Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session G10: Microscale Flows: Emulsions and Interfaces |
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Chair: Patrick Tabeling, ESPCI Room: 3005 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G10.00001: On tail formation during gravure printing of sessile drops Umut Ceyhan, S.J.S. Morris Kitsomboonloha et al.(2012) study the deposition of femtolitre drops by the gravure method. The substrate (gravure plate) passes under a stationary blade; liquid placed on the substrate upstream of the blade fills the engraved wells as they enter the blade--substrate gap. Motion of the substrate beneath the blade removes the excess, leaving liquid--filled wells. The resulting pattern can then be printed. As a well leaves the blade, some liquid is, however, subtracted from it and left as a tail between the well and blade. Tails are undesirable because they reduce the sharpness of printed features. It was proposed that tails form by a 3--dimensional mechanism involving lateral wicking of liquid from the wells along the blade--substrate intersection. Here, lubrication theory is used to show that the effect can be understood within the context of plane flow. As a well passes under the trailing edge of the blade, capillary suction causes the meniscus to rise on the blade, but once the well has left, the increased drag exerted by the substrate pulls the meniscus down. Liquid dragged from the meniscus forms the tail. We conclude that tail formation is a problem in plane Stokes flow. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G10.00002: Slip-accelerated falling drop along a vertical fiber Hsien-Hung Wei, David Halpern Effects of wall slip on the motion of a falling drop along a vertical fiber are investigated theoretically. Using lubrication theory, we derive an interfacial evolution equation to describe how the drop's travelling speed and height vary with the Bond number and the slip length. Our numerical results reveal that the drop can travel much faster than the one without slip due to the dramatic increase in the travelling speed with the slip length. The drop height is also found to rapidly increase with the slip length, which is due to enchanced capillary draining from the film into the drop. For Bond number above some critical value, however, capillary draining is suppressed and hence so is the drop height. We determine how the critical Bond number varies with the slip length. For a sufficiently large Bond number, the relevant Kuramoto-Sivashinsky equation is also derived to reveal how the suppression of the capillary instability is mediated by slip effects in the weakly nonlinear regime. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G10.00003: Film flow over an inclined plate: effects of solvent properties and contact angles Rajesh Singh, Janine Galvin The liquid film behavior on the structured packing is a key aspect to the overall efficiency of the column. In this context, the effects of solvent properties and contact angle ($\gamma$) on the hydrodynamics of film flow are systematically investigated. Specifically, multiphase flow simulations for film flow over an inclined plate are carried out using volume of fluid method. A scaling analysis for film thickness and interfacial area was performed. Accordingly, a theory for film thickness and wetted area in terms of Kapitza number (\textit{Ka}) is proposed. The advantage of the \textit{Ka} is that it only depends on fluid properties and independent of flow parameters. Therefore the \textit{Ka} becomes fixed for a given solvent and it decreases with increasing solvent viscosity. The results show that for a fully wetted plate the film thickness ($\delta$) decreases with increasing \textit{Ka} number as $\delta \sim 1/Ka^{1/4}$. For rivulet flow, the interfacial area ($A_{In}$) is found to decrease with increasing \textit{Ka} value. Indeed, scaling analysis shows the relation $A_{In} \sim 1/Ka^{1/2}$. The effect of varying contact angle on the hydrodynamics of rivulet flows was also investigated. The contact angle has no impact on the film thickness for a fully wetted plate but strongly influences the interfacial area for the case of partially wetted plate. For rivulet flow the interfacial area increases with increasing contact angle and is holds the relation $A_{In} \sim 1/(1-\cos \gamma)^{m}$ for a wide range of contact angle. The value of exponent $m$ depends on the \textit{Ka} number and shows two values, one for medium to high surface tension and another for low surface tension value. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G10.00004: Fluid structure in the immediate vicinity of an equilibrium contact line from first principles and assessment of disjoining pressure models Andreas Nold, David N. Sibley, Benjamin D. Goddard, Serafim Kalliadasis Predicting the fluid structure at a three-phase contact line of macroscopic drops is of interest from a fundamental fluid dynamics point of view. However, exact computations for very small scales are prohibitive. As a consequence, coarse-grained quantities such as interface height and disjoining pressure profiles are used to model the interface shape. Here, we evaluate such coarse-grained models within a rigorous and self-consistent framework based on statistical mechanics, in particular with a Density Functional Theory (DFT) approach. We examine the nanoscale behavior of an equilibrium three-phase contact line in the presence of long-ranged intermolecular forces by employing DFT together with fundamental measure theory. Our analysis also enables us to evaluate the predictive quality of effective Hamiltonian models in the vicinity of the contact line. We compare the results for mean field effective Hamiltonians with disjoining pressures defined through the adsorption isotherm for a planar liquid film, and the normal force balance at the contact line [Phys. Fluids, {\bf 26}, 072001, 2014]. Results are given for a variety of contact angles. An accurate description of the small-scale behavior of a three-phase conjunction is a prerequisite to understanding dynamic wetting phenomena. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G10.00005: Influence of surface properties and miscibility upon displacement flow in microchannels Yu Lu, Emilia Nowak, James Percival, Chris Pain, Mark Simmons Microfluidics have potential for a wide range of applications, yet successful operation will depend upon precise understanding of the fluid distribution in multiphase operations, particularly for cleaning of the system. Experiments and numerical studies have been conducted on the displacement process in a single microchannel geometry as a function of fluid dynamics and fluid properties. The microchannel has a near-semicircle cross-section of 205 $\mu$m width and 100 $\mu$m in depth. Miscible and immiscible fluid pairs (water/glycerol and water/silicon oil respectively) with different viscosity ratios and wettability of the channel walls are examined. Micro-Particle Image Velocimetry ($\mu$-PIV) and Planar Laser Induced Fluorescence (PLIF) are used to obtain velocity fields and interface profile respectively. Flow patterns, interfacial instabilities, displacement efficiency and possible secondary flows are examined. