreement with the dispersion relations. This confirms the convective nature of the jetting regime. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G20.00007: The mechanism for difference in effects of the change in viscosity by chemical reactions on viscous fingering depending on Damk\"ohler number Shohei Iwata, Yuichro Nagatsu, Yusuke Kondo, Kenji Matsuda, Yoshihito Kato, Yutaka Tada In our previous study (Nagatsu et al., \textit{J. Fluid Mech} 2007), we experimentally showed that an increase in the viscosity of the more-viscous liquid by a chemical reaction at a very high Damk\"ohler number, Da, (defined as the ratio between a characteristic time of fluid motion and that of a chemical reaction) made viscous fingering pattern more dense. In present study, we investigated the effects of the increased viscosity caused by chemical reaction at a moderate Da on the viscous fingering in a radial Hele-Shaw cell. We observed that present reaction decreased the area occupied by the fingering pattern around the injection hole. As a result, the fingering pattern became less dense following the reaction. These results indicate that the effects of the increased viscosity by the reactions on viscous fingering pattern are different depending on Da. We discuss the mechanism for the difference based on visualization of reacting flow field by means of pH indicators. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G20.00008: On the evolution of a retracting straight liquid sheet edge: experimental study Rouslan Krechetnikov, Hans C. Mayer The evolution of the initially straight edge of a retracting liquid sheet is still a subject of debate. Theoretical and numerical studies have provided conflicting results, and experimental efforts have, to our knowledge, never been attempted owing to the difficulty in achieving a uniform edge. However, recent advances in experimental techniques, specifically those presented in detail in Poster \#72 of APS-DFD 2010 (H.C. Mayer and R. Krechetnikov), have allowed us to uniformly detach a soap film from a straight edge using an impulsively heated wire frame. The detachment, retraction, and breakup of soap films (h $\sim$ 10 $\mu$m) is analyzed using high speed photography. Owing to the Plateau border that connects the uniform film to the wire frames (wire diameter 25-250 $\mu$m) - a feature not present when rupturing films from a point - the early stages of retraction are dominated by a relatively large rim mass. We explore the phenomena at very early times (t $<$ 100 $\mu$s) associated with the birth of these detached films which may add complexity to the problem of determining what instability mechanism(s) are responsible for their breakup. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G20.00009: Effects of gel properties produced by chemical reactions on viscous fingering Tomohiro Ujiie, Yuichiro Nagatsu, Mitsumasa Ban, Shuichi Iwata, Yoshihito Kato, Yutaka Tada We have experimentally investigated viscous fingering with chemical reaction producing gel. Here, two systems were employed. In one system, sodium polyacrylate (SPA) solution and ferric ion solution were used as the more and less viscous liquids, respectively. In another system, xthantan gum (XG) solution and the ferric ion solution were used as the more and less viscous liquids, respectively. For high concentration of ferric ion, viscous fingering pattern was changed into spiral pattern in the former system, whereas into fracture pattern in the latter system. We consider that the difference in the change of the patterns in the two systems will be caused by the difference in the properties of the gels. Therefore, we have measured the rheological properties of the gels by means of a rheometer. We have found that the gel in the former case is more elastic. Furthermore, we have discussed the relationship between the measured rheological properties and the observed spiral or fracturing patterns. [Preview Abstract] |
Monday, November 21, 2011 9:57AM - 10:10AM |
G20.00010: Suppression of viscous fingering by elastic membranes Draga Pihler-Puzovic, Pierre Illien, Matthias Heil, Anne Juel We report the suppression of interfacial fingering instabilities resulting from the injection of air into the narrow, fluid-filled gap between two parallel plates (a Hele- Shaw cell), when one of the boundaries is replaced by an elastic membrane. In a rigid cell, if the air is injected at a sufficiently fast flow rate, the interface is unstable, ultimately resulting in the development of a complex dendritic fingers. In the elastic- walled cell, interfacial instabilities develop at much larger air-injection rates because membrane deformations reduce destabilising pressure perturbations ahead of the propagating interface, and fundamentally alter the interfacial pattern that develops. We characterise the dependence of the instability on the parameters of the system, and present simple scaling arguments to explain the experimentally observed behaviour. [Preview Abstract] |
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