Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session T17: Drops: Coalescence |
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Chair: Sangjin Ryu, University of Nebraska Lincoln Room: North 131 AB |
Tuesday, November 23, 2021 12:40PM - 12:53PM |
T17.00001: Coalescence of viscous drops Paul R Kaneelil, Amir Pahlavan, Nan Xue, Howard A Stone The coalescence of two viscous drops that are spreading near one another is a phenomenon that is rich in physics. As the two drops meet, a liquid bridge forms along the axis connecting the centers of the two drops. The width of the bridge has been shown to exhibit a power law growth at early times and the height profile of the bridge along the symmetry axis has been shown to have a local self-similar profile. Here, we study experimentally and theoretically the coalescence of two viscous drops on a highly wetting substrate. We use interferometry to resolve the evolution of the height profile in space and time and further investigate its self-similarity. |
Tuesday, November 23, 2021 12:53PM - 1:06PM |
T17.00002: Lifetime of an oil droplet coalescing with a water bath Nikhil S Shirdade, Varun Kulkarni, Suhas R Tamvada, Venkata Yashasvi Lolla, Sushant Anand The extent of partial coalescence of an oil droplet when it gently deposits on a pool of water is often the rate determining step in its complete merger with the bulk. For oil spills, it translates not only to a longer time before the droplet forms a slick but also markedly different spreading behavior. In this work, using high speed video imaging complemented by theoretical modeling, we show for the first time that this early time spreading behavior of the droplet can be oscillatory or reach a constant value, asymptotically, differing from the well documented, power law, spreading behavior for an oil film. We attribute this to the strength of the capillary waves and role of inertia which is diminished greatly at later times and is dependent on the droplet viscosity. Our results are rationalized using a phase diagram demarcating the different regimes, scaling laws governing the size of the daughter droplet, and the effect of droplet viscosity on the total coalescence time. We envision our results to greatly expand our knowledge and understanding of oil spills, in particular, and oil-water interactions in general. |
Tuesday, November 23, 2021 1:06PM - 1:19PM |
T17.00003: Growth and arrest of the cusp formed by coalescing yield stress drops Vanessa R Kern, Torstein Sæter, Andreas Carlson Yield stress fluids are ubiquitous in everyday life and burgeoning in research. Here we experimentally investigate the evolution of the cusp formed by two coalescing yield stress fluid drops using high speed imaging technology. In the short-time post coalescence the formed liquid bridge grows self-similarly balanced by viscosity and surface tension. At long-time the drop's liquid-gas interface arrests in a non-equilibrium shape influenced by yield stress. The influence of yield stress, contact angle and contact angle symmetry on these initial coalescence dynamics and on the final non-equilibrium rest state of the post-coalesced drops will be discussed, and our results compared against a numerical model for the cusp evolution. |
Tuesday, November 23, 2021 1:19PM - 1:32PM |
T17.00004: Interplay of density ratio, viscosity ratio, and initial offset on the coalescence of droplet pairs in confined shear flow. S M Abdullah Al Mamun, Shima Parsa, Samaneh Farokhirad The hydrodynamic collision between droplet pairs dispersed in another continuous fluid matrix is critical to diverse engineering and real-life applications. Simulating such phenomena is more challenging due to the impact of small-scale interactions that determine large-scale behavior. We investigate the effect of density ratios (60 to 800), viscosity ratios (12 to 60), and the initial offset between two droplets on the collision mode in confined shear flow through simulation based on the free-energy binary-liquid lattice Boltzmann method. For all the case studies performed, Reynolds number and Capillary number were kept 1.0 and 0.01, respectively. There has been found both upper and lower critical initial offset which lead to three different modes of collision, namely reverse-back, coalescence, and pass-over. Our results further reveal that the effect of density and viscosity ratios is significant on such collision modes. Near the critical initial offset, competition between inertia and shear forces causes to show unusual revolution of each droplet around the other and eventual coalescence, as we vary density and viscosity ratios. The effect of all these parameters along with the confinement ratio considered in this study will be discussed in detail. |
Tuesday, November 23, 2021 1:32PM - 1:45PM |
