Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A34: Drops II: Drop Impact on Liquid Surfaces |
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Chair: Yongsheng Lian, Louisville University Room: 405 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A34.00001: The effects of droplet characteristics on the surface features in a rain field R. Liu, H. Brown, X. Liu, J.H. Duncan The characteristics of the shape of a water surface in response to the impact of simulated raindrops are studied experimentally in a 1.22-m-by-1.22-m water pool with a water depth of 0.3 m. A rain generator consisting of an open-surface water tank with an array of 22-gauge hypodermic needles (typical needle-to-needle spacing of about $L_0 = 3.5$ cm) attached to holes in the tank bottom is mounted 2 m above the water pool. The tank is connected to a 2D translation stage to provide a small-radius ($< L_0$) horizontal circular motion to the needles, thus avoiding repeated drop impacts at the same location under each needle. The droplet diameters ($d$) and number of drops per unit time per needle ($n$) are varied by changing the length of the needles while maintaining the same volumetric flow rate ($n \pi d^3/6$) through control of the water depth in the generator tank. The water surface features, including the crown, stalk and ring waves, due to the impacts of the drops are measured with a cinematic laser-induced- fluorescence (LIF) technique. The dependence of these features on the rain characteristics are discussed. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A34.00002: Numerical investigation of air film breakup and micro-bubble formation in liquid-liquid impact events Seyedshahabaddin Mirjalili, Ali Mani Experimental evidence shows that micro-bubbles can be generated when a droplet of the size of a few millimeters impacts a layer of the same liquid with a velocity of a few meters per second. This phenomenon, also known as Mesler entrainment is cumbersome to numerically simulate due to the small time and length scales involved. In order to gain a better understanding of the relevant scales, parameters, and regions, 2-D boundary element simulations inspired by M. Mani, Mandre, Brenner (JFM, vol. 647, p. 143, 2010) were performed. By developing treatments for topological changes, these simulations are extended to after impact events and finally depict the formation of micro-bubbles of sizes similar to entrapped bubbles in Mesler entrainment. Compressibility effects on final bubble size are discussed, and the requirements for a resolved CFD calculation are obtained. Thereafter, a 2-D two-phase flow calculation using a diffuse interface model is undertaken and based on grid-converged results, the statistics of the bubbles are examined and compared with available experimental data. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A34.00003: Dynamics of an air film entrapped by drop impact on liquid surface Ji San Lee, Byung Mook Weon, Su Ji Park, Ji Won Jung, Ji Tae Kim, Jaeyeon Pyo, Kamel Fezzaa, Jung Ho Je When a liquid drop impacts a liquid substrate, air is entrapped underneath, finally evolving into spherical bubbles. This phenomenon occurs robustly and plays an important role in various natural phenomena and industrial applications. In this study, we investigated the evolution of an entrapped air film during drop impact using ultrafast X-ray phase-contrast imaging. The evolution exhibits very complex and different behaviors depending on fluid properties. We reveal that the retraction dynamics of the air film crosses from a capillary-inertial regime to a capillary-viscous regime by the increase of Ohnesorge number (Oh). At Oh \textless 0.03, a daughter droplet is generated inside the air film, due to the convergence of the capillary waves generated during the retraction. We also find that the evolved bubble is broken up into two at low Oh, driven by its vertical stretching and capillary instability. We finally demonstrate a phase diagram for the formation of daughter droplet and bubble breakup in terms of Oh and Weber number (We). [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A34.00004: Surfactant effects on cumulative drop size distributions produced by air bubbles bursting on a non-quiescent free surface K. Parmar, X. Liu, J.H. Duncan The generation of droplets when air bubbles travel upwards from within a liquid and burst at a free surface is studied experimentally. The bubbles are generated in a glass water tank that is 0.91 m long and 0.46 m wide with a water depth of 0.5 m. The tank is equipped with an acrylic box at its bottom that creates the bubble field using filtered air injected through an array of 180 hypodermic needles (0.33 mm ID). Two different surface conditions are created by using clean water and a 0.4\% aqueous solution of Triton X-100 surfactant. Measurements of the bubble diameters as they approach the free surface are obtained with diffuse light shadowgraph images. The range of bubble diameters studied is 2.885 mm to 3.301 mm for clean water and 2.369 mm to 3.014 mm for the surfactant solution. A laser-light high-speed cinematic shadowgraph system is employed to record and measure the diameters and motions of the droplets at the free surface. This system can measure droplets with diameters $\leq$ 50 $\mu$m. The results show a clear distinction between the droplet distributions obtained in clean water and the surfactant solution. A bimodal droplet distribution is observed for clean water with at least two dominating peaks. For the surfactant solution, a single distribution peak is seen. