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 H4: Drops VI |
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Chair: Stephane Zaleski, Institut D'Alembert, CNRS and UPMC Room: 23C |
Monday, November 19, 2012 10:30AM - 10:43AM |
H4.00001: Effect of the liquid/solid friction on the drop impact dynamics Christophe Pirat, Henri Lastakowski, Anne-Laure Biance, Christophe Ybert In this experimental study, the problem of drop impact on a smooth solid surface is investigated, both in wetting and non-wetting configurations. The combination of high speed Particle Image Velocimerty and absorption method allows us to probe the flow in the lamella during an impact. The role played by the solid/liquid friction in the sheet spreading is clarified by comparing impacts in low friction (above the Leidenfrost temperature) and partial wetting (on a cold sustrate). In the latter case, the results show that the lamella reaches a thickness limited by the development of a viscous boundary layer, in very good agreement with recent theoretical and numerical results. The boundary layer is not observed in the low friction case. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H4.00002: An aerodynamical mechanism for droplet splashing Christophe Josserand, Zhen Jian, Stephane Popinet, Pascal Ray, Stephane Zaleski Using a numerical study of drop impact on a solid substrate, we propose an aerodynamical mechanism to explain the dependance of the impact on the gas properties. Varying both the viscous and density ratio between the drop liquid and the external gas in the incompressible limit, we describe the frontier between spreading and splashing. Although a jet is sometimes formed before contact, we show that the splashing dynamics is controlled by the aerodynamical pressure exerted by the gas on the expanding liquid sheet. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H4.00003: Splash Criteria for Liquid Drop Impact on Smooth, Dry Surfaces Cacey Stevens, Sidney Nagel It is important to find a criterion that predicts the transition from smooth deposition to splashing of low-viscosity liquid drops when they land on a smooth, dry surface. Using high-speed imaging, we have determined the threshold pressure, $P_T$, of the ambient gas for which a liquid drop splashes as a function of the relevant parameters (gas molecular weight $m_G$, liquid viscosity $\nu_L$, surface tension $\sigma$, drop diameter D, and impact speed $U_0$). There is a non-monotonic trend of $P_T$ versus $U_0$ [1]. We find this same trend as we systematically change other liquid and gas properties; they simply shift the curve. By defining a scaled pressure, $P_{T,scaled}=P_T D^{0.5} \nu_L^{0.25} m_G^{0.5} \sigma^{-0.25}$, and scaled impact speed $U_{0,scaled}=U_0 D^{0.3} \nu_L^{0.4} \sigma^{-0.35}$, we find a collapse of all data sets onto a single curve. This scaling applies to both high and low velocity regimes.\\[4pt] [1] {Xu} L. Xu, S. Nagel, and W. Zhang. Phys. Rev. Lett. 94, 184505 (2005). [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H4.00004: Substrate topography-mediated air film collapse below an impacting drop Jolet de Ruiter, Dirk van den Ende, Frieder Mugele Liquid drops hitting solid surfaces are slowed down by the ambient air layer that needs to be squeezed out before the liquid actually touches the solid. How does substrate topography mediate the collapse of the air film? For moderate velocity impacts (Weber number around unity) we show that a drop gently spreads on an undulated air layer that thins to a minimum of several hundreds of nanometers. Whether or not the air layer collapses (in a single spot nucleation) leading to wetting of the substrate, can be tuned with substrate topography. Introducing micro- and nanoscale defects of various topography, we observe the two different cases, i.e. drop bouncing on the air layer and directed nucleation of liquid-solid contact. Using quantitative dual wavelength reflection interference microscopy we reveal the evolution of the air film, and the influence of substrate defects on localized pressure build-up and film collapse. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H4.00005: Drop impact of suspensions Francois Boyer, Jacco Snoeijer, Frits Dijksman, Detlef Lohse Drop impact dynamics on solid surfaces is a classical subject of interfacial hydrodynamics, which occurs in many industrial and environmental situations. So far, most of the studies have concerned Newtonian fluids. Complex fluids (polymer dispersions, particle suspensions, gels, emulsions, ...) are however of considerable interest for a wide range of applications. From a fundamental point of view, how the non-Newtonian features of a complex liquid change (drastically in some cases) the drop impact dynamics is a challenging open problem. Newly observed phenomena will then be presented. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H4.00006: Dynamic Leidenfrost temperature for impact of droplets on micro-structured surfaces Hendrik J.J. Staat, Tuan Tran, Arturo Susarrey Arce, Tobias C. Foertsch, Arie van Houselt, Han Gardeniers, Andrea Prosperetti, Detlef Lohse, Chao Sun When a droplet impacts a surface heated above the liquid's boiling point, the droplet either contacts the surface and boils immediately, (contact boiling) or is supported by a developing vapor layer and bounces back (film boiling \& Leidenfrost state). We study the transition between these two different behaviors and how it is affected by the control parameters such as impact velocity and controlled roughness (i.e., micro-structures fabricated on silicone surfaces). Additionally, in the film boiling regime, we show that the resident time of the droplet impact is insensitive to the velocity, the surface temperature, and the structure's geometry. In the contact boiling regime, we show that the structured surfaces induce the formation of liquid jets emerging during the spreading stage of the impacting droplets. