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 H20: Interfacial/Thin Film Instability IV |
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Chair: Jacco Snoeijer, University of Twente Room: 323 |
Monday, November 21, 2011 10:30AM - 10:43AM |
H20.00001: Surfactant spreading on a thin liquid film: testing models with experiments Michael Shearer, Ellen Peterson, Antoine Le Bouil, Karen Daniels We report on recent experiments with NBD-PC lipid spreading on a thin layer of glycerin, in which small concentrations of molecules are visualized using fluorescence. The varying profile of the film surface is simultaneously captured using a laser line. In lubrication models, insoluble surfactant is passively transported by the free surface. This assumption is plausible in the regime of small surfactant concentrations, for which the spreading molecules form a monolayer, but is questionable for larger quantities. We record the time-dependent distribution of surfactant for various initial concentrations down to a monolayer, making direct comparisons between the experimental results and predictions from simulations of the model system of partial differential equations. [Preview Abstract] |
Monday, November 21, 2011 10:43AM - 10:56AM |
H20.00002: The size and shape of gas-focused viscous micro-jets C. Ferrera, A.M. Ganan-Calvo, J.M. Montanero, E.J. Vega, M.A. Herrada The size and shape of gas-focused viscous micro-jets are analyzed theoretically and experimentally. These micro-jets are shaped by the action of a co-flowing gas stream due to both the pressure drop in the axial direction occurring in front of the discharge orifice, and the tangential viscous stress caused by the difference between the velocities of the gas and jet behind the orifice. The slender approximation is used to describing the shape of the tapering meniscus and the emitted liquid ligament. Assuming that the driving force takes a uniform value over the entire liquid domain, a universal (self-similar) solution of the momentum equation can be obtained. Experiments were conducted to assess the validity of that solution for a wide range of liquid viscosities. A remarkable collapse into a single curve is obtained for of all jet diameters measured beyond the orifice. This result shows that the driving force mentioned above attains a rather homogeneous value at the region where the micro-jet develops. The universal solution also provides satisfactory results in front of the orifice for sufficiently slender liquid meniscus, provided that the ratio capillary-to-orifice distance to orifice diameter takes sufficiently small values. The approach used in this work can also be applied to study other microjet generation means (co-flowing, electrospray, electrospinning{\ldots}). [Preview Abstract] |
Monday, November 21, 2011 10:56AM - 11:09AM |
H20.00003: Air entrainment by viscous contact lines Tak Shing Chan, Antonin Marchand, Bruno Andreotti, Jacco H. Snoeijer We study the entrainment of air by moving contact lines, by plunging a solid plate into a reservoir of silicone oil. Above a critical plunging velocity we observe the formation of an air film, typically of 10 microns thickness, which subsequently destabilizes to form bubbles. The breakup of the air film occurs through the nucleation of ``rewetting'' holes, growing regions of liquid reestablishing contact with the surface -- the inverse process of the classical dewetting holes. Typical speeds for air entrainment turn out to be orders of magnitude larger than the speed for liquid film deposition. By varying the viscosity of the silicone oil, it is found that this critical speed depends on properties of both the liquid and the air. We propose an approximate hydrodynamic model that accounts for this two-phase nature of air entrainment. [Preview Abstract] |
Monday, November 21, 2011 11:09AM - 11:22AM |
