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 M4: Drops X |
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Chair: Eric Johnsen, University of Michigan Room: 23C |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M4.00001: ABSTRACT WITHDRAWN |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M4.00002: Explosion of Leidenfrost Droplets Florian Moreau, Pierre Colinet, Stephane Dorbolo When a drop is released on a plate heated above a given temperature, a thin layer of vapour can isolate the droplet so that it levitates over the plate. This effect was first reported by Leidenfrost in 1756. However, this fascinating subject remains an active field of research in both fundamental and applied researches. In this work, we focus on what happens when surfactant is added to the drop. The aim is to study the influence of a decrease of the surface tension. Surprisingly, as the droplet evaporates, suddenly it explodes. The evolution of the droplet and the resulting explosion are followed using a high speed camera. We show that when a critical concentration of surfactant is reached inside the drop, a shell of surfactant is formed leading to the explosion. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M4.00003: Experimental study of the oscillating interface of a falling drop Suhwan Choi, Thomas Ward The drop interface oscillation generated from detachment from a nozzle due to gravity are experimentally studied. The fluids used in the experiments are glycerol-water mixtures with viscosities ranging from 0.005 to 0.410 Pa s, mineral oil having a viscosity of 0.0270 Pa s, and DI water with viscosity of 0.0009 Pa s. The drop oscillating is taken by fast camera to make observations. For large drops, where the interface relative to a polar angle may be measured, the periodic deformation is plotted as a function of time. For smaller drops we measure the deformation as switching between an oblate and prolate drop as a function of time. The phenomenon is clearly a function of the fluid viscosity but we seek to propose a pinch-off mechanism for understanding the source of the observed oscillations. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M4.00004: ABSTRACT WITHDRAWN |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M4.00005: Interaction of droplets in recirculation regions within microfluidic systems Nastaran Ghazi, Ashkan Hosseini, Shahab Shojaei-Zadeh We investigate the interaction of oil droplets in continuous water phase as they travel across the streamlines of a recirculation region using microfluidic devices. Oil droplets are first generated using hydrodynamic focusing and then enter a recirculation region. The droplets then keep recirculating until they are pushed out by the incoming ones. We show that the frequency of droplet generation, viscosity contrast (oil to water), and geometry determine which droplets to stay in the recirculation region and which one to leave. Using flow field simulations, we investigate the migration of droplets and their trajectories based on the geometry of the recirculation region, the bubble size, and fluid properties. Under favorable conditions, when droplets interact within the recirculation region for long enough time, the film thickness that separates the two interfaces reduces and droplets will coalesce. The proposed design thus provides a suitable platform to study droplet coalescence within microfluidic devices. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M4.00006: Experimental Investigation of Gravity- and Wind-Forced Drop Stability Jason Schmucker, 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. To investigate this phenomenon, drops were placed on the rough aluminum floor of a tiltable wind tunnel and brought to critical conditions over a range of drop size, inclination angle, and flow speed. A technique capable of measuring full 3D drop profiles was used to investigate the drops' evolution toward runback. The measurement uses a comparison of the surface speckle pattern captured in an overhead drop image with a corresponding image of the dry surface to measure drop shape. Drops forced by airflow alone are found to shed at a Weber number of $8.0\pm0.5$ for this system with advancing and receding contact angles of $\theta_{a}=63.5^\circ\pm3.7^\circ$ and $\theta_{r} = 8.2^\circ\pm1.5^\circ$. Drops at larger surface inclinations shed at lower Weber number. From reconstructed drop profile sequences, the evolution of contact lines, drop profiles, and contact angle distributions are detailed. Contact line adhesion forces are integrated and related to the forcing air velocity. Drops whose stability limits are dominated by gravity are found to exhibit significantly different evolution toward runback than those dominated by airflow. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M4.00007: Physics of drop formation at a step Zhenzhen Li, Samuel Metais, Alex Leshansky, Len Pismen, Patrick Tabeling When a confined liquid stream surrounded by another immiscible liquid flows from a shallow channel into a deep reservoir, droplets can be generated at the entrance of the reservoir. We perform series of experiments in this geometry in a microfluidic system and compare our observations to a recent theory developed by A. Leshansky and L. Pismen. We observe that below a critical capillary number, Ca, small monodispersed droplets are formed. We give scaling explanation on their sizes. Above the critical Ca, the stream takes a quasi-steady shape feeding a large droplet formed in the reservoir. Theoretical arguments are presented explaining the transition between the two regimes and the shape of the interface, that separates the two liquids. The experiments confirm the theory at a quantitative level. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M4.00008: Janus droplet motion in an external flow Sergey Shklyaev, Andrey Ivantsov, Misael Diaz, Ubaldo M. Cordova-Figueroa We consider a hydrodynamics of a Janus droplet, which consists of two hemispherical domains occupied by different liquids. The simplest problem, a Janus droplet in a uniform at infinity flow, is analyzed. The interfaces are assumed weakly deformable. It is shown, that the velocity field can be represented as a superposition of two fields: for internal surface (i) normal and (ii) parallel to the external flow. In case (i) the flow is axisymmetric; the force imposed on the droplet is found by summation of the series. It is worth noting, that even for equal internal viscosities, the solution for a simple drop [1,2] is not reproduced. Indeed, the internal impermeable interface prohibits a flow of Hadamard-Rybczynski type. Weak deformation of the interfaces is found; it is shown that deformation of the internal surface is larger than that of the drop surface. In case (ii) expansion in Lamb's functions is applied; both the torque and force are found. It is also shown that stable configuration of a torque-free droplet corresponds to case (i) with less viscous fluid on the upstream face.\\[4pt] [1] J. S. Hadamard, Compt. Rend. Acad. Sci. 152, 1735 (1911).\\[0pt] [2] W. Rybczynski, Bull. Acad. Sci. Cracovie (ser. A), 40 (1911). [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M4.00009: ABSTRACT WITHDRAWN |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M4.00010: Air Entrapment for Liquid Drops Impacting a Solid Substrate Yuan Liu, Peng Tan, Lei Xu Using high-speed photography coupled with optical interference, we experimentally study the air entrapment during a liquid drop impacting a solid substrate. We observe the formation of a compressed air film before the liquid touches the substrate, with internal pressure considerably higher than the atmospheric value. The degree of compression highly depends on the impact velocity, as explained by balancing the liquid deceleration with the large pressure of compressed air. After contact, the air film expands vertically at the edge, reducing its pressure within a few tens of microseconds and producing a thick rim on the perimeter. This thick-rimmed air film subsequently contracts into an air bubble, governed by the complex interaction between surface tension, inertia and viscous drag. Such a process is universally observed for impacts above a few centimeters high. [Preview Abstract] |
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