Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D12: Drop Coalescence |
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Chair: Paul Steen, Cornell University Room: Georgia World Congress Center B217 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D12.00001: Study of inertial coalescence of droplets on a solid substrate using lattice Boltzmann modeling Nilesh D Pawar, Supreet Singh Bahga, Sunil R Kale, Sasidhar Kondaraju Whenever two liquid drops contact, they coalesce to form a single droplet to minimize the surface energy. It is a fundamental process and has a wide range of applications in droplet condensation, microfluidic devices, inkjet printing, and powder metallurgy. Earlier studies were focussed on the coalescence of free droplets. However, in many applications, e.g., dropwise condensation, coalescing drops are in contact with a solid surface. Besides, the coalescence dynamics of sessile drops on a surface deviates significantly from that of free drops. In the present work, we focus on the early-stage coalescence of drops on a solid substrate in the inertial regime. We use a pseudo-potential lattice Boltzmann method to simulate coalescence of equal size droplets and the coalescence of unequal size droplets. Our simulation results show that for a surface with contact angle below 90°, bridge height grows with time and follow h∼t2/3. We also show that the rescaled bridge profile at different times collapses into a single curve. We supplement our LBM simulation results with scaling analysis. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D12.00002: The role of inertia in coalescence of drops in liquid-liquid emulsions Vishrut Garg, Krishnaraj Sambath, Sumeet S Thete, Hariprasad J Subramani, Osman A Basaran The collision and coalescence of liquid drops immersed in a second liquid play a critical role in deciding the fate of liquid-liquid emulsions which are ubiquitous in nature, e.g. oil water emulsions and food products. We simulate the approach, collision, and eventual coalescence of two equal-sized drops of radius R immersed in an ambient liquid where both liquids are incompressible Newtonian fluids, using a Galerkin finite element based algorithm. The governing continuity and Navier Stokes equations are augmented to account for long range van der Waals interactions that become significant as the separation between the drops falls below the order of a few hundred nanometers. For ambient fluids with low capillary numbers Ca = μGR/σ, where μ is the viscosity of the ambient liquid, G is the strain rate of the compressional flow imposed on the ambient liquid, and σ is the interfacial tension, the drops are seen to rebound on first approach before coalescing on approaching each other again, leading to a departure from existing scaling theories for drainage times. We examine the significance of inertia in causing the drops to rebound, resulting in increased drainage times. |
(Author Not Attending)
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D12.00003: Abstract Withdrawn
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Sunday, November 18, 2018 3:09PM - 3:22PM |
D12.00004: Hindered coalescence in the presence of insoluble surfactants Carolina Vannozzi For moderate Capillary numbers, Ca=(0.05-0.3), simulations and scaling of two viscous drops undergoing a flow-induced head-on collision in a viscous matrix (Stokes flow limit) showed that coalescence is hindered, i.e. the film between the drops reached a steady state thickness hss. Here, we analyze this phenomenon in the presence of insoluble surfactants both for high and low surfactant interfacial concentrations (C), via the boundary integral method [1]. We find that hss in the presence of surfactant is smaller than hss for the clean interface case and it decreases as the surfactant interfacial diffusivity Ds decreases. Contrary to intuition, hss decreases with increasing C, i.e. high C systems are less stable to coalescence than low C ones, for the same Ds. Moreover, for high C , as Ds decreases, the film drains continuosly with time, without an oscillatory behavior. A comparison between surfactant cover drops and clean drops, characterized by different viscosity ratios with respect to the matrix, shows that drops with surfactants, even in trace amounts, behave like highly viscous drops, with viscosity ratios O(100). [1]Vannozzi, C. Physics of Fluids 2012, 24.
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Sunday, November 18, 2018 3:22PM - 3:35PM |
D12.00005: PLIF Studies on Surfactant Distribution during Drop/Interface Coalescence Teng Dong, Weheliye Hashi Weheliye, Panagiota Angeli In this work, High-speed Laser Induced Fluorescence was used for the first time to detect the distribution of the fluorescent surfactant NBD-PC during the coalescence of aqueous drops with liquid/liquid interfaces. The aqueous phase was a 46% glycerol solution while the organic phase was a 5 cSt silicone oil. To vary the surfactant concentration on the interface, the drop and the corresponding interface were allowed to stand for different times (0 ~ 900 min) before the experiments. After the drop contacted the interface, the surfactants on the contacted region were swept away by the draining liquid in the trapped film and peaked at a barrier ring area, 0.75Rh away from the centre, where Rh is the horizontal drop radius. Once the thin film retracted after the rupture, the surfactant concentration peaked at the neck tip and continued to increase during the film retraction. At the later coalescence stages, the drop took the shape of a cylinder due to the capillary waves on the drop surface. The surfactant concentration was found to be low at the top of the cylinder where the interface was stretched. As the liquid cylinder started to shrink at the final stages the surfactant concentration at the top increased significantly. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D12.00006: Drop Rest Phenomena on Moving Liquid/Liquid Interface Weheliye Hashi Weheliye, Teng Dong, Panagiota Angeli When a drop approaches the surface of its homophase through another immiscible liquid, it may rest on the interface to allow the trapped film of the other phase to drain out. In this work, the rest phenomena of drops on moving liquid/liquid interfaces prior to coalescence were studied. The experiments were carried out in a flow channel (5 cm × 5 cm square section and 1 m length) with a layer of aqueous phase at the bottom and an organic phase on the top. The channel had two openings at the inlet and the outlet with 5 mm height to enable the flow of the aqueous phase at the channel bottom. To vary the properties of the fluids, glycerol solutions at different concentrations (0%, 25%, 45%) were used as the aqueous phase while 5 cSt silicone oil and Exxsol D80 oil were used as the organic phase. The rest times were found to increase significantly with increasing interface speed; this increase was more prominent with the less viscous oil. The velocity fields near the interface and the structure of the thin film between the drop and the interface were studied with high-speed imaging to better understand the mechanism of film drainage.
