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 Q11: Drop Impact on Solids II |
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Chair: Xiang Cheng, University of Minnesota, Twin Cities Room: Georgia World Congress Center B216 |
(Author Not Attending)
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Q11.00001: Drop impact on rigid substrate with controlled motion Thanh-Vinh Nguyen, Isao Shimoyama We report a method to reduce the contact time of a droplet impacting on a solid substrate by controlling the substrate motion. This motion is sinusoidal and triggered by a detection of the moment of impact using a MEMS-based force sensor fabricated on the substrate. We show that substrate motion can reduce the contact time by ~50% for a flat superhydrophobic surface. The most efficient reduction of the contact time was reached when moving the substrate with a period similar to the spreading time of the droplet. Especially, combining substrate textures and its motion could lead to a reduction of approximately 80% of the contact time. |
Tuesday, November 20, 2018 1:03PM - 1:16PM |
Q11.00002: Impact of liquid droplets on heated surfaces Zhizhao Che, Song Rong, Shiquan Shen, Tianyou Wang The phenomenon of droplet impact on heated surface happens ubiquitously in nature and in a wide range of industrial applications which include but are not limited to spray cooling and fuel-spray impingement in internal combustion engines. The impact process could be significantly affected by the heat transfer or phase change. We experimentally study the impact dynamics by high-speed imaging using liquids with different viscosities within wide ranges of Weber number and substrate temperature. We identify a mode of droplet bouncing (bouncing-with-spray mode) that can reduce the residence time significantly compared with the traditional retraction-bouncing mode. A scaling law is proposed for the transition boundary between the retraction-bouncing mode and the bouncing-with-spray mode. This study not only provides physical insight into the mechanism of the impact dynamics, but also can be helpful in the optimization of this process in the relevant applications. |
Tuesday, November 20, 2018 1:16PM - 1:29PM |
Q11.00003: Impact of Leidenfrost Drops on Spherical Targets John Sharer Allen, Mitchel McLean The impact of Leidenfrost drops on planar surfaces has been an on-going topic of research. However, the impingement of these drops on spherical targets is less understood despite its important implications for petroleum processing such as in fluid catalytic cracking. In this work, an experimental system was designed to heat metallic targets beyond the Leidenfrost temperature for drop impingement studies. Water drops with Weber numbers ranging from 10 to 45 were impinged on planar and spherical targets with temperatures from 160 °C to 220 °C. Impinging drops covered a larger surface area on a spherical compared to a planar surface. In addition, it was observed that the Leidenfrost temperature depended on the ratio of the drop diameter to the spherical target diameter. For Weber numbers from 10 to 15, drops were observed to rebound off the target. At Weber numbers from 15 to 30, liquid hole formation was observed and analyzed. For greater Weber numbers and temperatures, liquid toroids developed. The toroidal drop dynamics were measured with high speed photography and these results were compared with existing models for planar surfaces. The toroids broke up into a discrete number of smaller drops as a result of a Plateau-Rayleigh instability.
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Tuesday, November 20, 2018 1:29PM - 1:42PM |
Q11.00004: Impact force of liquid drops Leonardo Gordillo, Ting-Pi Sun, Xiang Cheng By synchronizing high-speed photography with fast force sensing, we simultaneously measure the temporal evolution of the shape and impact force of impacting drops on solid surfaces over a wide range of Reynolds numbers (Re). We show that at high Re when inertia dominates the impact processes, the early time evolution of impact force follows a square-root scaling, quantitatively agreeing with a recent self-similar theory. The observation demonstrates the existence of upward propagating self-similar pressure fields during the initial impact of liquid drops at high Re. At intermediate Re when viscous forces set in, we analyze the early time scaling of the impact force of viscous drops using a perturbation method, which shows a similar square-root temporal scaling with the coefficient inversely proportional to the square root of Re. The analysis quantitatively matches our experiments and predicts the trends of the maximum impact force and the associated peak time with decreasing Re. Finally, we also discuss the influence of viscoelasticity on the temporal signature of impact forces. |
Tuesday, November 20, 2018 1:42PM - 1:55PM |
Q11.00005: Impact force of a water droplet impinging on a flat surface with different types of wettability Takahito Saruhashi, Koichi Sakata, Masafumi Yamazaki, Shigeo Kimura, Hiroyuki Abe, Naoki Sakai Localized downpour is one of the reasons causing disasters such as river overflowing and landslides. In order to rescue human lives over these happens, micro aircrafts have been developed to immediately identify risks of secondary calamity. When it flies in Localized downpour, the load applied by raindrops will be applied to the micro aircraft, but any accurate evaluation has not been done yet. For the estimation of the load caused by rain droplets, impact force of water droplets with equivalent diameter of 2.0 mm to 6.5 mm were measured. Super hydrophobicity and hydrophilicity were adopt as surface wettability, and evaluated which surface property was more effective for micro aircraft. |
Tuesday, November 20, 2018 1:55PM - 2:08PM |
Q11.00006: Entrapment of microbubbles during drop impact onto a nano-rough solid surface Kenneth R. Langley, Er Qiang Li, Ivan U. Vakarelski, Sigurdur T. Thoroddsen Drop impacts on solids are ubiquitous in natural and industrial processes. Air that is entrapped during the impact process can negatively impact the quality or effectiveness of coatings or processes. As a drop approaches a solid surface, the pressure in the intervening air layer becomes sufficient to deform the bottom of the drop entrapping a central disc of air upon impact, which has been well studied for smooth surfaces. We investigate the formation of a thick band of microbubbles that is formed surrounding the central air disc during drop impact onto microscope slides coated with a nano-particle based superhydrophobic coating (Rq ≈ 70-140 nm) using ultra-high-speed interferometry at rates up to 5 million fps. The effects of the size and structure of the nano-roughness on the central air disc and the mechanisms for the formation of the microbubbles will be presented. See Langley et al., Soft Matter, 2018, DOI: 10.1039/c8sm01070f |