H20.00004: Jetting-to-bubbling transition in planar coflowing air-water sheets R. Bola\~nos-Jim\'enez, A. Sevilla, C. Guti\'errez-Montes, E. Sanmiguel-Rojas, C. Mart\'Inez-Baz\'an We study the transition between the two regimes experimentally observed when a plane air sheet surrounded by a coflowing water sheet discharges into stagnant air: a bubbling regime, which leads to the periodic break-up of the air sheet, and a jetting regime, where both sheets evolve slowly downstream without breaking. A jetting-to-bubbling transition curve has been experimentally obtained for a fixed liquid-to-gas thickness ratio, $h = h_{w,0}^*/h_{a,0}^*\simeq 5.27$, as a function on the values of two control parameters, namely, the Weber number, $We=\rho_w\,u_{w,0}^{*2}\,h_{a,0}^*/\sigma$, and the velocity ratio, $\Lambda=u_{w,0}^*/\bar u_{a,0}^*$, where $u_{w,0}^*$ and $\bar u_{a,0}^*$ are the water velocity and the mean air velocity at the exit slit, respectively, and $h_{a,0}^*$ and $h_{w,0}^*$ are the half-thicknesses of the air and water sheets at the exit. A linear spatiotemporal stability analysis, contemplating the downstream evolution of the sheets, shows good agreement with the experiments if a sufficiently long region of absolute instability is postulated. [Preview Abstract] |
Monday, November 21, 2011 11:22AM - 11:35AM |
H20.00005: Nonlinear instabilities and Koopman Modes in axisymmetric multiphase shear flows Daniel Duke, Julio Soria, Damon Honnery The shear-driven atomisation of multiphase flows is a process about which relatively little is understood, as it is driven by complex, detailed and short-lived transient instabilites. Novel high-resolution surface velocimetry of axisymmetric annular sheets reveals new detail which has not been previously observed through spatially and temporally resolved velocity profiles. We hypothesise that three simultaneous and separate physical sources of instability may explain the complexity of this flow. A novel application of the Hilbert Transform permits the isolation of these distinct instabilities. The first two are the more well-known nozzle boundary layer and free shear layer driven instabilities. The third is due to nonlinear sheet distortion leading to rupture and may be directly driving the primary atomisation process. This instability has not been previously observed in isolation and is inherently non-linear and non-sinusoidal which has made its eduction challenging. A novel application of Koopman modes through use of the Dynamic Mode Decomposition suggests that traditional theoretical approaches to understanding these flows may need to be re-examined. [Preview Abstract] |
Monday, November 21, 2011 11:35AM - 11:48AM |
H20.00006: Stability of flow focusing: The minimum attainable flow rate J.M. Montanero, N. Rebollo, A. Acero, C. Ferrera, M.A. Herrada, A.M. Ganan-Calvo We analyze both theoretically and experimentally the stability of the steady jetting regime reached when liquid jets are focused by coaxial gas streams. In the low-viscosity case, viscous dissipation in the feeding capillary and liquid meniscus seem to be the origin of the instability. For high-viscosity liquids, the breakdown of the jetting regime takes place when the pressure drop cannot overcome the resistance force offered by surface tension. The characteristic flow rates for which the tapering menisci become unstable do not depend on the pressure drop applied to the system to produce the micro-jet. They increase (decrease) with viscosity for very low (high) viscosity liquids. Experiments confirmed the validity of the above conclusions. For each applied pressure drop, there is a minimum liquid flow rate below which the liquid meniscus drips. The minimum flow rates become practically independent of the applied pressure drop for sufficiently large values of this quantity. There exists an optimum value of the capillary-to-orifice distance for which the minimum flow rate attains a limiting value, which constitutes the lowest flow rate attainable with a given configuration in the steady jetting regime. A two-dimensional stability map with a high degree of validity is plotted on the plane defined by the Reynolds and capillary numbers based on the limiting flow rate. [Preview Abstract] |
Monday, November 21, 2011 11:48AM - 12:01PM |
H20.00007: Ring-waves dominate mass transport in air-driven core-annular flows Jeffrey Olander, Roberto Camassa, Greg Forest, Long Lee, John Mellnik, H. Reed Ogrosky To better understand the movement of mucus through the trachea that arises as a result of air flow, we perform a series of experiments 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. We present experiments and theory that demonstrate that these waves are actually vortex ring waves of annular fluid rolling up the tube between the liquid substrate layer and gas core. A long-wave asymptotic model is developed that exhibits this mass- transport wave behavior. The region of parameter space where such mass transport waves exist is found. In addition, wavelength, wave speed, and mean thickness of the oil lining the tube are found as a function of air speed. A comparison is made between the model and the experiments. These results give insight into the clearing of mucus in the trachea by air flows. [Preview Abstract] |
