Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session A22: Drops: Coalescence I |
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Chair: Sumeet Thete, Air Products Room: 604 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A22.00001: Internal Flows in Impacting and Coalescing Droplets of Different Surface Tension Thomas C. Sykes, Alfonso A. Castrejon-Pita, J. Rafael Castrejon-Pita, Mark C. T. Wilson, David Harbottle, Zinedine Khatir, Harvey M. Thompson Internal flows determine the extent of advective mixing within coalescing droplets, which can be enhanced by complex flow structures such as internal jets. Good mixing is needed in applications ranging from reactive inkjet printing to open-surface microfluidics, where coalescing droplets often consist of different fluids with distinct properties. The mixing of impacting and coalescing mm-sized droplets, with different properties, in contact with a substrate is studied using two colour high-speed cameras (side and bottom views). This arrangement allows internal and surface phenomena to be distinguished, and hence the true extent and mechanism of mixing to be determined. Given enough lateral droplet separation, the impacting droplet inertia generates a surface jet atop the originally sessile droplet, which can be enhanced or suppressed through Marangoni flow by modifying the surface tension difference between the droplets. Conditions promoting good mixing are established, with practical implications for lateral separation and deposition order. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A22.00002: Numerical investigation of collision of two liquid metal droplets under the influence of magnetic field Jiacai Huang, Jie Zhang, Kirti Sahu, Mingjiu Ni We investigate the collision dynamics between liquid GaInSn droplets under the influence of an applied magnetic field by conducting three-dimensional numerical simulations. The surface tension between liquid metal droplets is much larger than that of the normal liquid droplets. The collision dynamics is influenced by a large number of parameters, namely, the Weber number, Reynolds number, separation distance and the relative velocity of the droplets, along with the applied magnetic field. We have validated our solver by comparing with the earlier experimental results without the magnetohydrodynamics (MHD) effect. Different regimes of coalescence and separation dynamics are observed without the MHD effect and found to show excellent agreements with the earlier experiments. Then we study the effect of the external magnetic field and its direction on the droplet dynamics and regime map of the distinct coalescence behaviors. The underlying physics of the collision dynamics of liquid metal droplets has been analyzed using the resultant flow field. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A22.00003: Coalescence dynamics of a droplet on a sessile droplet Manish Kumar, Rajneesh Bhardwaj, Kirti Chandra Sahu The coalescence dynamics of an ethanol droplet freely falling on a sessile ethanol droplet resting on glass substrate is investigated experimentally using a high-speed imaging system. The static contact angle of sessile droplet on glass surface is found to be 24\textdegree with a standard deviation of 1.3\textdegree . The regime maps showing the partial and the complete coalescence behaviors in the plane of the normalized impact height and the volume of the sessile droplet with the diameter and the volume of the impacting droplet are presented. It is observed that the partial coalescence of satellite droplet occurs when the ratio of the volume of the sessile droplet to that of the impacting droplet is greater than two. The size of the satellite droplet is found to be about 0.1 times the size of that of the impacting droplet, which increases with the increase in the normalized impact height and normalized volume of the sessile droplet. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A22.00004: Coalescence of Liquid Metal Droplets with application to metallic 3D printing Ryan McGuan, Pirouz Kavehpour, Rob Candler In the field of additive manufacturing, printing functional metallic parts remains the penultimate challenge and goal of research and industry. Metals present many challenges due to high thermal conductivity, their tendency to oxidize and sensitivity to thermocapillary effects. Understanding and mitigating these variables is necessary for the process to become workable, scalable and economic. For droplet-based 3D printing, it is essential to study the liquid droplet coalescence on a planar surface presents a challenging crucible of physical forces including viscosity, capillarity, inertial and gravitational body force. This is further complicated in some substances, such as certain liquid and molten metals that undergo oxidation at the surface introducing a ``quasi-film'' confined to the air-metal or liquid-metal (in the case of surrounding the droplet with an immiscible liquid) interface. This oxidation layer introduces inhomogeneity of material properties at the surface, as well as interfacial phenomena that defy traditional models of surface tension or interfacial energy driven interactions. We investigated the coalescence of room temperature liquid metal and its unique features. The coalescence process is studied for metallic droplets with and without oxidation. We observed that many features of this phenomenon that differ from non-metallic drop coalescence. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A22.00005: Droplet propulsion on a superhydrophilic wire induced by coalescence. Allison O'Donnell, Youhua Jiang, Kyoo-Chul Park Prior research has reported that droplets can jump from both superhydrophobic flat surfaces and hydrophobic cylindrical wires upon coalescence. As the surface area of droplets reduces after coalescence, released surface energy transitions to kinetic energy, causing the droplet to jump. Our findings indicate that droplet propulsion induced by the same surface-to-kinetic energy transition also occurs on superhydrophilic wires. In this experiment, droplets are sequentially deposited on an inclined superhydrophilic wire at a controlled rate. As the droplets wet the wire and flow downward, adjacent droplets may merge. In such cases, droplet speeds before and after coalescence are measured. This is repeated with droplets of differing viscosities and surface tensions and on wires of varying diameters. The results suggest that the ratio of increased kinetic energy to available surface energy is less than 4{\%}. Additionally, it decreases as viscosity increases, surface tension decreases, and wire diameter increases. This trend can be attributed to the energy loss from viscous friction as droplets oscillate during coalescence. A scaling law is also provided to explain this trend. The phenomena reported in this study provide new insights into liquid transport on wires. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A22.00006: Self-Similar Coalescence of Liquid Lenses Michiel Hack, Walter Tewes, Kirsten Harth, Qingguang Xie, Jens Harting, Jacco Snoeijer The coalescence of droplets is of key interest to many industrial applications, such as inkjet printing and spray formation. Here, we study the initial stages of coalescence of liquid lenses, consisting of droplets floating on a liquid pool. Using high-speed imaging, we find that the bridge grows with self-similar dynamics, with different scaling laws at low and high lens viscosities, indicating the existence of two coalescence regimes. We provide an analytical description based on the slender geometry of the system, using the two-dimensional thin sheet equations. Excellent agreement is found with the experiments, capturing both the exponents and the detailed spatial structure of the similarity solution in both regimes. Finally, we show that all data collapse on a single curve capturing the full range of the coalescence dynamics. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A22.00007: Shear-induced non-coalescence for contactless droplet bearings Michela Geri, Bavand Keshavarz, Gareth McKinley Coalescence between a drop and a bath of the same liquid can be delayed or even completely suppressed if the lubricating air layer separating the approaching liquid surfaces is replenished. Thermal Marangoni stresses can promote such scenario if an initial temperature difference is imposed between drop and bath. Even when the droplet is left to thermally equilibrate coalescence can be delayed for many seconds, with the delay time being an increasing power law of the temperature difference. Under isothermal conditions coalescence can be suppressed if the droplet is externally forced to move over the liquid surface. This motion continuously replenishes the air cushion separating the droplet and bath while inducing fluid recirculation within both liquids. We use a custom setup that allows us to carefully control the speed at which the drop is kept in motion over the bath. By pinning the droplet to a force transducer we measure the friction force that is generated by the lubricating air flow for different velocities and different fluid viscosities. We discuss our results in view of a lubrication analysis for the air flow within the gap and investigate the possibility of using this technique to generate contactless droplet bearings. [Preview Abstract] |
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