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 A14: Drops: General I |
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Chair: John Bush, Massachusetts Institute of Technology Room: 3009/3011 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A14.00001: High Reynolds number droplet de-pinning on textured surfaces: theory and experiments Sungyon Lee, Benjamin Wilcox, Feng Xu, Edward White The stability of drops on surfaces subject to forcing by wind and gravity is relevant to heat exchangers, fuel cells, and aircraft icing, and it lacks understanding in a high Reynolds number regime. To experimentally investigate this phenomenon, water drops are placed on the rough aluminum floor of a tiltable wind tunnel and brought to critical conditions for varying drop sizes, inclination angles, and flow speeds. In particular, the evolving 3D droplet shapes under flow are reconstructed based on a laser-speckle interface measurement tool, while the critical flow rates of droplet depinning are also noted. By accounting for the contact angle hysteresis and the pressure build-up in a nearly turbulent boundary layer, the critical depinning flow rate is theoretically predicted and is compared to the experimental results. We also observe and explain the transition of the drop depinning behavior from inertia-dominated to gravity-dominated regimes at non-zero inclination angles. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A14.00002: Directional motion of liquid under mechanical vibrations Maxime Costalonga, Philippe Brunet, Hassan Peerhossaini When a liquid is submitted to mechanical vibrations, steady flows or motion can be generated by non-linear effects. One example is the steady \emph{acoustic streaming} one can observe when an acoustic wave propagates in a fluid. At the scale of a droplet, steady motion of the whole amount of liquid can arise from zero-mean periodic forcing. As It has been observed by Brunet \emph{et al.} (PRL 2007), a drop can climb an inclined surface when submitted to vertical vibrations above a threshold in acceleration. Later, Noblin \emph{et al.} (PRL 2009) showed the velocity and the direction of motion of a sessile drop submitted to both horizontal and vertical vibrations can be tuned by the phase shift between these two excitations. Here we present an experimental study of the mean motion of a sessile drop under slanted vibrations, focusing on the effects of drop properties, as well as the inclination angle of the axis of vibrations. It is shown that the volume and viscosity strongly affect the drop mean velocity, and can even change the direction of its motion. In the case of a low viscous drop, gravity can become significant and be modulated by the inclination of the axis of vibrations. Contact line dynamic during the drop oscillations is also investigated. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A14.00003: A capillary Archimedes' screw Baptiste Darbois Texier, Stephane Dorbolo As used by Egyptians for irrigation and reported by Archimedes, a screw turning inside a hollow pipe can pull out a fluid againt gravity. At a centimetric scale, an analagous system can be found with a drop pending on a rotating spiral which is tilted toward the horizontal. The ascent of the drop to the top of the spiral is considered and a theoretical model based on geometrical considerations is proposed. The climb of the drop is limited by the fluid deposition on the screw at high capillary number and by a centrifugation phenomenon. We find out the range of fluid proprities and spiral characteristics for which an ascending motion of the drop is possible. Finally we discuss the efficiency of such system to extract a fluid from a bath at a centrimetric scale. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A14.00004: ABSTRACT WITHDRAWN |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A14.00005: Simulation of flows with moving contact lines on curved substrates by immersed boundary methods Hang Ding, Hao-Ran Liu We propose an approach to simulate flows with moving contact lines (MCLs) on curved substrates. The approach combines an immersed boundary method with a three-component diffuse-interface model and a characteristic MCL model. The immersed boundary method circumvents the penetration of the gas and the liquid into the solid by convection while the three-component diffuse-interface model can prevent the diffusive fluxes of the gas and liquid from infiltrating into the solid substrate. The characteristic MCL model not only allows for the motion of contact lines, but makes the gas-liquid interface to intersect the solid object at an angle in consistence with the prescribed contact angle, even with tangent variation at the solid surface. We examine the performance of the approach through a variety of numerical experiments: mass conservation and interface shapes at equilibrium were tested through the simulation of drop spreading on a circular cylinder, while the dynamic behavior of MCLs on the curved boundaries was investigated by simulating water entry of and drop impact on a sphere, respectively. At last, we studied the penetration process of a drop into a cluster of circular cylinders. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A14.00006: Drop motion due to oscillations of an inclined substrate Yi Xia, Chun-Ti Chang, Susan Daniel, Paul Steen A sessile drop on a stationary inclined substrate remains pinned unless the angle of inclination is greater than some critical value. Alternatively, when shaken at even small angles of inclination, the drop undergoes shape deflections which may lead to drop translation. Translation occurs when large contact angle fluctuations, favored by oscillations at resonance, overcome contact angle hysteresis. In this study, resonance is triggered by substrate-normal oscillations. The drop translation is typically observed to be of constant speed for a given set of parameters. The speed is measured experimentally as a function of resonance mode, driving amplitude and drop volume. This technique of activating the motion of drops having a particular volume can be utilized for applications of droplet selection and transport. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A14.00007: Why a falling drop does not in general behave like a rising bubble Rama Govindarajan, Manoj Tripathi, Kirti Sahu Is a settling drop equivalent to a rising bubble? The answer is known to be in general a no, but we show that when the density of the drop is less than 1.2 times that of the surrounding fluid, an equivalent bubble can be designed for small inertia and large surface tension. Hadamard's exact solution is shown to be better for this than making the Boussinesq approximation. Scaling relationships and numerical simulations show a bubble-drop equivalence for moderate inertia and surface tension, so long as the density ratio of the drop to its surroundings is close to unity. When this ratio is far from unity, the drop and the bubble are very different. We show that this is due to the tendency for vorticity to be concentrated in the lighter fluid, i.e. within the bubble but outside the drop. As the Galilei and Bond numbers are increased, a bubble displays underdamped shape oscillations, whereas beyond critical values of these numbers, over-damped behaviour resulting in break-up takes place. The different circulation patterns result in thin and cup-like drops but bubbles thick at their base. These shapes are then prone to break-up at the sides and centre, respectively. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A14.00008: Self-crumpling elastomers: bending motion induced by a drying stimulus Fran\c{c}ois Boulogne, Howard A. Stone Capillary forces exerted by a liquid drop can bend elastic slender structures such as fibers or sheets. However, to successfully achieve capillary origami with sheets, it is important to make sure that the adhesion of the elastomer with the surface is low. We report an experimental study of the drying-induced peeling of a bilayer consisting of an elastomeric disk coated with a suspension of nanoparticles. We show that where capillary forces associated with the scale of the droplet can not compete with the adhesion of the elastomer on a surface, nevertheless large tensile stress developed in the coating, which resulted in a moment bending the bilayer. We attribute this stress to the nano-menisci in the pores of the colloidal material and we propose a model that describes successfully the early stage curvature of the bilayer. Thus, we show that the peeling can be conveniently controlled by the particle size and the coating thickness. We believe that such systems can be employed in various situations where delicate surfaces are involved such as in applications with optical and electronic components or in restoration of photographies, painting, wallpaper, fragile collectibles from contamination by dust, pollen, dirt, etc. [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A14.00009: A low-cost, precise piezoelectric droplet generator Tanya Liu, Daniel M. Harris, John W.M. Bush We present the design for a piezoelectric drop-on-demand generator capable of producing highly repeatable, millimetric droplets. The generator is low-cost, simple to fabricate, and easily reproduced. We demonstrate that droplet diameter can be controlled through variation of the piezoelectric driving waveform parameters. Our experiments demonstrate that if waveform amplitude is fixed, droplet diameter is directly dependent on waveform pulse width, allowing for a range of droplet sizes to be produced for a fixed nozzle diameter. Successful droplet generation occurs only within a finite range of pulse widths; however, outside of this range satellite droplets form or no droplet generation occurs. We also discuss the dependence of droplet size on other system parameters including pressure at the nozzle plane and nozzle diameter. These results make the generator design potentially applicable to a wide range of fluids experiments where repeatable millimetric droplets are required. [Preview Abstract] |
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