Monday, November 21, 2011 12:01PM - 12:14PM |
H20.00008: Surface tension gradient-induced gel fracture Constantine Spandagos, Paul Luckham, Omar Matar The spreading of surfactant-laden droplets on the surface of gels can be accompanied by the fracture of the gel surface and the subsequent formation of crack-like patterns. This intriguing phenomenon has been observed on the surface of different types of gels, and for the spreading of both convectional and superspreading surfactants. Understanding the mechanism behind this crack formation is believed to contribute significantly to the control and improvement of a large number of processes that involve systems of spreading liquids and gel-like materials. It is suggested that Maranagoni stresses generated by surface tension gradients between the surfactant and the gel are responsible for the gel surface fracture. The gel seems to ``unzip'' in a direction perpendicular to that of the crack propagation. Furthermore, our systematic study reveals that a crack can be formed only within the experimental conditions that allow $S/\Delta w$ to be greater than $G$', where $S$ is the surface tension difference between the surfactant and the underlying gel, $\Delta w$ is the change of the width of a crack and $G$' is the storage modulus of the substrate. [Preview Abstract] |
Monday, November 21, 2011 12:14PM - 12:27PM |
H20.00009: Investigation of partially dewetting flow at Re$\sim $O(100) and Ca$\sim $O(0.01) Hyoungsoo Kim, Christian Poelma, Jerry Westerweel We study a free surface flow problem with moving contact lines at a Reynolds number Re$\sim $O(100) and a capillary number Ca$\sim $O(0.01). This problem corresponds to the flow in an immersion drop applied in a liquid-immersion lithography machine; so-called a shear-driven confined liquid layer. At a certain critical condition, droplet break-up occurs. We quantitatively investigate this free surface flow problem by means of shadowgraphy measurement and tomographic PIV. For increasing shear rates, we observe four successive regimes at the rear end of the liquid layer. These regimes are identified based on the shape of the elongated tail and denoted as oval, corner, cusp, and pearling. Interestingly, the same qualitative behavior is also observed in a different geometry in the Stokes flow regime (Podgorski et al. 2001). Moreover, additional quantitative similarities are found by evaluating analytical results for sliding drops, based on Stokes flow: the Cox-Voinov model, the three-dimensional lubrication model and a self-similar flow pattern. Surprisingly enough, these results match with the observations even for the present intermediate Reynolds number free surface flow. To understand the underlying mechanism behind this agreement, we have performed a scale analysis based on the momentum equations. Finally, we present the flow structure in this problem is expressed in terms of the results of the lubrication equation. [Preview Abstract] |
Monday, November 21, 2011 12:27PM - 12:40PM |
H20.00010: Oscillatory bubbles induced by geometrical constraint Anne Juel, Mickael Pailha, Andrew Hazel Microscale process engineering requires precise control of bubbles and droplets. We investigate geometry-induced control and find that a centered constriction in the cross section of rectangular tubes can lead to new families of steadily propagating bubbles, which localize in the least-constricted regions of the cross section. Tuning the constriction geometry can cause a switchlike transition from centered to localized bubbles at a critical value of the flow rate: a mechanism for flow-rate-driven bubble control [1]. Striking and robust periodic oscillations develop on the advancing air-fluid interface that can dramatically reduce the volume of fluid extracted. The dynamics of the oscillations are consistent with their arising from a global homoclinic connection between the stable and unstable manifolds of a steady, symmetry-broken solution.\\[4pt] [1] A. de L\'{o}zar, A. Heap, F. Box, A.L. Hazel \& A. Juel {\em Partially-occluded tubes can force switch-like transitions in the behavior of propagating bubbles}, Phys.\ Fluids. {\bf 21}, 101702; doi: 10.1063/1.3247879, 2009. [Preview Abstract] |
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