Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session MJ: Surface Tension Effects II |
Hide Abstracts |
Chair: Carlos Hidrovo, University of Texas at Austin Room: 101I |
Tuesday, November 24, 2009 8:00AM - 8:13AM |
MJ.00001: Friction Reduction in Superhydrophobic Microchannels Tae Jin Kim, Carlos H. Hidrovo Superhydrophobic surfaces are surfaces with fluid contact angles larger than 150$^{\circ}$. Superhydrophobicity can be achieved by means of surface texturing through either a Wenzel or Cassie state. It is widely known, however, that drag reduction is closely related to Cassie state surfaces with low degrees of adhesion and several studies have been widely conducted on the topic. In this research we investigate the effects of surface texturing on superhydrophobic microchannels. Both PDMS and silicon based samples were fabricated and used to experimentally characterize the effects that microtexturing geometry has on the friction reduction behavior. We developed a layered-two-fluid system model to simulate the slip velocity condition and approximate the drag reducing behavior of the microtexturing. A surface energy formulation was also introduced to account for the effects that pressure has on the transition from a Cassie (non-wetting) to Wenzel (wetting) state of the microtexturing. Pressure effects in the wetting of the microtexturing are essential since the flow is driven by a pressure gradient. Experimental results on both friction reduction and pressure induced microtexturing wetting were compared against the models. [Preview Abstract] |
Tuesday, November 24, 2009 8:13AM - 8:26AM |
MJ.00002: Hydraulic Jumps on Superhydrophobic Surfaces Exhibiting Ribs and Cavities Michael Johnson, Benton Russell, Daniel Maynes, Brent Webb We report experimental results characterizing the dynamics of a liquid jet impinging normally on superhydrophobic surfaces spanning the Weber number (based on the jet velocity and diameter) range from 100 to 1400. The superhydrophobic surfaces are fabricated with both silicon and PDMS surfaces and exhibit micro-ribs and cavities coated with a hydrophobic coating. In general, the hydraulic jump exhibits an elliptical shape with the major axis being aligned parallel to the ribs, concomitant with the frictional resistance being smaller in the parallel direction than in the transverse direction. When the water depth downstream of the jump was imposed at a predetermined value, the major and minor axis of the jump increased with decreasing water depth, following classical hydraulic jump behavior. When no water depth was imposed, however, the total projected area of the ellipse exhibited a nearly linear dependence on the jet Weber number, and was nominally invariant with varying hydrophobicity and relative size of the ribs and cavities. For this scenario the Weber number (based on the local radial velocity and water depth prior to the jump) was of order unity at the jump location. The results also reveal that for increasing relative size of the cavities, the ratio of the ellipse axis (major-to-minor) increases. [Preview Abstract] |
Tuesday, November 24, 2009 8:26AM - 8:39AM |
MJ.00003: Sinking of a sphere into viscous liquid Duck-Gyu Lee, Ho-Young Kim A dense solid sphere gently released on an air-liquid interface slowly sinks into the liquid due to gravity while the motion is resisted by liquid viscosity and interfacial tension. We study the sinking velocity of the sphere both theoretically and experimentally. The viscous drag force on the sphere is determined by solving the Stokes equation. To find the retarding force due to interfacial tension, we obtain the meniscus profile by solving the dynamic boundary condition that relates the jump of normal stresses across the air-liquid interface to the surface tension. The predicted sinking velocity, a function of the sphere density and radius, liquid density, viscosity and surface tension, and the dynamic contact angle, is in good agreement with the experimental measurements. The current work expands our knowledge of the sinking process of small spheres which mainly concerned the sinking of completely immersed spheres so far. [Preview Abstract] |
Tuesday, November 24, 2009 8:39AM - 8:52AM |
MJ.00004: Capillary aggregates of floating particles as an attractive granular media Michael Berhanu, Arshad Kudrolli Aggregation of cohesive particles floating in a medium is a very broad physical phenomena occurring in colloidal systems, soot particles, and intergalactic dust under gravitation. We investigate the geometrically constrained dynamics of aggregation with new experiments using floating spheres (3 mm) at the air-liquid interface. A short range attractive force can be induced by the combination of buoyancy and capillarity to create self-assembled particle structures which can be tracked by imaging. First, the particles are placed randomly at the interface, and then aggregation is induced by smoothly decreasing the area of the interface which causes the particles to come within the attractive force range caused by capillarity. We study the aggregation phenomena due to the fusion of initial small clusters in one large cluster. Once it is formed, we are interested by its mechanical properties, and the question of the cluster rigidity is discussed. Finally we study the structural properties of aggregates. Thus we can exhibit significant differences with a classical two dimensional granular media due to attraction between particles. [Preview Abstract] |
Tuesday, November 24, 2009 8:52AM - 9:05AM |
MJ.00005: Forced Spreading and Coalescence of Viscous Drops Shelley Anna, Pilgyu Kang, Shahab Shojaei-Zadeh, Christine Appleby This study investigates the dynamics of spreading and coalescence of sessile droplets on a surface, a process important in applications such as inkjet printing, spray coating, and flooding of fuel cells. We use a simple microfluidic device to control the spreading and merging processes. Droplet shape, diameter and maximum height are monitored as functions of time. We compare the dynamics with existing scaling models modified to incorporate time dependent volume, and we extend these models to describe the scaling behavior of the liquid bridge growing between merging droplets on a surface. The experiments agree well with the expected scaling incorporating capillary, gravity, and viscous forces. [Preview Abstract] |