T17.00005: Coalescence of trapped droplets using a microfluidic device Chinmayee Panigrahi, Cari S Dutcher The phenomenon of coalescence of droplets is crucial to destabilization of emulsion systems. For emulsions stabilized by surfactants, the adherence of a droplet to a solid boundary can greatly affect the time scales of coalescence. A microfluidic device with a T-junction and a hydrodynamic trap is employed to characterize the coalescence of trapped droplets. Micrometer sized water droplets are first formed in a T-junction by the shearing of a surfactant-laden hydrocarbon continuous phase. The size of the droplet can be controlled by altering the ratio of the flow rates of both phases. A passive hydrodynamic trap is utilized to capture the newly generated droplet. The continuous generation of droplets allows for coalescence to occur at the trap. The series of coalescence is captured with a high-speed camera and the film drainage time is measured against the varying concentration of surfactant, degree of confinement, and incoming droplet velocity. Coalescence is quantified by the dimensionless drainage time plotted against the capillary number, and consideration is given to the appropriate scaling for the different experimental regimes. |
Tuesday, November 23, 2021 1:45PM - 1:58PM |
T17.00006: Non-liquid dynamics during the coalescence of colloidal and cellular aggregates Haicen Yue The coalescence of liquid-like aggregates made of mesoscopically large units – such as colloids or epithelial cells – is ubiquitous in industrial applications dealing with emulsions, and in studies of living biological tissues. In these systems inertia plays little role, thus we numerically study the coalescence of both particulate and biological aggregates via Brownian dynamics. We find that the physics of aggregate coalescence is different from liquid droplet coalescence, both at early stages (where thermal capillary effects are important) and at intermediate and late stages (where we can compare with direct solutions of the Navier-Stokes equation). We also find quantitatively different scaling laws for the neck growth in our two microscopic models, which we attribute to the importance of “metric” vs “topological” interaction potentials. We use these discrete models to connect both the microscopic force laws and the equations of motion to the macroscopic behavior of these unconventional soft matter systems. By comparison with the well-understood properties of conventional liquid drops, we uncover a range of previously unidentified possibilities for how meso-scale droplets can coalesce. |
Tuesday, November 23, 2021 1:58PM - 2:11PM |
T17.00007: Confinement Effects on Drop Coalescence: An Experimental Study Using Hele-Shaw Cell Haipeng Zhang, Ko Okumura, Sangjin Ryu When an initial contact occurs between two drops, a liquid bridge is formed between them and grows rapidly. Although the evolution of the liquid bridge is driven by surface tension during the coalescence process, there exist regimes dominated by viscous or inertial force, which are identified by scaling relations of the temporal growth of the bridge. These scaling relations have been obtained by studying unconfined drops (i.e., spherical drops) or partially confined drops (i.e., drops resting on a plane). Considering drop coalescence can happen in a narrow confinement, we experimentally investigated drop coalescence in Hele-Shaw cell devices, which were formed by two parallel hydrophobic surfaces with controllable spacing. The growing bridge of drops coalescing in the Hele-Shaw cell was captured using high-speed video microscopy, and the temporal evolution of the bridge width was measured by using image processing. Obtained scaling relations of the bridge width showed noticeable differences from the unconfined and partially confined cases, which is expected to contribute to unveiling effects of the confinement on drop coalescence. |
Tuesday, November 23, 2021 2:11PM - 2:24PM |
T17.00008: Droplet coalescence in air: formation and expansion of the bridge Philippe Tordjeman, Véronique Chireux, Frédéric Risso In numerous natural or man-made processes, from clouds formation to emulsions separation, coalescence drives the size growth of droplets. It is a complex phenomenon that involves several mechanisms at various scales and time. In this talk we will describe the main two mechanisms that control the two stages of droplet coalescence in air. The first one concerns the jump-to-contact instability which has been evidenced by Atomic Force Microscopy (AFM). This hydrodynamic instability due to van der Waals forces occurs at the nanoscale and is responsible for the bridge formation between two droplets when the two facing surfaces are separated by a distance of around 10 nm. Specific experiments by Frequency Modulation-AFM have been developed to measure this critical distance and point out that its value is fixed by the Hamaker constant and the surface tension of the liquid. |
Tuesday, November 23, 2021 2:24PM - 2:37PM |
