Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E33: Drops: Wetting and Spreading II |
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Chair: Sungyon Lee, Texas A&M University Room: Ballroom A |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E33.00001: Drop stability in wind: theory Sungyon Lee Water drops may remain pinned on a solid substrate against external forcing due to contact angle hysteresis. Schmucker and White investigated this phenomenon experimentally in a high Reynolds number regime, by measuring the critical wind velocity at which partially wetting water drops depin inside a wind tunnel. Due to the unsteady turbulent boundary layer, droplets are observed to undergo vortex-shedding induced oscillations. By contrast, the overall elongation of the drop prior to depinning occurs on a much slower timescale with self-similar droplet shapes at the onset. Based on these observations, a simple, quasi-static model of depinning droplet is developed by implementing the phenomenological description of the boundary layer. The resultant model successfully captures the critical onset of droplet motion and is the first of on-going studies that connect the classical boundary layer theory with droplet dynamics. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E33.00002: Drop stability in wind: effect of solid protrusion Alireza Hooshanginejad, Benjamin Wilcox, Edward White, Sungyon Lee We experimentally investigate the inertia-driven onset of droplet depinning behind a solid protrusion inside a wind tunnel. In the high Reynolds number regime, the separation and reattachment of the boundary layer in the presence of the solid protrusion directly lead to complex behavior of the partially wetting water drop as a function of its position. For varying droplet volumes and droplet positions from the protrusion, we measure the critical wind velocity at which the drop starts to depin. In particular, drops in a certain volume range are observed to reverse their depinning direction at a critical distance from the solid. By coupling the boundary layer characteristics with droplet dynamics, we explain the physical mechanism of the resultant droplet behavior. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E33.00003: Simulations of contact angle induced pearling for sliding drops Scott McCue, Lisa Mayo, Timothy Moroney Droplets sliding down an incline can develop a corner or a cusp at their rear, or undergo a pearling transition whereby the tail breaks up into a number of smaller satellite droplets. These phenomena have been of interest since the experimental work of Podgorski et al. (2001) {\em Phys Rev Lett} {\bf 87}, 036102. It appears that the experimental investigation of this problem is limited due to the inherent difficulty of minimising contact angle hysteresis, whereby physical or chemical heterogeneities of the substrate cause pinning and distortion of the droplet. By applying a lubrication model with a disjoining pressure term, we investigate these flows numerically in order to further shed light on how certain conditions (such as contact angle) affect the corner-cusp-pearling transition. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E33.00004: Superspreading: molecular dynamics simulations and experimental results Panagiotis Theodorakis, Nina Kovalchuk, Victor Starov, Erich Muller, Richard Craster, Omar Matar The intriguing ability of certain surfactant molecules to drive the superspreading of liquids to complete wetting on hydrophobic substrates is central to numerous applications that range from coating flow technology to enhanced oil recovery. Recently, we have observed that for superspreading to occur, two key conditions must be simultaneously satisfied: the adsorption of surfactants from the liquid–vapor surface onto the three-phase contact line augmented by local bilayer formation. Crucially, this must be coordinated with the rapid replenishment of liquid–vapor and solid–liquid interfaces with surfactants from the interior of the droplet. Here, we present the structural characteristics and kinetics of the droplet spreading during the different stages of this process, and we compare our results with experimental data for trisiloxane and poly oxy ethylene surfactants. In this way, we highlight and explore the differences between surfactants, paving the way for the design of molecular architectures tailored specifically for applications that rely on the control of wetting. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E33.00005: Thermocapillary flow of droplets: gravity and disjoining potential effects Juan M. Gomba, Jonatan R. Mac Intyre, Carlos A. Perazzo In a previous work (Gomba and Homsy, JFM, 2010) one of us studied the flow of a droplet on a rigid substrate under the effect of a constant temperature gradient, and under partial wetting and zero-gravity conditions. Three different regimes of flow, that depends on the contact angle and volume of the droplets were reported. Here we introduced the gravity and different substrate-liquid molecular interactions to study its effect on the flow. We observe that for small contact angles, the asymptotic behaviour of the droplet is similar to the one observed for none gravity conditions, no matter the molecular interaction modeled. For larger contact angles, the velocity of the droplet and the eventual occurrence of breakup into smaller droplets are analyzed. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E33.00006: Dancing droplets: Chemical space, substrates, and control Nate Cira, Adrien Benusiglio, Manu Prakash Previously we showed that droplets of propylene glycol and water display remarkable properties when placed on clean glass due to an interplay between surface tension and evaporation. (Cira, Benusiglio, Prakash: Nature, 2015). We showed that these mechanisms apply to a range of two-component mixtures of miscible liquids where one component has both higher surface tension and higher vapor pressure on a variety of high energy surfaces. We now show how this rule can be cheated using a simple trick. We go on to demonstrate applications for cleaning, and show how this system works on substrates prepared only with sunlight. We finish by demonstrating active control of droplets, allowing access to a host of new possibilities. [Preview Abstract] |
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