Tuesday, November 24, 2009 9:05AM - 9:18AM |
MJ.00006: Computational Study of Moving Striple Lines Neeharika Anantharaju, Mahesh Panchagnula, Srikanth Vedantam Wetting of chemically heterogeneous surfaces is modeled using phase field theory. This model studies the one-dimensional kinetic processes involved in wetting a substrate similar to a Wilhelmy technique. The experimental technique provides a complete validation due to its capability to capture the triple line kinetics in addition to measuring the contact angles during the advancing and receding processes. A chemically heterogeneous surface is said to be composed of a predetermined arrangement of two materials. The novelty in the current approach lies in the fact that each of the component materials is constitutively allowed to exhibit hysteresis. We investigate the local shape of the triple line which plays an important role in determining the macroscopic contact angle due to its ability to be pinned at various defect locations on real surfaces. We also demonstrate that the shape of the advancing and receding triple line is sensitive to the specific arrangement of the two materials. [Preview Abstract] |
Tuesday, November 24, 2009 9:18AM - 9:31AM |
MJ.00007: A simplified hydrodynamic study of painting Jungchul Kim, Ho-Young Kim The use of a paintbrush by mankind is known to have started 2000 BC in the Chinese ancient empire, Jin. Despite the long history of painting, attempts to physically and mathematically understand the process of painting seem sparse so far. Here we consider how paint is applied on a solid surface by studying the behavior of a viscous drop sheared between moving plates simulating a canvas and a paintbrush. Dimensional analysis reveals that the behavior is determined by the Capillary number (a ratio of the viscous force to the surface tension force), the receding contact angle and the drop aspect ratio. We experimentally find three distinct drop behaviors, intact dragging, dripping and spreading, and construct a regime map using the foregoing dimensionless parameters. We also give scaling laws to determine the boundaries on the regime map, which agree well with experiment. [Preview Abstract] |
Tuesday, November 24, 2009 9:31AM - 9:44AM |
MJ.00008: Combined Gravitational and Thermocapillary Interactions of Spherical Drops with Incompressible Surfactant Michael Rother Collision efficiencies are calculated by a trajectory analysis for two contaminated spherical drops under the combined influence of buoyancy and a constant temperature gradient at low Reynolds number and with negligible thermal convection in the limit of nearly uniform surfactant coverage. As in the case of clean drops, a region in the parameter space exists where collisions are forbidden when the driving forces are opposed. However, because of the increased effect of thermocapillary repulsion when surfactant is present, coalescence can be inhibited even when the driving forces are aligned in the same direction. In addition to trajectories leading to coalescence and separation of the drops, closed trajectories are also observed. At parameter values where the asymmetric mobility function is zero, retrograde motion can occur, where the angle between vertical and the drops' line of centers decreases as the drops come into contact. This retrograde motion requires alteration to the closed form expression for the collision efficiency. The effect of incompressible surfactant on dilute dispersions of two physical systems is also considered. [Preview Abstract] |
Tuesday, November 24, 2009 9:44AM - 9:57AM |
MJ.00009: Surfactant Effects on Drops Rising in Tubes Yuanyuan Cui, Nivedita Gupta We present our numerical results for the motion of viscous drops rising in cylindrical capillaries at finite Reynolds numbers. A hybrid volume-of-fluid method with a front-tracking scheme is implemented to explore the effects of Bond number, Weber number, and viscosity of the drop phase on the steady shapes and terminal velocities of drops. As the Bond number increases, drops become more deformed. At higher Weber numbers, a reentrant cavity is found at the rear of the drop. Increasing the viscosity of the drop phase reduces the terminal velocity since the resistance of the drop to the ambient fluid increases. We also present the results of adding soluble surfactants in the adsorption-desorption limit in the two-phase system to study the effects of surfactant concentration on the drop motion. The surfactants are modeled using a Langmuir adsorption framework. The non-uniform distribution of surfactants along the fluid interface generates Marangoni stresses that retards the motion of drops and depends on the initial coverage of surfactants. We investigate the effect of mass transfer kinetics on drop deformation and mobility. [Preview Abstract] |
Tuesday, November 24, 2009 9:57AM - 10:10AM |
MJ.00010: Surfactant-induced migration of a drop in Stokes flow James Hanna, Petia Vlahovska In Stokes flows, symmetry considerations dictate that a neutrally-buoyant spherical particle will not migrate laterally with respect to the local flow direction. We show that a loss of symmetry due to flow-induced surfactant redistribution leads to cross-stream drift of a spherical drop in Poiseuille flow. We derive analytical expressions for the migration velocity in the limit of small non-uniformities in the surfactant distribution, corresponding to weak-flow conditions or a high-viscosity drop. The analysis predicts that the direction of migration is always towards the flow centerline. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2023 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700