Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session QY: Instability: Interfacial and Thin Film VI |
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Chair: Mathieu Sellier, University of Canterbury Room: Hyatt Regency Long Beach Regency E |
Tuesday, November 23, 2010 12:50PM - 1:03PM |
QY.00001: Fingering Patterns of Ferrofluid Droplets In a Radial Field Ching-Yao Chen, W.-L. Wu, Y.-S. Yang, Jose Miranda Complex pattern formation abounds in nature and has been actively studied in many different physical, chemical, and biological systems. One major point of interest is to understand the morphology of the rising patterns. In this context, the investigation of growth phenomena in ferrofluids has drawn considerable attention during the last few decades. Due to its unique response to applied magnetic fields, this fluid material has become a prototypical dipolar system for the study of a number of pattern-forming processes and interfacial instabilities. Pattern formation in a ferrofluid system under an in-plane radial magnetic field is experimentally investigated. Visually striking patterns are obtained. For miscible ferrofluids, the morphologies change from circular at a zero field to complex starburst-like structures at a finite field. Less vigorous fingering patterns evolve if the fluids are immiscible. The number of fingers can be tuned by applying a perpendicular field to introduce desired initial perturbation before the switching to in plane field. The evolution of ferrofluid droplets of various initial diameters, subjected to different magnetic field strengths is considered to investigate their influences. [Preview Abstract] |
Tuesday, November 23, 2010 1:03PM - 1:16PM |
QY.00002: On the breakup of nanoscale metalic rings melted via laser pulses Javier Diez, Lou Kondic, Yueying Wu, Jason Fowlkes, Philip Rack We apply a hydrodynamic model based on lubrication approximation with the inclusion of van der Waals forces to study the instability and breakup of nanolithographically patterned copper rings. The initially solid metal is transformed into liquid phase via nanosecond pulsed laser heating above the melt threshold. We show that the resultant average distance between droplets can be correlated to the dewetting flow and instability growths that occur during the liquid lifetime of the melted copper rings. For rings with 13nm height, experimental data give a sudden change in the nanoparticle spacing relation with the ring width. This behavior is attributed to a transition from a Raleigh-Plateau instability to a thin film instability due to the competition between the cumulative transport and instability timescales. To explore the competition between instability mechanisms further, we carried out experiments with rings of height 7 nm. These results were recently published in Langmuir (26(14), 11972, 2010). [Preview Abstract] |
Tuesday, November 23, 2010 1:16PM - 1:29PM |
QY.00003: Models for metallic foam lamellae Michael B. Gratton, Stephen H. Davis We consider a pure liquid film with two liquid-gas interfaces --- a free film --- in two dimensions. Assuming that the aspect ratio of the film thickness to the arc length of the center-line is small, we develop a set of models using lubrication theory for the evolution of the film including the effects of different gas pressures above and below the liquid as well as strong surface tension. These models show a separation of timescales between center-line relaxation, thickness averaging, and drainage due to an applied pressure gradient along the film. Interpreted in the case of surfactant-free foams, these results show that the lamella separating two bubbles in an unstable foam will quickly assume a center-line that is an arc of a circle. Thereafter, the film will become uniform in thickness and drain due to capillary suction from adjoining Plateau borders. [Preview Abstract] |
Tuesday, November 23, 2010 1:29PM - 1:42PM |
QY.00004: A network model for foam dynamics Peter Stewart, Michael Gratton, Michael Davis, Stephen Davis We present a large-scale network model for the dynamics and stability of a planar metallic foam, composed of polygonal gas bubbles separated by thin liquid films. In particular, we track the positions of the bubble vertices, where most of the liquid volume is concentrated, and incorporate a direct coupling between the pressure and volume of the bubbles, surface-tension forces on the gas-liquid interfaces and draining and elongational flows in the films. We invoke a van-der-Waals instability criterion due to Anderson, Brush and Davis [to appear in {\it J. Fluid Mech.}] and present numerical simulations of the resulting topological re-arrangements within the foam. [Preview Abstract] |
Tuesday, November 23, 2010 1:42PM - 1:55PM |
QY.00005: Electrically Driven Motion of Thin Films of Dielectric Liquids Pilnam Kim, Camille Duprat, Scott S.H. Tsai, Howard A. Stone In electrohydrodynamic (EHD) pumping, fluid forces are generated by the interaction of electric fields with the charges they induce in the fluid. Here, we investigate the effect of a tangential electric field on the motion of a thin film of a dielectric liquid in a wedge-shape geometry. We first present an experiment study. We find that the fluid is driven to high electrical potential regions due to the tangential field. In addition, the liquid interface undergoes an instability in the form of a conical shape (a Taylor cone) induced by a normal electric field to the interface. We propose a thin film model using lubrication theory to describe the film thickness-velocity relationship and characterize the jetting processes that accompany the interface instability. [Preview Abstract] |
