Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D13: Drops: Superhydrophobic Surfaces IDrops FSI
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Chair: Arun Kota, Colorado State University Room: 506 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D13.00001: Takeoff, flight and landing of self-jumping microdroplets Pierre Lecointre, Timothee Mouterde, Christophe Clanet, David Quere Drops on highly repellent materials jump after they coalesce. The jumping velocity was shown for millimetric drops to arise from a balance between capillarity and inertia and we first extend this result down to 5 micrometers, that is, in the range of dew. Such micrometric droplets are expelled at velocity as high as 80 cm/s. In contrast, smaller drops are slower, which we describe by taking into account viscous dissipation. Our model also explains why asymmetric coalescences lead to slower jumps. We finally investigate the flight, jumping height and trajectory of these microdroplets until they fall back on the substrate. Although these drops were strongly expelled upwards, it is found that they never bounce when they come back, which is shown to result from a lack of kinetic energy for retaking off. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D13.00002: Coalescence induced self-propulsion of droplets on superomniphobic surfaces. Hamed Vahabi, Wei Wang, Seth Davies, Joseph M.Mabry, Arun K.Kota We utilized superomniphobic surfaces to systematically investigate the different regimes of coalescence-induced self-propulsion of liquid droplets with a wide range of droplet radii, viscosities and surface tensions. Our results indicate that for all the liquids studied, the transition from the inertial-capillary regime to the visco-capillary regime occurs over a narrow range of Ohnesorge number \textit{Oh} $\approx $ 0.02 to 0.05. The non-dimensional jumping velocity $V_{j}^{\ast }$ is nearly constant ($V_{j}^{\ast }$ $\approx $ 0.2) in the inertial-capillary regime and decreases in the visco-capillary regime as the Ohnesorge number \textit{Oh} increases, in agreement with prior work. Within the visco-capillary regime, decreasing the droplet radius $R_{0}$ results in a more rapid decrease in the non-dimensional jumping velocity $V_{j}^{\ast }$ compared to increasing the viscosity $\mu $. This is because decreasing the droplet radius $R_{0}$ increases the inertial-capillary velocity $V_{ic}$ in addition to increasing the Ohnesorge number \textit{Oh}. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D13.00003: An Optical-Based Aggregate Approach to Measuring Condensation Heat Transfer. Kimberly A. Stevens, Julie Crockett, Daniel R. Maynes, Brian D. Iverson Condensation heat transfer is significant in a variety of industries including desalination, energy conversion, atmospheric water harvesting, and electronics cooling. Recently, superhydrophobic surfaces have gained attention as a possible condensing surface due to their potential for high droplet mobility and coalescence-induced, out-of-plane jumping of the condensate droplets, both of which contribute to higher rates of condensate removal and thus higher thermal transport rates. Several studies involving condensation on superhydrophobic surfaces have quantified metrics which indirectly indicate the relative rate of heat transfer on a surface, such as maximum droplet diameter, drop size distribution, and individual droplet growth rates. In this study, an optical-based method is used to monitor growth and departure of individual condensate drops for the entire viewing area to obtain full-field, aggregate heat transfer measurements. This approach offers several advantages relative to traditional heat transfer measurement methods such as heat flux sensors and thermocouples, including the ability to provide a link between macroscopic heat transfer rates and the more indirect measures of heat transfer traditionally reported in the literature. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D13.00004: Suppression of Coalescence-Induced Droplet-Jumping on Superhydrophobic Surfaces by Microstructures Yota Maeda, Peng Zhang, Fengyong Lv, Alexandros Askounis, Biao Sheng, Yasuyuki Takata, Daniel Orejon Superhydrophobic surfaces are receiving increasing attention due to their enhanced condensation performance, which is owed to the ability to shed-off the condensate either by gravity or by coalescence-induced droplet-jumping. In this work we study the effect of microstructures on coalescence-induced droplet-jumping phenomenon. The frequency of the droplet-jumping events is found to decrease when increasing the density and the size of the microstructures. In addition, important differences in the droplet size distribution and in the number of coalescing droplets involved in the jumping events are reported. In the presence of microstructures, bigger and greater number of droplets are required to coalesce for the jump to ensue. Furthermore, we report the suppression of the droplet-jumping performance of droplets in size similar to that of the microstructures. We propose that microstructures introduce a droplet an angular deviation from the main surface normal so, upon coalescence, droplets sitting on the side of the microstructures will not fully contribute to the jump in the out-of-plane direction. As consequence the heat transfer performance due to small jumping droplets is reduced in the presence of microstructures. The authors acknowledge the support of WPI-I2CNER and KAKENHI JSPS. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D13.00005: Superhydrophobicity enhancement through substrate flexibility Thomas Vasileiou, Julia Gerber, Jana Prautzsch, Thomas Schutzius, Dimos Poulikakos Inspired by manifestations in nature, micro/nanoengineering superhydrophobic surfaces has been the focus of much work. Generally, hydrophobicity is increased through the combined effects of surface texturing and chemistry; being durable, rigid substrate materials are the norm. However, many natural and technical materials are flexible, and the resulting effect on hydrophobicity has been largely unexplored. Here, we show that the rational tuning of flexibility can work collaboratively with the surface micro/nanotexture to enhance liquid repellency performance, defined by impalement and breakup resistance, contact time reduction, and restitution coefficient increase. Reduction in substrate stiffness and areal density imparts immediate acceleration and intrinsic responsiveness to impacting droplets, mitigating the collision and lowering the impalement probability by $\sim$60$\%$ without the need for active actuation. We demonstrate the above discoveries with materials ranging from thin steel or polymer sheets to butterfly wings. