Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session EG: Drops III |
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Chair: Marc K. Smith, Georgia Institute of Technology Room: 101A |
Sunday, November 23, 2008 4:10PM - 4:23PM |
EG.00001: Model-dependence of Marangoni forces for volatile sessile drops Nebojsa Murisic, Lou Kondic The problem of volatile sessile drops, although apparently simple, involves a complex interplay of several physical mechanisms -- the conduction of heat through the solid and the liquid, the phase transition with associated cooling of the evaporating interface, and the diffusion of the vapor through the surrounding gas phase. In this work we discuss the predictions of two commonly used evaporation models regarding Marangoni forces along the interface of evaporating drops. The material parameters required for careful modeling are extracted from our own experimental data. We find striking differences between the two models -- in particular, the qualitatively different temperature profiles at the evaporating interface. These predicted temperature profiles remain to be measured experimentally. [Preview Abstract] |
Sunday, November 23, 2008 4:23PM - 4:36PM |
EG.00002: Evaporation of femtoliter sessile droplets Thierry Ondarcuhu, Julien Arcmone, Erik Dujardin, Gemma Ruis, Francesc Perez-Murano The evaporation of sessile microdroplets with diameter in the millimeter range has been studied for a long time both theoretically and experimentally. However, experimental data are lacking on evaporation in the micron range despite its importance in the development of micro and nanofluidics. We show here that this problem can be addressed by a combination of two newly developed techniques. We recently demonstrated that droplets in the femto to attoliter range can be deposited in a surface using an atomic force microscope-based method so-called NADIS[1]. Using nanopositioning technique such ``femtodroplets'' could be deposited on a quad-beam resonator (QBR), ultrasensitive mass sensor with a mass resolution more that 1000 times better than quartz microbalance. During evaporation, we monitored temporal evolution of the droplets mass down to 10 fg (10 attoliters volume) resolution. The results obtained on glycerol droplets with initial volumes ranging from 0.2 fL to 20 fL are interpreted in the framework of existing models [2]. \\[0pt] [1] A.Fang, E. Dujardin, T.Ondarcuhu, NanoLett. 6 (2006) 2368. \\[0pt] [2] J. Arcamone et al, J.Phys.Chem.B 111 (2007) 13020. [Preview Abstract] |
Sunday, November 23, 2008 4:36PM - 4:49PM |
EG.00003: Leidenfrost Drop on a Step Guillaume Lagubeau, Marie Le Merrer, Christophe Clanet, David Quere When deposited on a hot plate, a water droplet evaporates quickly. However, a vapor film appears under the drop above a critical temperature, called Leidenfrost temperature, which insulates the drop from its substrate. Linke \& al (2006) reported a spontaneous movement of such a drop, when deposited on a ratchet. We study here the case of a flat substrate decorated with a single micrometric step. The drop is deposited on the lower part of the plate and pushed towards the step at small constant velocity. If the kinetic energy of the drop is sufficient, it can climb up the step. In that case, depending on the substrate temperature, the drop can either be decelerated or accelerated by the step. We try to understand the dynamics of these drops, especially the regime where they accelerate. Taking advantage of this phenomenon, we could then build a multiple-step setup, making it possible for a Leidenfrost drop to climb stairs. [Preview Abstract] |
Sunday, November 23, 2008 4:49PM - 5:02PM |
EG.00004: Vibration-induced Wenzel to Cassie Transition on a Superhydrophobic Surface Jonathan Boreyko, Chuan-Hua Chen A drop on a roughened superhydrophobic surface will exhibit either the Cassie state, where the drop sits on the air-filled textures, or the Wenzel state, where the drop wets the cavities of the textures. The superhydrophobic Cassie state is the preferred state with a small contact angle hysteresis and high drop mobility. Although superhydrophobic surfaces can be designed to make the Cassie state energetically more favorable, drops often end up trapped in a metastable Wenzel state and an energy barrier has to be crossed for reversal to the desired Cassie state. We show that mechanical vibration can be used to cross this energy barrier without resorting to liquid-vapor phase change. A complete Wenzel to Cassie dewetting transition is accomplished by vibrating a metastable Wenzel drop on a superhydrophobic lotus leaf. A water drop in a metastable Wenzel state is obtained by exploiting the differential evaporation rate of water and ethanol. The dewetting transition requires minimum external forcing when the vibration is in resonance with the drop. The vibration-induced Wenzel to Cassie transition is a plausible mechanism used by water-repellant plants to maintain superhydrophobicity, and is applicable to a variety of engineering systems requiring sustained superhydrophobicity. [Preview Abstract] |
Sunday, November 23, 2008 5:02PM - 5:15PM |
EG.00005: Rapid Drop Dynamics During Superhydrophobic Condensation Xiaodong Zhang, Jonathan Boreyko, Chuan-Hua Chen Rapid drop motion is observed on superhydrophobic surfaces during condensation; condensate drops with diameter of order 10~$\mu $m can move at above 100G and 0.1 m/s. When water vapor condenses on a horizontal superhydrophobic surface, condensate drops move in a seemingly random direction. The observed motion is attributed to the energy released through coalescence of neighboring condensate drops. A scaling analysis captured the initial acceleration and terminal velocity. Our work is a step forward in understanding the dynamics of superhydrophobic condensation occurring in both natural water-repellant plants and engineered dropwise condensers. [Preview Abstract] |