T17.00009: Dynamics of non-coalescence in multi-drops suspended in insulating medium under electric field Subhankar Roy, Rochish M Thaokar An intriguing experimental observation in electrocoalescence of emulsions is the very low critical electric field (Ca) beyond which chaining and shorting is observed unlike the experimental and theoretical predictions in the case of two water droplets in oil. A numerical, analytical and experimental study on interaction between multiple conducting drops in a dielectric medium under an electric field is presented here. Non-coalescence of two conducting drops in a dielectric medium has been studied before. However, the presence of a third drop drastically changes the physics of approach, contact and further coalescence or non-coalescence. The interacting drops are bridged using an analytical model and numerically solved for asymmetrical double cones coming in contact. Thereafter their evolution under electric field is studied to observe the non-coalescence phenomenon. Findings suggest that a smaller drop between two bigger droplets hinder coalescence, effectively changing the critical Ca, and affecting the impact of drop sizes. Phase diagrams showing regimes of coalescence and non-coalescence are plotted as a function of the said parameters. These results suggest that theoretical studies should be able to predict the very low critical Ca observed in electrocoalescence of emulsions |
Tuesday, November 23, 2021 2:37PM - 2:50PM |
T17.00010: Surfactant-induced Marangoni stresses in drop-interface coalescence Cristian Ricardo Constante Amores, Assen Batchvarov, Lyes Kahouadji, Seungwon Shin, Jalel Chergui, Damir Juric, Omar K Matar We study the effect of surfactants on the dynamics of a drop-interface coalescence using fully-three-dimensional direct numerical simulations. We employ a hybrid interface-tracking/level-set method, which takes into account Marangoni stresses that arise from surface tension gradients, interfacial and bulk diffusion, and sorption kinetic effects. We validate our predictions against available experimental data, and perform a parametric study that demonstrates the delicate interplay between the flow fields and those associated with the surfactant bulk and interfacial concentrations. The results of this work unravel the crucial role of the Marangoni stresses in the flow physics of coalescence with particular attention paid to their influence on neck reopening dynamics in terms of stagnation-point inhibition, and near-neck vorticity generation. We demonstrate that surfactant-laden cases feature a rigidifying effect on the interface compared to the surfactant-free case, a mechanism that underpins the observed surfactant-induced phenomena. |
Tuesday, November 23, 2021 2:50PM - 3:03PM |
T17.00011: Drop deposition on wetted surface Laurent Duchemin, Christophe F Josserand When a liquid drop is gently deposited on a miscible thin liquid film, the coalescence dynamics is driven by the lubrication of both the gas interlayer and the liquid film. It induces in particular a slip velocity at the film-air interfaces that accelerates the coalescence process by comparaison with deposition on a solid substrate. Here, we develop and investigate numerically a model based on the coupling of two lubrication equations with the drop deformation. We observe interfaces deformation in good agreement with existing experiments. Finally, we characterize how the coalescence is influenced by the slipping of the lubrication layer, through the analysis of the contact time. |
Tuesday, November 23, 2021 3:03PM - 3:16PM |
T17.00012: Film drainage between a moving droplet and liquid layer Athanasios Boutsikakis, Pierre Elyakime, Roel Belt, Dominique Legendre In a pipe flow with crude oil and water, a dense packed layer of water droplets in oil can form at the interface which can strongly impact pressure losses, especially when oil viscosity increases. The water in the dense packed layer reduces the amount of water in the free water layer and therefore the ratio between oil-wetted and water-wetted perimeter (hence wall friction). It is experimentally observed that a flowing dense packed layer is more stable than a static one. A dense packed layer is rather complex, so we simplify the problem by looking at a single droplet in contact with a liquid layer. Experiments have showed that the coalescence between a moving droplet and a liquid layer is delayed with respect to a static droplet and liquid layer. The purpose of this work is to explain this behavior by DNS of the aforementioned multiphase configuration. Two effects are in play: The first one is a flow-induced lift force exerted on the droplet in the direction away from the liquid layer. The second one is a lubrication force, induced by the motion of the droplet itself, pushing the droplet away from the liquid layer (Reynolds' lubrication theory), similarly to a slider bearing. An attempt to quantify and understand the physical mechanism with the strongest impact is then made. |
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