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D13.00006: Droplet impact dynamics on flexible superhydrophobic surfaces Jeong-Hyun Kim, William Gorman, Jessica Shang In this talk, we will present a series of droplet impact experiments performed on elastic superhydrophobic surfaces. A commercial superhydrophobic paint, WX2100, was sprayed on smooth PDMS surfaces that were prepared by a standard soft lithography technique. The droplet spreading and retraction dynamics, trajectory, and substrate response will be presented for different surface lengths and Weber numbers. The elasticity of the superhydrophobic surfaces was found to affect dynamics of the droplets and the substrate. The contact time of the droplet on an elastic superhydrophobic surface was reduced by as much as 22{\%} compared to the rigid superhydrophobic surface. The reduction in the contact time was even observed at low Weber number, \textit{We}\textasciitilde 20, which was much lower than the critical Weber number reported in literature. A variety of surface deflection behavior was observed after the second impact of the rebounding droplet. When the droplet motion was in phase with the surface motion, the deflection of the surface was found to deviate and increase from the original decay of the surface deflection. However, when the droplet motion was out of phase with the surface, the displacement of the surface was reduced and dampened quickly by the droplet body force. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D13.00007: Maximal expansion and retraction dynamics in drop-on-drop impacts Maher Damak, Kripa Varanasi The phenomenon of a droplet impacting another droplet on a surface is ubiquitous in agricultural sprays, thermal sprays and rain impact on surfaces. Here we experimentally study drop-on-drop impacts on superhydrophobic surfaces and characterize the maximum expansion diameter and retraction time. We extend previous arguments for single drop impacts to model the droplet diameter and thickness at its maximal expansion in the capillary and viscous regimes. We show that our generalized model captures drop-on-drop impacts over a large range of viscosities, impact velocities, droplet sizes and size ratios, and that previous single drop impact models are a particular case of our model. We then investigate the retraction dynamics and show that the retraction phase has no memory of the initial impact dynamics and that single drop retraction times can be used with a new effective length scale, in both the inertial and viscous regimes. Our results expand previous models of droplet impacts and suggest that many results from the single drop impact literature may be extended to multiple drop impacts. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D13.00008: Drop Impact on Hairy Surfaces Alice Nasto, Anette Hosoi Using a combination of experiments and theory, we investigate the effect of a millimeter-scale hairy texture on impact of liquid drops. By varying the speed of the drop at impact and the spacing of the hairs, we observe a variety of behaviors. For dense hairs and low impact velocity, the liquid drop sits on top of the hair, similar to a Cassie-Baxter state. For higher impact velocity, and intermediate to high density of hairs, the drops penetrate through the surface, but the hairs resist their spreading. For low hair density and high impact velocity, the drops impact and splash. [Preview Abstract] |
Sunday, November 19, 2017 3:59PM - 4:12PM |
D13.00009: Numerical study of droplet impact and rebound on superhydrophobic surface Xuan Cai, Yanchen Wu, Martin Woerner, Bettina Frohnapfel Droplet impact and rebound on superhydrophobic surface is an important process in many applications; among them are developing self-cleaning or anti-icing materials and limiting liquid film formation of Diesel Exhaust Fluid (DEF) in exhaust gas pipe. In the latter field, rebound of DEF droplet from wall is desired as an effective mean for avoiding or reducing unwanted solid deposition. Our goal is to numerically study influence of surface wettability on DEF droplet impact and rebound behavior. A phase-field method is chosen, which was implemented in OpenFOAM by us and validated for wetting-related interfacial flow problems. In the present contribution we first numerically reproduce relevant experimental studies in literature, to validate the code for droplet impact and rebound problem. There we study droplet-surface contact time, maximum/instantaneous spreading factor and droplet shape evolution. Our numerical results show good agreement with experimental data. Next we investigate for DEF droplets the effects of diameter, impact velocity and surface wettability on rebound behavior and jumping height. Based on Weber number and equilibrium contact angle, two regimes are identified. We show that surface wettability is a deciding factor for achieving rebound event. [Preview Abstract] |
Sunday, November 19, 2017 4:12PM - 4:25PM |
D13.00010: Atomization of Impinging Droplets on Superheated Superhydrophobic Surfaces Preston Emerson, Julie Crockett, Daniel Maynes Water droplets impinging smooth superheated surfaces may be characterized by dynamic vapor bubbles rising to the surface, popping, and causing a spray of tiny droplets to erupt from the droplet. This spray is called secondary atomization. Here, atomization is quantified experimentally for water droplets impinging superheated superhydrophobic surfaces. Smooth hydrophobic and superhydrophobic surfaces with varying rib and post microstructuring were explored. Each surface was placed on an aluminum heating block, and impingement events were captured with a high speed camera at 3000 fps. For consistency among tests, all events were normalized by the maximum atomization found over a range of temperatures on a smooth hydrophobic surface. An estimate of the level of atomization during an impingement event was created by quantifying the volume of fluid present in the atomization spray. Droplet diameter and Weber number were held constant, and atomization was found for a range of temperatures through the lifetime of the impinging droplet. The Leidenfrost temperature was also determined and defined to be the lowest temperature at which atomization ceases to occur. Both atomization and Leidenfrost temperature increase with decreasing pitch (distance between microstructures). [Preview Abstract] |
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