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G10.00006: Dynamics of liquid bridges inside microchannels subject to pure oscillatory flows Majid Ahmadlouydarab, Jalel Azaiez, Zhangxin Chen We report on 2D simulations of liquid bridges' dynamics in microchannels of uniform wettability and subject to external oscillatory flows. The flow equations were solved using the Cahn-Hilliard diffuse-interface formulation and the finite element method with unstructured grid. It was found that regardless of the wettability properties of the microchannel walls, there is a critical frequency above which the bridge shows perpetual periodic oscillatory motion. Below that critical frequency, the liquid bridge ruptures when the channel walls are philic and detaches from the surface when they are phobic. This critical frequency depends on the viscosity ratio, oscillation amplitude and geometric aspect ratio of the bridge. It was also found that the flow velocity is out of phase with the footprint/throat lengths and that the latter two show a phase difference. These differences were explained in terms of the motion of the two contact lines on the substrates and the deformation of the fluid-fluid interfaces. To characterize the behavior of the liquid bridge, two quantitative parameters; the liquid bridge-solid interfacial length and the length of the throat of the liquid bridge were used. Variations of the interfacial morphology development of the bridge were analyzed to understand the bridge response. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G10.00007: Confined Selective Withdrawal Alvaro Evangelio, Francisco Campo-Cortes, Jose Manuel Gordillo It is well known that the controlled production of monodisperse simple and composite emulsions possesses uncountable applications in medicine, pharmacy, materials science and industry. Here we present both experiments and slender-body theory regarding the generation of simple emulsions using a configuration that we have called Confined Selective Withdrawal, since it is an improved configuration of the classical Selective Withdrawal. We consider two different situations, namely, the cases when the outer flow Reynolds number is high and low, respectively. Several geometrical configurations and a wide range of viscosity ratios are analyzed so that the physics behind the phenomenon can be fully understood. In addition, we present both experiments and theory regarding the generation of composite emulsions. This phenomenon is only feasible when the outer flow Reynolds number is low enough. In this case, we propose a more complex theory which requires the simultaneous resolution of two interfaces in order to predict the shape of the jet and the sizes of the drops formed. The excellent agreement between our slender-body approximation and the experimental evidence fully validates our theories. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G10.00008: A semi-implicit finite element method for viscous lipid membranes Diego Rodrigues, Roberto Ausas, Fernando Mut, Gustavo Buscaglia We propose a robust simulation method for phospholipid membranes. It is based on a mixed three-field formulation that accounts for tangential fluidity (Boussinesq-Scriven law), bending elasticity (Canham-Helfrich model) and inextensibility. The unknowns are the velocity, vector curvature and surface pressure fields, all of which are interpolated with linear continuous finite elements. The method is semi-implicit, it requires the solution of a single linear system per time step. Conditional time stability is observed, with a time step restriction that scales as the square of the mesh size. Mesh quality and refinement are maintained by adaptively remeshing. Another ingredient is a numerical force that emulates the action of an optical tweezer, allowing for virtual interaction with the membrane. Extensive relaxation experiments are reported. Comparisons to exact shapes reveal the orders of convergence for position (5/3), vector curvature (3/2), surface pressure (1) and bending energy (2). Tweezing experiments are also presented. Convergence to the exact dynamics of a cylindrical tether is confirmed. Further tests illustrate the robustness of the method (six tweezers acting simultaneously) and the significance of viscous effects on membrane's deformation under external forces. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G10.00009: Numerical simulations of interacting surfactant-laden jets in microfluidic channels Garvit Goel, Junfeng Yang, Joao Cabral, Omar Matar We consider the dynamics of jets of surfactant solution in oil under microfluidic confinement. Previous experimental work has demonstrated the occurrence of ``jetting'' and ``dripping'' flow regimes depending on the choice of oil and water flow rates, viscosity ratio, and surfactant concentration. To take into account the influence of soluble surfactant on the behaviour of the jets, we present a computational fluid dynamics (CFD) approach which uses the Volume-of-Fluid method capturing the interface topology accurately with minimal mass loss. This approach accounts for sorption kinetics, Marangoni stresses, diffusion, and surface dilation. This method is incorporated into a CFD code to study the jetting and dripping regimes in a microfluidics channel. The modelling results are validated against experimental measurements. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G10.00010: The fluid dynamics of microjet explosions caused by extremely intense X-ray pulses Claudiu Stan, Hartawan Laksmono, Raymond Sierra, Despina Milathianaki, Jason Koglin, Marc Messerschmidt, Garth Williams, Hasan Demirci, Sabine Botha, Karol Nass, Howard Stone, Ilme Schlichting, Robert Shoeman, Sebastien Boutet Femtosecond X-ray scattering experiments at free-electron laser facilities typically requires liquid jet delivery methods to bring samples to the region of interaction with X-rays. We have imaged optically the damage process in water microjets due to intense hard X-ray pulses at the Linac Coherent Light Source (LCLS), using time-resolved imaging techniques to record movies at rates up to half a billion frames per second. For pulse energies larger than a few percent of the maximum pulse energy available at LCLS, the X-rays deposit energies much larger than the latent heat of vaporization in water, and induce a phase explosion that opens a gap in the jet. The LCLS pulses last a few tens of femtoseconds, but the full evolution of the broken jet is orders of magnitude slower -- typically in the microsecond range -- due to complex fluid dynamics processes triggered by the phase explosion. Although the explosion results in a complex sequence of phenomena, they lead to an approximately self-similar flow of the liquid in the jet. [Preview Abstract] |
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