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 B21: Drops: Temperature and Marangoni Effects II |
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Chair: Kirti Sahu, Indian Institute of Technology Hyderabad India Room: 603 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B21.00001: Drop impact on heated nanostructures Lihui Liu, Guobiao Cai, Peichun Amy Tsai Drop impact on a heated surface not only displays fascinating dynamics but also plays a crucial role in many industrial processes, such as spray cooling, metallurgy, and fuel injection. We experimentally investigate the impact dynamics of water droplets on both polished and nanostructured heated silica surfaces, under a wide range of surface temperature and impact speed (i.e., Weber number). For the polished surface, at a low surface temperature (below 200$^{\circ}$C) water drop gently spread on the surface. Whereas, at a higher temperature (above the Leidenfrost point) the droplets completely rebound, with accompanied boiling, atomizing, and eventually splash as the Weber number increases. We show that the nanostructures significantly affect the impact dynamics, compared to that on the polished surface. In particular, the nano-textures can easily prompt splashing, vapor bursting, and jetting. We also found that the surface roughness and packing fraction of the nano-structures influence the impact outcomes. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B21.00002: Marangoni spreading and contracting three-component droplets on completely wetting surfaces Nate Cira, Dieter Baumgartner, Shayandev Sinha, Stefan Karpitschka Marangoni flows are a well-established mechanism for inducing droplet spreading and contraction. In this work, we study the behavior of a three-component mixture (ethanol, water, and propylene glycol) on high energy surfaces. Evaporation driven concentration differences give rise to surface tension gradients and Marangoni flows responsible for three categories of behavior across the ternary concentration space -- contraction, enhanced spreading, and sequential spreading then contraction. Based on the combined effects of each component's evaporation on surface tension, we predict boundaries for each of these behaviors that align well with experimental data across a wide range of concentrations and humidities. We present additional data on how internal flow structures impact droplet shape. Self-expansion and contraction make these droplets suitable for cleaning high energy surfaces. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B21.00003: Solutal Marangoni Flows in Premixed Alcohol-water Solution Hyoungsoo Kim, Han Seo Ko To date, the origin of solutal Marangoni flows in a premixed alcohol-water solution has not been investigated although the solutal Marangoni effects are widely used for various applications including material migration, mixing, coating and cleaning. It is reported that there is a flow transition during the evaporation of the premixed alcohol-water solution. Not from the pre-mixed alcohol-water mixture, we also show that the multiple irregular vortices in an evaporating water drop were created where the cloud of alcohol molecules in the air covered the droplet, which induces a local surface tension variation. Interestingly, the flow pattern is also very similar to the case of the premixed water-alcohol solution droplet. By comparing these two different cases, we tried to explain how the solutal Marangoni flow occurs in the alcohol-water mixture solution. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B21.00004: Bouncing behaviors of an oil droplet in a stratified liquid Yanshen Li, Christian Diddens, Lijun Thayyil Raju, Xuehua Zhang, Kai Leong Chong, Andrea Prosperetti, Detlef Lohse As we had found in Li et al, \textit{Phys. Rev. Lett.} (2019), an oil droplet is able to repeatedly first sink and then bounce up in a vertically stratified ethanol-water mixture. The Marangoni flow at the droplet-liquid interface, caused by the solute (ethanol) gradient, provides the propulsion for the upwards jump. We now further elucidate the mechanism and explore the phase space. Specifically, we find that as the droplet jumps up to the lighter ethanol-rich region, it gets harder for the propelling droplet to overcome its own weight, until finally it stops. At this maximum height, the Marangoni flow continues to propel, homogenizing its surrounding liquid, thus decreasing the propulsion itself, until it ceases. The droplet then sinks while dragging down this uniform liquid layer with it. We find that this uniform ``shielding'' layer eventually vanishes, mainly because of diffusion, so that the droplet bounces up again, continuing the cycle. It is also found that the maximum height increases with decreasing the droplet size. