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 G14: Drops: Bouncing, Impact and Dynamic Surface Interactions II |
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Chair: Yongsheng Lian, University of Louisville Room: 3009/3011 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G14.00001: Resonant and antiresonant bouncing droplets Maxime Hubert, Damien Robert, Herv\'e Caps, St\'ephane Dorbolo, Nicolas Vandewalle Droplets may bounce on a oscillating liquid surface. Because of the regeneration of the air layer between the drop and the surface, the bouncing droplets may never coalesce with the liquid underneath. We propose here a simple model for millimetric droplets of low viscosity, bouncing on a highly viscous bath. This model consists of two masses, linked together by a spring and a damper, bouncing onto a rigid and oscillating surface. We use this model to understand the shape of the bouncing threshold, the minimal bath acceleration required to sustain the bouncing dynamics. We show that the droplet is submitted to resonant and anti-resonant behaviors. We also show that those two phenomena are size-dependant and do not occur at the same frequencies for droplets of different radii. By means of resonance and antiresonance, we propose a new microfluidic technique to control precisely the droplets size, which only relies on the surface forcing parameters. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G14.00002: Deceleration of free aqueous droplets skirting across the surface of a pool of the same fluid Jacob Hale, Caleb Akers The non-coalescence of a free droplet atop a pool of the same fluid can be greatly enhanced when the drop has an initial horizontal velocity relative to the pool surface. The glancing impact and viscous interaction between the droplet and the pool impart a significant rotation to the droplet causing it to roll and thus entraining air between the two fluids. The translational speed of such a droplet is shown to decrease exponentially in time but with a time constant that increases linearly in time. This complex deceleration of the drop is in part due to the drop's rotational deceleration, visualized with suspended, neutrally buoyant microbeads. The observed motion is described in terms of viscous dissipation of the rotating drop and a viscous shear force between the droplet and bath. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G14.00003: Dynamics of bouncing droplets in annular cavities Zachary Louis Lentz, Mir Abbas Jalali, Mohammad-Reza Alam In a cylindrical bath of silicon oil, vertically excited by a frequency of 45 Hz, we trace the motion of bouncing droplets as they fill an annular region. We compute the mean tangential and radial velocity components of the droplets and show that the maximum tangential velocity is larger than the maximum radial velocity by one order of magnitude. Velocity dispersions have almost equal levels in the radial and tangential directions, and their mean values are 1/4 times smaller than the mean tangential velocity. These results show that bouncing droplets undergo random motions within annular cavities determined by the interference patterns of self-induced circumferential waves. We derive analytical relations between the velocity dispersion and the wavelength of surface waves, and calculate the mean tangential velocity of droplets using the random kicks that they experience at the boundaries of the cavity by inward and outward traveling waves. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G14.00004: Dynamic trapping of sliding drops on wetting defects Andrea Cavalli, Michiel Musterd, Dieter 't Mannetje, Dirk van den Ende, Frieder Mugele We present a numerical analysis of the dynamic interaction of a sessile sliding drop with a wetting defect. Our three-dimensional model, developed with OpenFOAM, allows us to describe inertial and viscous effects, as well as the internal degrees of freedom of the droplet. We observe that the ability of a drop to deform and stretch enhances the strength and range of the wetting defect, in comparison to a simplified analytic description. We further investigate the role of the strength, size and steepness of the defect in retaining the drop. Finally, we compare our simulations with trapping experiments on electrowetting obstacles, observing a quantitative agreement. This shows that the trapping of sliding drops follows a universal behavior, which is not significantly affected by the nature of the defect. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G14.00005: Coalescence-induced jumping motion on non-wetting surfaces: The mechanism of low energy conversion efficiency Fangjie Liu, Giovanni Ghigliotti, James J. Feng, Chuan-Hua Chen When two drops coalesce on a non-wetting substrate such as a superhydrophobic surface, the merged drop spontaneously jumps away from the surface. The self-propelled jumping is powered by the surface energy released upon drop coalescence. However, the translational kinetic energy associated with the jumping is much smaller than the released surface energy. The mechanism of this low energy conversion efficiency is elucidated with 3D phase field simulations which have been experimentally validated. The coalescing drops can be viewed as a two-lobed perturbation to the merged drop with a larger equilibrium radius. The large-amplitude perturbation induces the capillary-inertial oscillation of the merged drop, and the symmetry of the oscillation is broken by the non-wetting substrate. Since the substrate intercepts only a small fraction of the merged drop, a small translational momentum is imparted by the symmetry-breaking substrate, giving rise to the low jumping velocity of 0.2 when nondimensionalized by the capillary-inertial velocity and consequently a low energy conversion efficiency of less than 4\%. Other than this small fraction of translational kinetic energy, the majority of the kinetic energy is oscillatory and eventually dissipated. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G14.00006: Wave-driven Frictionless Stick-Slip of a self-propelled drop Vincent Bacot, Matthieu Labousse, St\'ephane Perrard, Yves Couder, Emmanuel Fort Mechanical parts often move not smoothly but in jerks. This well-known phenomenon, called Stick-Slip, is usually associated with friction, since sudden variations of the friction between solid objects, associated with microscopic shear storage, account for the stop and go motion. We report the observation of a similar phenomenon, not controlled by friction, but by propulsion, through the storage of propulsion sources in a surface wave. Our system is made of a liquid droplet, bouncing on a vibrated bath of the same liquid. The vibration of the bath both inhibits the drop coalescence and sustains standing waves on the bath surface around each bouncing spot, for a certain amount of time. These standing waves act as propulsion sources for the drop which thus moves in the horizontal plane. We show how the dynamic interferences of the standing waves created along the way lead to transitory jerks upon starting the motion of the drop. We show how the inhomogeneous dynamic storage of propulsion sources can lead to a steady stop and go motion. In this wave piloted stick-slip motion, the roles of the propulsion and friction are reversed and the spatio-temporal periodicity is tunable. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G14.00007: Air mediated dynamics of droplet impact on a smooth, solid surface John Kolinski, L. Mahadevan, Shmuel Rubinstein Before a falling drop can contact a solid surface, it must displace the air beneath it. Recent calculations and experiments show that as the drop approaches the surface, the air fails to drain, and instead compresses. As the air compresses, the pressure in the gas layer deforms the surface of the drop, thus inhibiting liquid-solid contact. Ultimately, the liquid droplet skates over a nanometer-thin film of air at a strikingly high velocity. These dynamics take place at fleeting timescales and diminutive length-scales, and are obscured by the bulk of the drop, making experimental observation difficult. We directly image the dynamics of the liquid-air interface, and use a novel form of TIR microscopy to study the dynamics and stability of the thin film of air beneath the drop. We show that the stability of the air film governs a novel transition in droplet impact events. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G14.00008: Lift-Off Instability During the Impact of a Drop on a Solid Surface Shmuel Rubinstein, John Kolinski, L. Mahadevan We directly measure the rapid spreading dynamics succeeding the impact of a droplet of fluid on a solid, dry surface. Upon impact, the air separating the liquid from the solid surface fails to drain and wetting is delayed as the liquid rapidly spreads outwards over a nanometer thin film of air. We show that the approach of the spreading liquid front toward the surface is unstable and the spreading front lifts off away from the surface. Lift-off ensues well before the liquid contacts the surface, in contrast with prevailing paradigm where lift-off of the liquid is contingent on solid-liquid contact and the formation of a viscous boundary layer. Here I will discuss the dynamics of liquid spreading over a thin film of air and its lift-off away from the surface over a large range of fluid viscosities. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G14.00009: Droplet impact patterns on inclined surfaces with variable properties Michael Lockard, G. Paul Neitzel, Marc K. Smith Bloodstain pattern analysis is used in the investigation of a crime scene to infer the impact velocity and size of an impacting droplet and, from these, the droplet's point and cause of origin. The final pattern is the result of complex fluid mechanical processes involved in the impact and spreading of a blood drop on a surface coupled with the wetting properties of the surface itself. Experiments have been designed to study these processes and the resulting patterns for the case of a single Newtonian water droplet impacting a planar, inclined surface with variable roughness and wetting properties. Results for Reynolds numbers in the range of (9,000 -- 27,000) and Weber numbers in the range of (300 -- 2,600) will be presented. Transient video images and final impact patterns will be analyzed and compared with results from traditional bloodstain pattern-analysis techniques used by the forensics community. In addition, preliminary work with a new Newtonian blood simulant designed to match the viscosity and surface tension of blood will be presented. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G14.00010: Shaping/Launching Droplets Impacting on Wettability-Patterned Surfaces M. Elsharkawy, T. Schutzius, G. Graeber, J. Oreluk, R. Ganguly, C. Megaridis We present experimental results of droplet impact on wettability-patterned surfaces specifically designed to perform various liquid handling tasks. Such surfaces are implemented for converting droplets from spheres to complex shapes ($e.g.,$ annuli or squares) and laterally launching the droplets even under orthogonal impact. The procedure harnesses the naturally occurring contact line pinning mechanisms at sharp wettability changes to influence droplet impact outcome, or even mobilize the fluid asymmetrically post impact. In the launching scenario, droplets impact orthogonally on a superhydrophobic surface and come in contact with a patterned hydrophilic region upon maximum spread. The end result is the launch of the droplet in the lateral direction due to contact line pinning on the hydrophilic region, and the resultant asymmetric disruption of the receding droplet dynamics. We analyze this phenomenon and explain the underlying physical conditions driving the lateral launch post impact. For such patterned surfaces, we show that there exist three possible regimes of dewetting, which depend on total contact time and location of droplet launch from the point of first contact. A model is put forth that predicts the horizontal launch velocity and relates the forces at play to discern between the three dewetting regimes. The study presents a new approach to control impacting droplet outcome and offers new insight on droplet impact behavior on wettability patterned surfaces. [Preview Abstract] |
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