Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G8: Drops V |
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Chair: Paul Steen, Cornell University Room: 25A |
Monday, November 19, 2012 8:00AM - 8:13AM |
G8.00001: Surface Patterns of Parametrically-Excited Sessile Drops Chun-Ti Chang, Joshua Bostwick, Susan Daniel, Paul Steen A mechanically-excited sessile water drop exhibits surface patterns that vary with driving amplitude and frequency. At small amplitudes, drops exhibit axisymmetric wave patterns. At sufficiently large amplitudes and at particular frequencies, symmetry breaks and there appear subharmonically-resonating sectoral and tesseral mode shapes. We report observations from experiment and compare to results from modeling the parametric excitation of the drops. The modeling uses the spectrum and eigenmodes from a linear stability analysis of the inviscid sessile Rayleigh drop with moving/pinned contact-line. Manipulating drop motion can be important to a variety of applications. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G8.00002: Sessile Rayleigh drop instability Paul Steen, Josh Bostwick Rayleigh (1879) determined the mode shapes and frequencies of the inviscid motion of a free drop held by surface tension. We study the inviscid motions of a sessile Rayleigh drop -- a drop which rests on a planar solid and whose contact-line is free to move. Linear stability analysis gives the modes and frequencies of the droplet motions. In this talk, we focus on the ``walking instability,'' an unstable mode wherein the drop moves across a planar substrate in an inviscid rocking-like motion. The mode shape is non-axisymmetric. Although the experimental literature has hinted at such a mode, this is the first prediction from linear stability analysis, as far as we are aware. The ``walking instability'' of the drop converts energy stored in the liquid shape into the energy of liquid motion -- which represents a heretofore unknown pathway of energy conversion of potentially wide significance for a broad range of applications. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G8.00003: Motion and coalescence of sessile drops driven by substrate wetting gradient and external flow Majid Ahmadlouydarab, James J. Feng We report finite-element simulations of the motion and coalescence of sessile drops driven by solid substrate wetting gradient and external flow. When the external flow and wetting gradient favor motion in opposite directions, their competition determines the behavior of a single sessile drop. If two drops are placed on a solid substrate with wetting gradient, the trailing drop may catch up with the leading one and coalesce with it. This is owing to the greater viscous friction on the leading drop, which is more spread out on a more hydrophobic area of the substrate. An external flow alone can induce coalescence, thanks to differential drags on the two drops. When both wetting gradient and flow effects coexist and compete, more complex scenarios arise, with either coalescence or separation depending on the strength of each. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G8.00004: Measuring the force of drag on air sheared sessile drops Andrew J.B. Milne, Brian Fleck, Alidad Amirfazli To blow a drop along or off of a surface (i.e. to shed the drop), the drag force on the drop (based on flow conditions, drop shape, and fluid properties) must overcome the adhesion force between the drop and the surface (based on surface tension, drop shape, and contact angle). While the shedding of sessile drops by shear flow has been studied [Milne, A. J. B. {\&} Amirfazli, A. \textit{Langmuir} 25, 14155 (2009).], no independent measurements of the drag or adhesion forces have been made. Likewise, analytic predictions are limited to hemispherical drops and low air velocities. We present, therefore, measurements of the drag force on sessile drops at air velocities up to the point of incipient motion. Measurements were made using a modified floating element shear sensor in a laminar low speed wind tunnel to record drag force over the surface with the drop absent, and over the combined system of the surface and drop partially immersed in the boundary layer. Surfaces of different wettabilities were used to study the effects of drop shape and contact angles, with drop volume ranged between approximately 10 and 100 microlitres. The drag force for incipient motion (which by definition equals the maximum of the adhesion force) is compared to simplified models for drop adhesion such as that of Furmidge [Furmidge, C. G. L., \textit{J. Colloid Sci.} 17, 309 (1962).] [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G8.00005: Drop Stability on Leaves Will Blackmore, Yongkang Chen, Chris Hinojosa, Ryan Jenson, Duc Nguyen, Andrew Wollman, Mark Weislogel The stability of drops on surfaces has implications to many natural and industrial processes. Critical configurational drop stability is defined herein as any change in a control parameter (i.e. drop volume) that leads to movement to a new location on the surface and/or detachment from the surface. The configurational stability of `wall-bound drops'' is enhanced by contact line pinning at sharp edges. In this work an extensive array of computations are performed for ``wall-edge-vertex bound drops'' (a.k.a. drops on blade tips or drops on leaf tips which they resemble). The numerical approach applies the Surface Evolver algorithm through implementation of a new file layer and a multi-parameter sweep function. As a consequence, thousands of critical drop configurations are efficiently computed as functions of contact angle, blade edge vertex half-angle, and g-orientation. Simple experiments are performed to benchmark the computations which are then correlated for ease of application. It is shown that sessile, pendent, and wall-edge bound drops are only limiting cases of the more generalized blade-bound drops, and that the ubiquitous ``dry leaf tip'' is observed for a range of the critical geometric and wetting parameters. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G8.00006: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 9:18AM - 9:31AM |
G8.00007: The shape of a drop between two rigid fibers Suzie Protiere, Camille Duprat, Howard A. Stone Wetting of fibrous media is observed in many engineered systems, e.g. filters, textiles, paper etc. and may also be found in Nature (e.g. hair or feathers). To understand the basic response of such material when interacting with a liquid we study the model system of a finite volume of liquid on two parallel rigid fibers. A liquid wetting the fibers can adopt two distinct equilibrium shapes: a compact hemispherical drop shape or a long liquid column of constant cross-section. These two morphologies depend on the inter-fiber distance, the liquid volume, the fiber radius and the liquid-fiber contact angle. We study the transitions between a drop shape and a column by incrementally varying the inter-fiber distance and find that the transition depends on the global geometry of the system as well as on the volume of liquid. More surprisingly we find that these two morphological states may coexist for certain parameter values. These switches in morphologies may be used to manipulate or transport liquid at a small scale. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G8.00008: A closed-form analytical solution for both hanging and sitting droplets Juan Manuel Gomba, Carlos Alberto Perazzo We present a new analytical solution for the shape of both hanging and sitting droplets under the effects of gravity, surface tension and molecular forces arising between the liquid and the substrate. These molecular London/van der Waals and electrostatics forces, which are described by means of potential functions, are responsible for the existence of a nanometric precursor film that surrounds the droplet. The analytical solution describes pancake-type profiles and also droplets elongated in the vertical direction. We find novel expressions that relate microscopic and nanoscopic aspects, such as the strength of the molecular forces and the thickness of the nanometric precursor film, with macroscopic quantities, e. g., the cross sectional area, the height and the width of the droplet. Note: No pancakes will be served. [Preview Abstract] |
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