Tuesday, November 23, 2010 1:55PM - 2:08PM |
QY.00006: Viscous fingering of a miscible reactive A+B$\to $C interface with an infinite Damk\"ohler number: Nonlinear simulations Yuichiro Nagatsu, Anne De Wit Nonlinear simulations of miscible viscous fingering are performed for a reactive system where a simple infinitely fast A+B$\to $C chemical reaction takes place when a solution containing the reactant A is displacing another miscible solution containing the reactant B. The viscosity of the fluid depends on the concentration of the chemicals B and C. The various nonlinear fingering dynamics are analyzed numerically for an infinite Damk\"ohler number $D_{a}$ as a function of the log-mobility ratios $R_{b}$ and $R_{c}$ quantifying the viscosity ratios of the solutions of B and C versus that of the solution of A respectively. If $R_{b}>$0, i.e. if the system is genuinely viscously unstable because the displaced solution of B is more viscous than the displacing solution of A, we analyze the changes to classical non-reactive viscous fingering induced by the reaction. [Preview Abstract] |
Tuesday, November 23, 2010 2:08PM - 2:21PM |
QY.00007: Emulation of Mucus Propulsion in the Trachea Driven by Constant Air Flow Reed Ogrosky, Roberto Camassa, Michael Jenkinson, Jeffrey Olander, Shreyas Tikare To better understand the movement of mucus through the trachea that arises as a result of air flow, we design an experiment to emulate mucus movement by an air-driven vertical flow of high-viscosity silicone oil through a thin glass tube. When a constant flux of air is delivered through the bottom of the tube, instabilities arise, generating upward moving waves at the oil/air interface. These constitute a main mechanism of momentum transfer from air to oil, whereby oil is transported upward against gravity. We test this mechanism with several different flow rates of both air and oil. Specifically, increasing the air speed results in shorter wavelengths, lower wave speed, a smaller mean thickness of oil lining the tube, and smaller displacements by arriving waves at the wetting front when oil is advancing in a dry tube. In particular, we quantify the role of waves in advancing this front, and show how waves play a dominant role in this advancement. These results give insight into the clearing of mucus in the trachea by air flows. [Preview Abstract] |
Tuesday, November 23, 2010 2:21PM - 2:34PM |
QY.00008: Moving contact lines in a vapor-liquid system: a singularity-free description in the sole framework of classical physics Alexey Rednikov, Pierre Colinet When one is lead to think about a theoretical treatment of moving contact lines in the sole framework of classical physics, the first associations coming to mind are most probably those of singularities intractable unless ``regularizing'' effects, beyond the classical approach, are taken into account, such as the disjoining pressure or a slip at the wall. Here we show that, contrary to such preconceptions, no contact-line singularities arise, even in the absence of these regularizing effects, in a system consisting of a liquid, its pure vapor and a superheated substrate (of interest, in particular, in boiling applications). Furthermore, no thermal singularities typically associated with this system are encountered either, even in the absence of the thermal regularizing effects such as a finite rate of the evaporation kinetics or a finite heat conductivity of the substrate. We consider, in the framework of the lubrication theory and a classical one-sided model, a contact line moving at a constant velocity (advancing or receding) and starting abruptly at a (formally) bare solid surface, the micro- contact angle being either equal to zero or finite. [Preview Abstract] |
Tuesday, November 23, 2010 2:34PM - 2:47PM |
QY.00009: Steady contact lines in a vapor-liquid system: truncated versus extended adsorbed microfilms Pierre Colinet, Alexey Rednikov The classical microscale theory of evaporating contact lines is revisited (for the case of a one-component liquid and its pure vapor) in the framework of a continuum lubrication-type model allowing the prediction of the apparent contact angle and of the microscale evaporation flux. The analysis is restricted to perfectly flat and homogeneous substrates maintained at constant temperature. While the classical theory, used for perfectly wetting situations, assumes the existence of an adsorbed microfilm extending all over the apparently dry portions of the superheated substrate, we here show that such regime may actually become metastable against a new regime with a truncated microfilm, ending up at a bare surface. Consideration of this new regime requires introducing the spreading coefficient into the picture, hence in some sense unifying two apparently unrelated ways of modeling contact line microstructures. In particular, the analysis also applies to partial wetting situations, and shows that the apparent contact angle only weakly deviates from Young's law in that case. Finally, while most of this theory is based on the usual inverse cubic law for the disjoining pressure, as in the classical case, slightly more general (non-singular) forms are also considered. [Preview Abstract] |
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