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 L35: Drops: Pinch-off and Coalescence |
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Chair: Firat Testik, University of Texas at San Antonio Room: Ballroom B |
Monday, November 23, 2015 4:05PM - 4:18PM |
L35.00001: Binary Raindrop Collisions Firat Testik, Kalimur Rahman In this talk, we will present first-time observations of binary raindrop collisions in natural rainfall and discuss the observed raindrop collision outcomes (i.e. breakup, coalescence, and bounce). Binary raindrop collisions have long been hypothesized as a key process in shaping of the raindrop size distribution, an important quantity for a number of meteorological and hydrological applications. Testik (2009) developed a regime diagram to determine the outcomes of a raindrop collision based upon the collision kinetic energies and surface energies of the colliding raindrops. This regime diagram has been validated previously using two different laboratory datasets for the collision of simulated raindrops. A new instrument that we have developed for precipitation microphysical observations, called High-speed Optical Disdrometer (HOD), made raindrop collision observations possible in natural rainfall and provided a valuable small dataset. In the light of these first time field observations of raindrop collisions, we will discuss Testik's diagram in this talk. Testik, F. Y., 2009. Outcome regimes of binary raindrop collisions. \textit{Atmos. Res.}, \textbf{94}, 389--399. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L35.00002: ABSTRACT WITHDRAWN |
Monday, November 23, 2015 4:31PM - 4:44PM |
L35.00003: Scaling vs simulations in the head-on collision of viscous drops with insoluble surfactants Carolina Vannozzi Scaling arguments are presented to show the effect of the surface diffusivity Ds on the head-on collision of two equal-sized viscous drops in a viscous matrix with insoluble surfactants. The scaling arguments are compared to simulations [1] of the experimental system studied by Yoon et al [2] where the drops are Polybutadiene(PBD) in PDMS, stabilized by block copolymers surfactants. Overall, the scaling could predict the effect of the different parameters on the drainage time (the surface Peclet number, the Marangoni number and the pushing force due to the external flow), but could not predict the experimental or simulated values. We tested our simulations against the scaling argument of [3], that claimed that emulsions stabilized by small molecule surfactants can be described with the assumption of non-diffusing surfactants. Here, however, following the same arguments, but without using the Stokes-Einstein expression for the surfactant surface mobility employed in Ref. [3] and by simply substituting the parameters for different emulsion systems, we show that Ds can be neglected only for oil in water emulsions, not for water in oil emulsion [1]. \\[4pt] [1] Vannozzi PoF (2012)\\[0pt] [2] Yoon, Hsu, Leal PoF (2007)\\[0pt] [3] Cristini, Blawzdziewicz, Loewemberg JFM (1998) [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L35.00004: Oscillations of a liquid bridge resulting from the coalescence of two droplets Veronique Chireux, David Fabre, Frederic Risso, Philippe Tordjeman, Sebastien Cazin We study the inertial oscillations of a bridge of liquid maintained between two disks, both experimentally and theoretically. In the experiment, the bridge is formed by the coalescence of two droplets. After coalescence, the bridge performs weakly damped oscillations until it reaches its equilibrium shape. Four modes of oscillations can be extracted from digital processing of images recorded by means of a high-speed camera. Their frequency and damping rate have been determined and found to be independent of the initial conditions. Concurrently, the eigen modes of oscillations of a non-cylindrical bridge have been computed by assuming inviscid flow and small amplitude oscillations, and their characteristics turn out to be significantly different from that of a cylindrical bridge. The agreement between theoretical and measured frequencies confirms that the experimental modes correspond to the eigenmodes of the linear inviscid theory. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L35.00005: Dynamics of drop coalescence on under-liquid substrates Surjyasish Mitra, Sushanta Mitra Theoretical understanding of drop coalescence on under-liquid substrates is a challenging problem due to the presence of a surrounding viscous medium. Though, most work till date have focused on coalescence in air medium, the presence of a surrounding viscous medium is a significant extension to this classical coalescence problem. Such instances are often found in physical systems such as oil-spills, wetting of marine ecosystem, etc. In the present work, a modified one-dimensional lubrication equation has been developed to describe the early coalescence behavior of two symmetric sessile drops for under-liquid substrates, which takes into account the viscosities of both the drop and the surrounding medium. We found a new time scale which governs the process and there exist a cross-over time between the universal scaling of the bridge height growth $\hat{{h}}\sim \hat{{t}}$ (valid for both under-liquid and air) and a much slower bridge growth $\hat{{h}}\sim \hat{{t}}^{0.24}$occurring at a later time. It is also found that the evolving bridge profile has a self-similarity, which breaks up much earlier for under-liquid substrates as opposed to symmetric coalescence in air. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L35.00006: Partial coalescence of soap bubbles Daniel M. Harris, Giuseppe Pucci, John W. M. Bush We present the results of an experimental investigation of the merger of a soap bubble with a planar soap film. When gently deposited onto a horizontal film, a bubble may interact with the underlying film in such a way as to decrease in size, leaving behind a smaller daughter bubble with approximately half the radius of its progenitor. The process repeats up to three times, with each partial coalescence event occurring over a time scale comparable to the inertial-capillary time. Our results are compared to the recent numerical simulations of Martin and Blanchette [Phys. Fluids {\bf 27}, 012103 (2015)] and to the coalescence cascade of droplets on a fluid bath. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L35.00007: Coalescence of surfactant-laden drops in liquids Emilia Nowak, Mark Simmons Whilst coalescence of droplets in air is much studied, the mechanism of merging surfactant-laden drops in other liquids is less well understood. The dynamics of the coalescence of droplets in presence of surfactants was investigated focusing on the curvature and progression of the width of the neck that bridges the drops (up to millimetres) as well as the mixing patterns and surface flows driven by Marangoni stresses. Coalescence of different composition droplets revealed difference in the curvature of the meniscus on either side of the growing bridge which was more pronounced for the lower viscosities of the surrounding oils and related to the different local values of the surface tension. With the aid of a dye present in one of the drops, the visualisation of bulk flow was possible and different patterns were observed with increasing viscosity of the surrounding oil that led to formation of ‘mushroom’-like structures inside the droplets. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L35.00008: In-flight surface tension and viscosity measurements of inkjet printed droplets Hendrik Staat, Arjan van der Bos, Marc van den Berg, Hans Reinten, Herman Wijshoff, Michel Versluis, Detlef Lohse In modern drop-on-demand inkjet printing, the jetted liquid is a mixture of solvents, pigments and surfactants. In order to predict the droplet formation process, it is of importance to know the liquid properties. Surface tension is not constant at the timescale of droplet formation for a liquid that contains surfactants, making it non-trivial to determine the surface tension of the ink directly. Therefore we developed a technique to measure the surface tension of liquids during inkjet printing. We use high speed imaging to record the shape oscillation of a microdroplet within the first few hundred microseconds after droplet pinch-off. The frequency of oscillation depends on the surface tension, so by determining this frequency, we can measure the surface tension. The decay of oscillation amplitude is set by the viscosity, so we can also determine the viscosity with this technique. We use this technique to study the effect of surfactants on the surface tension of ink during the inkjet printing process. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L35.00009: Assessment of Droplet Collision Models in Pulsed Sprays Giles Brereton, Farid Roshanghalb The electronic control of fuel injectors allows multiple ensembles of the same pulsed spray event to be measured at different locations in the spray, and to be ensemble averaged for data analysis. In this talk, we present experimental laser-diffraction measurements of droplet size distributions in planes through the pulsed spray, at different locations downstream of the regions of primary and secondary breakup. The measured size distribution closest to the injector serves as an initial condition for droplet collision simulations whereas the measured downstream distributions serve as target data. Lagrangian simulations of a population of spherical fuel droplets matching the measured near-injector size distribution and velocity are then carried out using different collision and satellite-droplet-generation models and compared with downstream size-distribution measurements. Collision models which account rationally for the relative vector velocities of colliding droplet pairs yield downstream size distributions in good agreement with measurements whereas `ad hoc' models and those which assume random collision angles fair poorly. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L35.00010: Stability and motion of liquid bridges between non-parallel surfaces Mohammadmehdi Ataei, Huanchen Chen, Tian Tang, Alidad Amirfazli Squeezing and stretching liquid bridges formed by approaching upper surface to a sessile drop deposited on a lower surface, is frequently observed in nature and industry, e.g. printing. However, most literature focuses on liquid bridges between two parallel surfaces. In practice, bridges can also be formed between surfaces with an angle $\alpha $between them. Here, the effect of $\alpha $ on the stability and motion of the bridge was studied experimentally. Different pairs of surfaces from hydrophilic to hydrophobic, along with different contact angle hysteresis (CAH) values, were used to study the effect of surface contact angle (SCA) and CAH on the bridge stability and motion. Unlike bridges between parallel surfaces, a stable bridge may not be formed when $\alpha $ is larger than a threshold value $\alpha_{\mathrm{c}}$. Instead, when bridge forms, it can undergo unstable movement towards the ends of surfaces. Shown in this study, $\alpha_{\mathrm{c}}$ is governed by both SCA and CAH (typically missed in literature). Also, during the squeezing and stretching cycles, because of $\alpha ,$ bulk motion of the liquid bridge along the surfaces can be observed. The direction and magnitude of the bulk motion is found to be related to SCA, CAH and $\alpha $. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L35.00011: Surfactant effect on drop coalescence and film drainage hydrodynamics Weheliye Weheliye, Maxime Chinaud, Victor Voulgaropoulos, Panagiota Angeli Coalescence of a drop on an aqueous-organic interface is studied in two test geometries A rectangular acrylic vessel and a Hele-Shaw cell (two parallel plates placed 2mm apart) are investigated for the experiments. Time resolved Particle Image Velocimetry (PIV) measurements provide information on the hydrodynamics during the bouncing stage of the droplet and on the vortices generated at the bulk fluid after the droplet has coalesced. The velocity field inside the droplet during its coalescence is presented. By localizing the rupture point of the coalescence in the quasi two dimensional cell, the film drainage dynamics are discussed by acquiring its flow velocity by PIV measurements with a straddling camera. The effect of surface tension forces in the coalescence of the droplet is investigated by introducing surface active agents at various concentrations extending on both sides of the critical micelle concentration. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L35.00012: Small drops from large nozzles Alfonso Arturo Castrejon-Pita, Ahmed Said Mohamed, Jose Rafael Castrejon-Pita, Miguel Angel Herrada We report experimental and numerical results of the generation of drops which are significantly smaller than the nozzle from which they are generated. The system consists of a cylindrical reservoir and two endplates. One plate is a thin metal sheet with a small orifice in its centre which acts as the nozzle. The other end consists of a piston which moves by the action of an elecromechanical actuator which in turn is driven by sine-shape pull-mode pulses. The meniscus (formed at the nozzle) is thus first overturned, forming a cavity. This cavity collapses and a thin and fast jet emerges from its centre. Under appropriate conditions the tip of this jet breaks up and produces a single diminutive drop. A good agreement between the experimental and numerical results was found. Also, a series of experiments were performed in order to study the effects that the pulse amplitude and width, together with variations in the liquid properties, have over the final size of the droplet. Based on these experiments, a predictive law for the droplet size has been derived. [Preview Abstract] |
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