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A34.00005: Sparkling Droplets: Aerosol Dispersion Resulting from Drop Impingement on Porous Surfaces Young Soo Joung, Cullen Buie We have investigated aerosol generation from droplets hitting wettable porous surfaces. Aerosols have been widely investigated due to their significant impact on the environment. To date, bubbles breaking at air/water interfaces have been considered the chief mechanism of aerosol dispersion. Here, we demonstrate that droplets can release aerosols when they impact porous surfaces. At the moment of impact, tiny bubbles are formed inside the droplet, fed by air escaping the porous media. The tiny bubbles break when they meet the droplet/air interface, releasing tiny water-jets, the sizes of which are in the range of tens-of-micrometers. Interestingly, within a specific range of impact velocity and surface properties, we observed frenzied ejection of tiny water-jets producing aerosol clouds above the surface. With knowledge of the surface properties and the impact conditions we can predict when the frenzied aerosol generation will occur. This study will produce novel experimental methodologies for further investigation of the environmental impact of aerosols. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A34.00006: Rain Drops and Oil Slicks: Impact of Water Droplets on a Surface Oil Layer David Murphy, David Morra, Joseph Katz Petroleum spills in aquatic environments form oil slicks on the water surface. These slicks, the thickness of which ranges from microns to several millimeters, negatively impact the natural environment and economic resources. While dispersion of these slicks as small droplets by breaking waves has long been investigated, the dispersive power of another environmental flow, rainfall, has not been considered. The impact of a water drop on a floating layer of immiscible fluid introduces a challenging flow physics problem. Our experimental observations examine processes occurring when falling water droplets impact on floating layers of sweet petroleum crude oil of various thicknesses and dispersant concentrations. The latter alter the surface tension by orders of magnitude. Impact events recorded at high-speed, using UV light to cause oil fluorescence, show the expected formation of modified multiphase Worthington jets, air cavities, as well as breakup of the slicks into clouds of oil droplets and oil-coated bubbles. The latter rise back to the surface and pop. Results include droplet size and spatial distributions as a function of rainfall momentum, oil properties, and processes involved. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A34.00007: Droplet splashing on a wet moving wall Jing Lou, Ming Cheng The splash of a liquid drop onto a moving solid wall covered with a thin film of the same liquid is simulated numerically for an impact Reynolds number of 1000 and a Weber number of 400. A two-phase flow lattice Boltzmann model is employed for the simulations. The characteristics of drop splashing on the wall are investigated by systematically varying the impact angle and wall speed. The angle of incidence $\theta$ ranges from 0 to 60 degrees, while the wall speed to the initial drop velocity ratio Uw varies from 0 to 3. In case of Uw equals to 0 and $\theta $large than 0 degree, the splashing is asymmetric. The increase of the angle leads to the transition from splash to partial splash. The present results indicate the existence of a critical $\theta $c value of about 55 degrees, which is determined by the liquid film thickness, Reynolds and Weber numbers. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A34.00008: Droplets motion by Dissipative particle dynamics on inclined flat plate with oil film covered Chuanjin Lan, Yanbao Ma Understanding the incipient motion of a droplet adhered to a surface has wide applications, from self-cleaning of superhydrophobic surfaces to electromagnetic drop actuation inmicrofluidic systems. In this paper the dissipative particle dynamics (DPD) is introduced to study the droplet motion at the meso-scale along the inclined flat plate covered with oil film at different tilt angles. The flow motion can be totally different with and without the existence of the oil film. The effect from tilt angle and oil film thickness is studied on the motion of the droplet. [Preview Abstract] |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A34.00009: Can consecutive droplet deposition yield uniform liquid films? Anne Juel, Alice Thompson, Carl Tipton, Andrew Hazel, Mark Dowling Ink-jet printing is being used to manufacture highly customizable electronic components through the successional deposition of overlapping droplets of conductive ink onto a substrate. For vanishing contact angles, the fluid drops merge together on the substrate to form a wide, uniform thread. However, in practical operating regimes involving moderate contact angles, the fluid is redistributed to form non-uniform patterns, characterized by the development of an initial wide bulge (larger than any individual drop). This ``head'' is sometimes followed by a sequence of secondary bulges connected by narrow rivulets. We conduct new experiments with high-speed imaging to examine the morphological development of the thread resulting from the deposition of a moderate number of successive micro-scale drops ($<80$) that may form up to two secondary bulges. We demonstrate that the spatial structure of the ink formation resulting from this complex deposition process can be predicted quantitatively with a simple physical model that incorporates contact angle hysteresis. The head formation occurs robustly due to an inviscid mechanism, while the secondary bulge formation arises from viscous effects. [Preview Abstract] |
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