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Sunday, November 18, 2018 3:48PM - 4:01PM |
D12.00007: Sweeping by Sessile Drop Coalescence Jonathan M. Ludwicki, Paul H. Steen Condensation is a ubiquitous heat transfer process that manifests as either dropwise or filmwise, depending on the wettability of the condensing surface. In practice, the condensation mode is often filmwise owing to easier control. However, dropwise condensation has gained recent attention due to its higher heat transfer coefficient. During dropwise condensation onto a cooled surface, vapor condenses as drops. Maximal heat transfer is favored by condensing onto fresh surface since smaller drops have a higher heat flux per unit footprint. An important mechanism of fresh surface generation is the sweeping up of nearby drops by a coalescence event. In this talk, we report on sweeping induced by sessile drop coalescence. Our focus is on how static contact angle influences the area swept by the dynamic motions of the coalescence process. Experiments are used in conjunction with numerical simulations to show how different surface types affect coalescence sweeping for a solid-water-air system. Larger swept areas are observed as static contact angle increases, which is explained in terms of surface energy conversion to kinetic energy. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D12.00008: Coalescence and non-coalescence of two aqueous drops in insulating oil under an electric field Vikky Anand, Vinay A Juvekar, Rochish Thaokar The coalescence of water drops, dispersed in oil, is critical to the destabilization of a water-in-oil emulsion. We report some new results, (i) The cone angle for the non-coalescence of drops can be significantly smaller (as small as 19o) than the reported value of 30.8o by Bird et al. (2009). (ii) A surprising observation of the dependence of the mode of coalescence/non-coalescence on the type of insulating oil is seen. A cone-cone mode for silicone oil is observed as against cone-dimple for castor oil (iii) The critical capillary number and the critical cone angle are found to be sensitive to the conductivity of the dispersed phase. (iv) The critical capillary number independent on the applied frequency, and (v) The apparent bridge during non-coalescence is indeed transitory and not permanent. The physical mechanism and thereby numerical calculations to explain the occurrence of very small non-coalescence angles, as well as the dependence of the phenomenon on the conductivity of the insulating oil and the water droplets, remain unexplained. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D12.00009: Enhancing coalescence of water-immersed oil drops with oil films via microtexturing of solid surfaces. Chan Jin Park, Ho-Young Kim Controlling the coalescence behavior of oil drops dispersed in water plays an important role in industrial processes, such as oil separation and oil spill recovery, as well as in our mundane activities of cooking. A typical experimental setup to study the oil-oil interactions in water involves a flat solid surface covered with a thin film of oil that faces downward to meet a rising oil drop. The collision does not lead to immediate coalescence of the drop and the film, but rather a significant wait time should elapse before the water film separating the oil masses is drained to cause the oil interfaces to meet eventually. Here we show that oil drops colliding with oil films covering microtextured, instead of flat, solid surfaces exhibit immediate coalescence. Our high-speed imaging experiments visualize the oil-oil coalescence initiating at the edges of the micropillars where the oil-water interface is displaced by an oil-oil interface. We rationalize this immediate coalescence by considering water drainage rate facilitated by the gaps of micropillars. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D12.00010: Satellite formation during the coalescence of unequal sized drops Hiranya Deka, Gautam Biswas, Amaresh Dalal Contrary to intuitive expectations, the coalescence of two unequal sized drops does not necessarily result in their complete convergence into a single larger mass in an effort to minimize the surface energy. Rather, the process often pinches off a smaller daughter drop, also known as satellite drop. We have performed comprehensive numerical investigations using Coupled Level Set and Volume of Fluid (CLSVOF) method to unveil the mechanism of partial coalescence. The capillary waves are generated owing to the sharp curvature produced near the contact region. We reveal that the deformation of the bottom drop because of the downward pull generated by the capillary waves propagating along its interface affect the pinch-off dynamics. Local curvature of the neck plays a crucial role in the pinch-off dynamics. A sharper axial curvature of the neck increases the local capillary pressure and restricts the pinch-off. The critical diameter ratio above which a satellite pinches off during the coalescence of two free falling drops increases with increasing relative strength of the viscous force and gravity force. The critical diameter ratio for satellite generation can be as low as 1.2 at lower strength of gravity and viscous forces. |
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