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B21.00005: Marangoni flow influences apparent contact angle of single component volatile liquids on completely wetting surfaces Shayandev Sinha, Dieter Baumgartner, Samira Shiri, Xingkun Man, Masao Doi, Nate Cira Volatile single-component liquids can exhibit apparent contact angles, even on completely wetting surfaces. Numerous reports have established that evaporation, capillarity, and viscous dissipation contribute to establishing this angle. A largely separate body of work has explored thermal Marangoni flows in evaporating droplets on partially wetting surfaces. Recent work has considered the role of both evaporation-driven stabilization and Marangoni flow, and found that both are important for droplet shape when evaporation is fast (Tsoumpas, \textit{et al}, 2015). Here we show how manipulating the substrate alone can alter Marangoni flow and impact the apparent contact angle. We present data on scenarios where faster evaporation leads to a lower apparent contact angle, opposing the prediction from evaporation-driven stabilization alone. Paired data from visualizing internal flows for various liquids and substrates shows that these effects on angle are consistent with observed Marangoni flows. These results underscore the importance of considering Marangoni flow in understanding the shape of evaporating highly volatile droplets on fully wetting surfaces.~ [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B21.00006: Motion of self-rewetting drop on a substrate with a linear temperature gradient Zelai Xu, Junyuan Chen, Haoran Liu, Hang Ding, Kirti Sahu We numerically investigate the dynamic of a self-wetting drop placed on a substrate with a linear temperature gradient. Two distinct drop behaviours, namely, the displacement and the stretching, are observed when it is placed at the location of the minimum surface tension. In the displacement regime, the drop spreads slightly and moves towards one side of the substrate. In contrast, in the stretching regime, the drop continues to spread about its initial location as it experiences a higher surface tension gradient with the increase in the wetting area. Theoretical models with and without viscous drag calculated using the lubrication approximation have been developed in order to predict the critical condition at which the drop undergoes the transition between the displacement and the stretching regimes. By changing the initial position of the drop, it is found that in the displacement regime, initial the migration of the drop obeys an exponential function with time, but it diverges at the later stage due to the increase in the drop deformation. On the other hand, in the stretching regime, we observe two scaling laws, with the wetted length of the drop increasing with time, $t$ as $t^{1/2}$ and $t$, for different combinations of the contact angle and the themocapillary influence. [Preview Abstract] |
Saturday, November 23, 2019 5:58PM - 6:11PM |
B21.00007: Lubrication model for vapor absorption into hygroscopic liquid desiccant droplets Zhenying Wang, George Karapetsas, Prashant Valluri, Adam Williams, Khellil Sefiane, Yasuyuki Takata Liquid desiccant is a hygroscopic aqueous solution widely used in dehumidification processes. In this work, we develop a lubrication type model to describe the vapor absorption process into hygroscopic liquid desiccant droplets. Typically, the mass diffusion at the liquid phase is 10$^{\mathrm{3\thinspace }}$- 10$^{\mathrm{4}}$ times lower than that at the gas phase, therefore we consider a liquid-side model to capture dominant mechanisms. We consider a thin axisymmetric droplet on a hydrophilic substrate which is initially in a thermal-equilibrium state. A precursor film is assumed to exist in front of the contact line, which permits contact line movement avoiding the singularity near the contact line. The absorptive mass flux is approximated combining the Hertz-Knudsen equation and the vapour-liquid thermodynamic equilibrium relationships across the interface. The simulation results predict the evolution of droplet profile in a wide range of air conditions. In the case of vapor desorption, the model indicates the formation of thin film in front of the triple contact line along with water depletion, and provides a plausible explanation for the radiating dendritic crystal patterns that form during the crystallization process of aqueous solution droplets (\textit{Shahidzadeh-Bonn et al. 2008; Hadj-Achour and Brutin, 2014}). In the case of vapor absorption, the model explains the droplet spreading along with water uptake, and indicates the important role of hysteresis force on contact line motion. [Preview Abstract] |
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