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H4.00007: Impact of droplet on superheated surfaces Detlef Lohse, Hendrik J.J. Staat, Tuan Tran, Andrea Prosperetti, Chao Sun At impact of a liquid droplet on a smooth surface heated way above the liquid's boiling point, the droplet spreads without any surface contact, floating on its own (Leidenfrost-type) vapor layer, and then bounces back. We show that the dimensionless maximum spreading factor $\Gamma$, defined by the ratio of the maximal spreading diameter and the droplet diameter, shows a universal scaling $\Gamma\sim We^{\gamma}$ with the Weber number $We$ -- regardless of surface temperature and of liquid properties -- which is much steeper than that for the impact on non-heated (hydrophilic or hydrophobic) surfaces, for which $\gamma = 1/4$. Based on the idea that the vapor shooting out of the gap between the droplet and the superheated surface drags the liquid outwards, we derive scaling laws for the spreading factor $\Gamma$, the vapor layer thickness, and the vapor flow velocity. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H4.00008: Microdroplet impact at very high velocity Claas Willem Visser, Yoshiyuki Tagawa, Chao Sun, Detlef Lohse At APS-DFD 2011, we presented preliminary data of water microdroplet impact at velocities up to 100 m/s and droplet diameters from 12 to 100 $\mu$m. Now we place these results into context and use them to improve understanding of droplet spreading. The parameter range covers the transition from capillary-limited to viscosity-limited spreading of the impacting droplet. The maximum spreading radius is compared to several existing models. The model by Pasandideh-Fard et al. (1996) agrees well with the measured data, indicating the importance of a thin boundary layer just above the surface. Here, most of the viscous dissipation of the spreading droplet takes place. As explained by the initial air layer under the impacting droplet, a contact angle of 180 degrees is used as model input. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H4.00009: How Does Air Evolve into a Bubble During Drop Impact? Ji San Lee, Byung Mook Weon, Su Ji Park, Ji Tae Kim, Jaeyeon Pyo, Jung Ho Je, Kamel Fezzaa When a liquid drop impacts on a solid substrate, a tiny air film is generally entrapped between the drop and the substrate and eventually evolves into a bubble by surface energy minimization. We investigated how air evolves into a bubble during drop impact using ultrafast x-ray phase-contrast imaging that enables us to track the detailed morphological changes of air with high temporal and spatial resolutions. We found that the evolution takes place through complicated three stages: inertial retraction of the air film, collapse of the top air surface into a bubble, and pinch-off of a daughter droplet in the bubble. The collapse and the pinch-off can be explained by energy convergence that is associated with Ohnesorge number (Oh) regarding capillary waves and viscous damping. We measured a critical Oh number, Oh* $\sim $ 0.026 $\pm$ 0.003, above which the generation of the daughter droplet is suppressed. Interestingly we found that the bubble is detached favorably from wettable surfaces, which suggests a feasible way to eliminate bubbles for many applications by controlling surface wettability. The threshold angle for bubble detachment was measured as $\sim $ 40 $\pm$ 5 deg. for water, which agrees with a geometrical estimation. [Preview Abstract] |
Monday, November 19, 2012 12:27PM - 12:40PM |
H4.00010: Pinning of a perfectly wetting volatile liquid at a sharp edge - experiment and theory Yannis Tsoumpas, Sam Dehaeck, Alexey Rednikov, Mariano Galvagno, Uwe Thiele, Pierre Colinet It is well known that contact lines of drops, even those of wetting liquids, stay pinned at sharp edges of the substrate until the apparent contact angle exceeds a critical value. In the present study, we show that evaporation influences this effect. The edge of a circular groove is used as an example. Experiments with wetting liquids of different evaporation rate show indeed that not only the spreading of the liquid is adequately halted, but also that the pinning is enhanced for the more volatile cases. The experimental results are qualitatively compared with predictions of a thin film model in two dimensions. The approach employs an evolution equation for the height profile of an evaporating thin film (small contact angle droplet) on a substrate with a rounded edge, and enables one to predict the dependence of the apparent contact angle on the position of the contact line. The calculations confirm our experimental observations, namely that there exists a dynamically produced critical angle for depinning that increases with the evaporation rate. This suggests that one may introduce a simple modification of the Gibbs criterion for pinning that accounts for the non-equilibrium effect of evaporation. [Preview Abstract] |
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