Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B25: Drops: Sessile and Static Surface Interactions |
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Chair: Edward White, Texas A&M University Room: 607 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B25.00001: Air flow structure over a sessile droplet: Wettability and size effects Reza Yaghoubi Emami, Yewon Kim, Hyungmin Park, Alidad Amirfazli A PIV study on the airflow over a sessile water droplet exposed to a shearing boundary layer flow was done in a wind tunnel. The free stream velocity, U$_{\mathrm{\infty }}$ was set just below the air velocity that causes the droplet to move. Re number based on U$_{\mathrm{\infty }}$ and the height of the droplet (h) was 500 $\le $ Re$_{\mathrm{h\thinspace }}\le $ 1500. We studied the effects of the substrate wettability (PMMA, PEMA, PS, and Teflon) and droplet size (10-30 $\mu $l) on flow characteristics (flow structure, vorticity, etc.) in the plane that bisects the droplet streamwise. Common to all systems was formation of a strong eddy flow in the bisect-plane. The location of the eddy depended on the surface wettability, and to a certain degree droplet size. Droplet surface oscillations caused instabilities in the eddy observed in case of drops on a PMMA surface. Also, flow structure showed a bifurcation that was not seen when 2D objects were examined (this is due to intrusion of flow from sides into the wake area). We will also discuss the effect of surface oscillations by comparing results for flow around a sessile droplet and a rigid mockup of the droplet. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B25.00002: Wind-driven drop depinning on uphill surfaces and with variable contact-angle hysteresis Eleazar Herrera Hernandez, Edward White Contact angle hysteresis enables liquid drops to adhere to solid surfaces when surfaces are tilted or the drops are exposed to gas flow. Previous work (2017.DFD.Q11.006) has identified critical depinning Weber numbers at which contact-angle hysteresis is unable to resist forcing. $We_{\mathrm{crit}}$ is constant for non-inclined surfaces and varies systematically with Bond number for inclined surfaces. This work continues to investigate and develop a model for critical depinning conditions using tests with different contact angle hysteresis values and negative surface inclinations for which gravity acts in the direction opposite to wind forcing. Results show that under gravity-dominated forcing the surface tension adhesion force is well predicted by the contact angle hysteresis, $\Delta(\cos\theta)_{a,r}$. It is found that surface tension adhesion is more effective under wind-dominated forcing than under gravity-dominated forcing. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B25.00003: Water Interface Unsteadiness and Vortex Shedding in Wind-Driven Droplet Wakes Roger Simon, Edward White Water drops adhere to solid substrates but can depin when wind forcing exceeds the surface adhesion tension force. We hypothesize that, at high Reynolds numbers, depinning is assisted by coupling between air vortex shedding in the separated wake and the natural frequency of the air-water interface. Simon and White (2018.DFD.G11.4) measured significant vortex shedding for drop protuberances at different downstream locations. This work seeks to better understand the near-field airflow conditions and dynamic motion of a water drop just before depinning. Simultaneous measurements of the unsteady interface shape and airflow fluctuations are used to determine whether the unsteady vortex shedding in a drop wake and interface oscillations are coupled. Depinning limits across a wide range of drop volumes are analyzed in conjunction with the unsteady interface and airflow measurements. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B25.00004: Capillary transport of droplets on 3d printed conical structures Josephine Van Hulle, Floriane Weyer, Stephane Dorbolo, Nicolas Vandewalle In most arid regions like deserts, plants and animals have developed specific strategies (such as spines) in order to survive long periods without water. Recent works have proposed that conical shapes could trigger the motion of droplets wrapping this fiber towards the plant thanks to some capillary asymmetry. The aim of this research is to show that it is possible to achieve such capillary transport with macroscopic 3d printed conical structures. The motion of silicon oil droplets on these cones is recorded with a CCD camera in order to collect the droplet positions and speeds. A change in the geometry of the droplet from a barrel to a clam-shell shape is observed depending on the half-angle of the cone and the droplet volume. This induces different dynamics of the droplet along the cone. In fact, this change suggests a transition in the dissipation from a bulk to a contact line dissipation. We present experimental results as well as a model for describing these dynamics. The results obtained herein could be used to develop smart ways for manipulating droplets. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B25.00005: Numerical study on sessile droplet evaporation Cecile Lalanne, Jose-Maria Fullana, Florence Lequien In marine atmosphere, sprayed droplets containing chloride particules are carried by the wind from the sea. Droplets deposit on the exposed surface and become saline sessile droplets. This creates an electrolyte of finite size that can react with the metallic surface, leading to atmospheric corrosion. In order to investigate the effect of evaporation on corrosion, we propose a numerical model on evap- oration dynamics of salty sessile drops, built with Basilisk, a free software. Because of the complexity of the problem, the model is being developed step by step. At first, we were interested in the evaporation of pure water sessile droplets deposited on non- corrodable substrat to understand the dynamics of evaporation on a simple case. The different flow mechanisms inside and outside the droplet were studied and compared with litterature and experi- mental observations. As a second step, we analyze the Marangoni flow due to the variation of surface tension along the drop interface, caused by the non-homogenenous salt concentration. At the end, we hope to be able to simulate the realistic salt deposit patterns after the complete drop evaporation to identify the potential sites of corrosion. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B25.00006: Shape bifurcations of evaporating droplets on smooth patterned surfaces Matthew Haynes, Gary Wells, Rodrigo Ledesma-Aguilar, Marc Pradas With the recent development of smooth, pinning-free surfaces it has become important to understand the evolution process of an evaporating droplet resting on such a surface. A two-dimensional study of this problem has recently reported a new mode of evaporation in which the droplet follows a reproducible sequence of configurations, consisting of a quasi-static phase-change interrupted by out-of-equilibrium snaps that are triggered by shape bifurcations. Here, we shall introduce a three-dimensional model of this problem, where we use a thin film approximation to reduce the Young-Laplace equation to a Poisson equation. Solutions are found by means of a Fourier series expansion. We shall present evidence of snap evaporation for a variety of chemical patterns, and use bifurcation diagrams to quantify the dynamic evolution of the droplet. [Preview Abstract] |
Saturday, November 23, 2019 5:58PM - 6:11PM |
B25.00007: Is humidity a key factor in controlling the wetting and evaporation of a binary sessile drop? Senthil Kumar Parimalanathan, Sam Dehaeck, Alexey Rednikov, Pierre Colinet Studies on the wetting phenomenon are of vital importance for industrial applications such as coating, lithography, and so on. Most of the fundamental aspects for these applications have been formulated through observing a sessile drop spreading on a substrate. It is of great interest to study the wetting characteristic in case of a drop with binary composition, especially the cocktails of alcohol [e.g., Iso-Propanol (IPA)] and water. It has been observed experimentally that in these cases, the ambient relative humidity (RH) and the initial drop composition seems to play a very important role in the wetting dynamics. On varying the composition of IPA/water mixture and RH, with the help of optical interferometry, the droplet spreading behavior has been studied. For better understanding, an analytical model has been developed to characterize the dependence of these parameters on wetting behavior. The model has been conveniently used to generate so-called a map of regime (MOR) diagram which clearly demarcates the different wetting zones depending on the relative evaporation (or adsorption/absorption) of water in the drop. [Preview Abstract] |
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