Tuesday, November 20, 2018 2:08PM - 2:21PM |
Q11.00007: Abstract Withdrawn
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Tuesday, November 20, 2018 2:21PM - 2:34PM |
Q11.00008: Computing the viscous effect in early-time drop impact dynamics Shruti Mishra, Shmuel M Rubinstein, Christopher Rycroft The impact of a liquid drop on a solid surface involves many intertwined physical effects, being influenced by drop velocity, surface tension, ambient pressure and liquid viscosity, among others. Recent experiments by Kolinski et al. (Phys. Rev. Lett. 112, 134501 (2014)) show that the liquid--air interface begins to deviate away from the solid surface even before contact. This lift-off of the interface starts at a critical time that scales with the square root of the kinematic viscosity of the liquid. To understand this, we study the approach of a liquid drop towards a solid surface in the presence of an intervening gas layer. We solve the Navier--Stokes equations for the liquid, coupled to the compressible lubrication equations for the gas, in two dimensions. With this approach, we recover the experimentally captured early time effect of liquid viscosity on the drop shape at early time, before the drop contacts the surface. By using computation, we are in a position to further probe the flow in the drop, towards understanding the origin of this viscous effect. |
Tuesday, November 20, 2018 2:34PM - 2:47PM |
Q11.00009: Air film failure mechanisms of a drop impacting an inclined surface Allison Kaminski, Ying Sun Drop impact studies have applications in industrial, agricultural and natural settings. Two air film failure mechanisms for drop impact on horizontal lubricated surfaces have been reported. For intermediate We (~1<We<10) drop-film contact is initiated at the center due to the downward motion of the drop’s top surface from impact-induced capillary waves. For larger We (We>O(10)) increased air pressure under the drop causes air film failure to occur much earlier, at a location surrounding the center dimple where the air film is very thin and van der Waals interactions become important. This study examines the development of the air layer beneath a drop approaching an inclined and atomically smooth surface using high-speed total internal reflection microscopy integrated with side-view imaging. A thin film of high viscosity silicone oil was spin coated onto a glass slide to create an atomically smooth surface. Surface tilt angles ranging from 0º to 60º and We ranging from 1 – 100 were studied under various pressures. Both normal and tangential components of We were examined. By varying We, tilt angle, and ambient pressure, different impact regimes, air film failure patterns, and drop-film interactions are established, and results are compared with those of horizontal surfaces. |
Tuesday, November 20, 2018 2:47PM - 3:00PM |
Q11.00010: A numerical study of retracting thin gas films and their role in micro-bubble entrainment Shahab Mirjalili, Wai Hong Ronald Chan, Ali Mani Studying the mechanism of micro-bubble generation in air entrainment processes is important because of its role in numerous industrial and natural flows. These bubbles, ranging between 10-100 microns, can remain underwater for long times due to their low buoyancy and mass transfer rates. The exact mechanism of entrainment is unclear and quantitative information on bubble size distribution and dependence on flow parameters is very limited. Experimental evidence from drop-pool impact events has led researchers to hypothesize that impact events are the main contributor to the generation of micro-bubbles. Namely, in a phenomenon known as Mesler entrainment, very thin air films are entrapped between the pool and the impacting drop. These films have very high aspect ratios and after puncture, shed micro-bubbles while retracting on timescales much smaller than the outer flow timescales. Understanding the effects of such dynamics on micro-bubble entrainment has motivated us to numerically study this problem in 2D and 3D. Using a diffuse interface method, we perform two-phase simulations of retracting thin gas films in initially static liquid backgrounds to gain understanding and gather data that can be used in subgrid-scale modeling of micro-bubble generation. |
Tuesday, November 20, 2018 3:00PM - 3:13PM |
Q11.00011: Film dynamics and lubricant depletion by droplets moving on lubricated surfaces Michael J Kreder, Dan Daniel, Adam Tetreault, Zhenle Cao, Baptiste Lemaire, Jaakko V. I. Timonen, Joanna Aizenberg Lubricated surfaces have shown promise in numerous applications where impinging foreign droplets must be removed easily; however, before they can be widely adopted, the problem of lubricant depletion must be solved. Despite recent progress, a quantitative mechanistic understanding of depletion is still lacking. In this work, we showed that the lubricant film thicknesses beneath, behind, and wrapping around a moving droplet change dynamically with the droplet's speed -- analogous to the classical Landau-Levich-Derjaguin problem. The interconnected lubricant dynamics result in the growth of the wetting ridge around the droplet, which is the dominant source of lubricant depletion. We then developed an analytic expression for the maximum amount of lubricant that can be depleted by a single droplet. Counter-intuitively, faster moving droplets subjected to higher driving forces deplete less lubricant than their slower moving counterparts. The insights developed in this work will inform future work and the design of longer-lasting lubricated surfaces. |
Tuesday, November 20, 2018 3:13PM - 3:26PM |
Q11.00012: Droplet gliding Kartik Regulagadda, Shamit Bakshi, Sarit Kumar Das Recently, it is shown that linear millimetric ridge-like structures on a superhydrophobic surface can significantly reduce the contact time of a bouncing drop (up to 50% during normal impacts). However, the contact time rises when the impacts take place far away from the ridge. Here, we investigate the influence of a ridge on contact time during an oblique impact. The droplet starts to glide resembling a skier in a ski-jump (for particular offset impact configurations) with a remarkable reduction in contact time (up to 70%). We discuss the mechanism of droplet gliding which is very different from normal impacts onto the ridge and relate it to the contact time reduction. |
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