Sunday, November 23, 2008 5:15PM - 5:28PM |
EG.00006: The Vibration of an Inviscid Incompressible Sessile Drop Marc K. Smith The fundamental frequencies and modes of vibration of a free spherical drop of inviscid incompressible fluid were computed 129 years ago by Lord Rayleigh. The analysis was possible because of simplifications resulting from the use of spherical coordinates. These same simplifications don't occur for a sessile drop, i.e., when the drop is supported on a horizontal planar surface, except for the case of a hemispherical drop. The present work describes an integrated analytical and numerical technique for the computation of the fundamental frequencies and modes of vibration of a supported sessile drop. Spherical coordinates are used to describe the interface shape, but the flow field inside the drop is computed numerically using the finite element method. Combining these techniques produces a linear eigenvalue problem that is solved numerically. Results will be presented for sessile drops with different contact angles without gravity and compared to experimental data. This technique can also be extended to sessile drops with gravity, in which the drop shape is flattened, and to substrate geometries that are not planar, such as a drop in a shallow cavity or hole. [Preview Abstract] |
Sunday, November 23, 2008 5:28PM - 5:41PM |
EG.00007: Flow in an evaporating sessile drop Hassan Masoud, James D. Felske Exact analytical solutions are derived for the Stokes flows within evaporating sessile drops of cylindrical and spherical cap shapes. The results are valid for arbitrary contact angle. The field equations, $E^4\psi =0$ and $\nabla ^4\psi =0$, are solved for the spherical and cylindrical cases, respectively. For the spherical cap, when the contact angle lies in the range $0\le \theta _c <\pi /2$, the boundary condition defined by the solution to the vapor phase transport must be modified since it becomes non-physical (singular) at the contact line. Solutions are obtained for any physically meaningful distribution of mass flux along the free surface as long the flux approaches a constant value at the contact line. The vapor phase transport has been written to include the mean flow velocity induced by the mass transfer. This leads to advective transport in the vapor phase in contrast to the simple diffusive transport which has been previously treated. The computed solutions demonstrate that the streamlines for viscous flow lie farther from the substrate than the corresponding streamlines for inviscid flow. [Preview Abstract] |
Sunday, November 23, 2008 5:41PM - 5:54PM |
EG.00008: Stability of toroidal drops Alberto Fernandez-Nieves, Ekapop Pairam Using co-flowing liquids we are able to generate drops with the shape of a torus. We will present experimental results on the stability of these unusual drops and on how they evolve into spheres. We find two distinct scenarios depending on the torus aspect ratio, which we quantify using optical imaging. [Preview Abstract] |
Sunday, November 23, 2008 5:54PM - 6:07PM |
EG.00009: Stability of the Taylor--Culick receding rim: surprising observations Henri Lhuissier, Emmanuel Villermaux When punctured, a uniform liquid sheet is known, since Taylor and Culick, to recess at a constant speed balancing surface tension and inertia. For planar soap films, this steady solution holds until the initially smooth receding rim is violently destabilized, exhibiting deep indentations from which droplets are ejected. A surprising new three dimensional mechanism explaining this destabilization and resulting wavelength has been evidenced~: because of the shear between the still outer medium and the receding liquid, the film flaps through a Kelvin--Helmholtz instability, itself inducing an acceleration perpendicular to the film, which intensifies with the flapping amplitude. To this acceleration is associated a classical Rayleigh--Taylor mechanism, promoting the rim indentations. The same mechanism holds for a punctured round bubble, for which the relevant acceleration is the Culick velocity squared divided by the bubble radius. The bearing of this phenomenon on aerosols formation in Nature will be underlined. [Preview Abstract] |
Sunday, November 23, 2008 6:07PM - 6:20PM |
EG.00010: Stability of a stationary moving droplet in porous medium Dmitriy Lyubimov, Sergey Shklyaev, Tatyana Lyubimova, Oleg Zikanov We consider sedimentation of a drop in a porous medium saturated by another fluid. Brinkman model is applied, whereas the interface of drop is assumed sharp and the capillary forces are neglected. The velocity of stationary sedimentation for the spherical drop is obtained. The resulting expression matches both the Hadamard-Rybczynski formula and the result for the conventional Darcy model in corresponding asymptotic limits. Stability of the stationary motion is studied. It is shown, that the drop is always unstable to perturbations that lead to formation of a cusp in the vicinity of the rear stagnation point. This behavior is similar to that for a drop in the absence of the porous matrix. Thus, the Brinkman model eliminates the unphysical features inherent to the Darcy model. [Preview Abstract] |
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