Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M33: Drops XIII: Drop Impact on Dry Surfaces |
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Chair: Irmgard Bischofberger, University of Chicago Room: 404 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M33.00001: Fast microdroplet impact: a high-detail investigation using novel experimental methods Claas Visser, Philipp Frommhold, Sander Wildeman, Chao Sun, Detlef Lohse Optimization of everyday applications (e.g. diesel engines and spray cleaning) requires full control of high-speed microdroplet impact. However, experimental data in this regime is scant. We present a novel method to visualize impact of droplets with a diameter of 50 micometers on hydrophillic and -phobic surfaces, at frame rates beyond 10Mfps. This allowed us to study in high detail the impact dynamics for velocities up to 50 m/s. In addition, the exact droplet shape during spreading was determined by a bottom-view interferometry technique. The study was complemented with numerical simulations, providing a complete and detailed picture of the 3D-flow field during impact. The physics of microdroplet spreading are scale-independent (they are governed by the Weber- and Reynolds numbers alone). This allows to describe and optimize the impact of droplets in a range of industrial applications. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M33.00002: Drop splash on a smooth, dry surface Guillaume Riboux, Jose Manuel Gordillo, Alexander Korobkin It is our purpose here to determine the conditions under which a drop of a given liquid with a known radius $R$ impacting against a smooth impermeable surface at a velocity $V$, will either spread axisymmetrically onto the substrate or will create a splash, giving rise to usually undesired star-shaped patterns. In our experimental setup, drops are generated injecting low viscosity liquids falling under the action of gravity from a stainless steel hypodermic needle. The experimental observations using two high speed cameras operating simultaneously and placed perpendicularly to each other reveal that, initially, the drop deforms axisymmetrically, with $A(T)$ the radius of the wetted area. For high enough values of the drop impact velocity, a thin sheet of liquid starts to be ejected from $A(T)$ at a velocity $V_{jet}>V$ for instants of time such that $T\geq T_c$. If $V_{jet}$ is above a certain threshold, which depends on the solid wetting properties as well as on the material properties of both the liquid and the atmospheric gas, the rim of the lamella dewets the solid to finally break into drops. Using Wagner's theory we demonstrate that $A(T)=\sqrt{3RVT}$ and our results also reveal that $T_c\propto We^{-1/2}=(\rho V^2 R/\sigma)^{-1/2}$ and $V_{jet}\propto We^{1/4}$. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M33.00003: Drop splash on a dry, smooth surface: theory Jose Manuel Gordillo, Guillaume Riboux, Alexander Korobkin In this presentation we develop a theoretical model that faithfully predicts, in wide ranges of values of the Ohnesorge and Reynolds numbers, the initial instant at which a high speed sheet is ejected as a result of the impact of a drop onto a dry, smooth substrate. Moreover, the model is able to faithfully reproduce the temporal evolution of the tip of the sheet. We also find that, while the role of the entrapped air bubble can be neglected in the sheet ejection process, the role of air is critical in the dewetting process of the tip of the sheet from the substrate. The splash transition predicted in the Re-Oh and gas to liquid viscosity ratio, agree well with experimental observations. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M33.00004: Numerical Simulation of Droplet Impact on Dry Solid Surfaces Using the Moment of Fluid Method Yisen Guo, Yongsheng Lian, Mark Sussman The impact of liquid droplets on solid surfaces is a ubiquitous phenomenon in nature and industries. The understanding of the underlying physics involved is critical to many industrial problems such as spray cooling, ink-jet printing, and fuel injection. In this work, we study the droplet impacts on solid surfaces using a Navier-Stokes solver based on the moment of fluid surface representation method. Both dynamic contact angle model and static contact angle model are used. The impacts on both hydrophobic and normal substrates are simulated. The droplet spreading, receding, and rebounding are investigated. Numerical results are compared with experimental results in terms of the droplet base diameters and droplet shapes. Our simulations show that the numerical method can accurately capture the droplet impact phenomena. The simulations also indicate that the dynamic contact models give better match than the static contact angle model. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M33.00005: Viscous boundary layer in splashing drops Michael Chemama, Ravi Singh, Michael Brenner, Shreyas Mandre The discovery that ambient pressure could control the splash of a drop on a solid surface generated renewed efforts to understand the physical mechanisms at work. A recent theoretical analysis [Mandre and Brenner; JFM, 690, 148, (2012)] predicted an initial self-similar evolution governed by the drop's inertia and the viscous drainage of the thin layer of air below. This solution breaks down after surface tension or non-linear inertia terms become important. Viscous effects in the drop were computed and shown to be asymptotically negligible. Here we show that the viscous boundary layer approximation, on which this result relies, can become invalid as there is a crossover between the boundary layer thickness and the typical dynamical length of the self-similar evolution. Whether this happens before or after surface tension sets in can lead to different behaviors. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M33.00006: Swirls and splashes: pressure dependence of the airflow created by drop impact Irmgard Bischofberger, Kelly W. Mauser, Bahni Ray, Taehun Lee, Sidney R. Nagel A drop impacting a solid surface with sufficient velocity will splash and emit many small droplets. However, removing the ambient air suppresses splashing completely. The transition between splashing and non-splashing occurs gradually: decreasing the air pressure systematically delays and eventually fully inhibits the occurrence of a splash. The mechanism by which the surrounding gas affects the drop dynamics remains unknown. We use modified Schlieren optics combined with high-speed video imaging to visualize the airflow created by the rapid spreading of the drop after it hits the substrate. We observe the generation of a vortex ring that is initially bound to the outer edge of the spreading liquid and subsequently detaches from the liquid to form a beautiful toroidal vortex sheet that expands and curls up into a roll. We have studied the dynamics of this vortex as a function of gas pressure and find that the sheet gets progressively smaller as the air pressure is decreased. This suggests a weakening of the vortex strength at low pressure. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M33.00007: Janus surfaces reveal the hidden face of splashing Andrzej Latka, Michelle Driscoll, Sidney Nagel When a drop impacts a dry solid surface, a rapidly moving contact line is created. Subsequently, a thin liquid sheet is ejected from the vicinity of this contact line. The thin sheet then breaks apart to form a splash. Previous work has shown that if the solid surface has a micron-scaled roughness, the thin sheet fails to eject and splashing is suppressed. A striking phenomenon can be observed if the drop impacts a hybrid surface comprised of a rough region, where the initial liquid-solid contact takes place, and a smooth region that is reached by only part of the spreading drop: splashing occurs only where the liquid-solid contact line encounters the smooth surface. Consequently, one observes part of the drop splashing, while the other part spreads on the rough surface undisturbed! The splashing outcome is sensitive to the location of the roughness boundary. Crucially, if the velocity of the contact line as it crosses over into the smooth region is below a threshold velocity, u$_{\mathrm{stop}}$, the drop will not splash even though it has left the rough surface. We describe how this hitherto unidentified characteristic velocity depends on other experimental parameters, such as the liquid viscosity and ambient gas pressure, and discuss the insights it provides into the physical mechanisms underlying splashing. [Preview Abstract] |
Tuesday, November 26, 2013 9:31AM - 9:44AM |
M33.00008: Multiscale liquid drop impact on wettable and textured surfaces Samaneh Farokhirad, Rui Zhang, Joel Koplik, Taehun Lee We present the impact of microscopic liquid droplets on solid surfaces which are flat, or pillared, with either homogeneous interactions or cross-shaped patterns of wettability using numerical simulations. The focus is on relatively low impact velocities leading to spreading or bouncing drops, rather than splashing. Lattice Boltzmann and Molecular dynamics methods are used for nanometer-sized and continuum droplets, respectively, and the results of the two methods are compared in terms of scaled variables. In most situations we find similar droplet behavior at both length scales. The agreements between the methods are reasonable at low impact velocities on wettable surfaces while some discrepancies are observed for strongly hydrophobic surfaces and for higher velocities. [Preview Abstract] |
Tuesday, November 26, 2013 9:44AM - 9:57AM |
M33.00009: Disintegration of a Round Liquid Jet due to Impact on a Superhydrophobic Surface Maziyar Jalaal, Boris Stoeber Liquid jet breakup has several applications such as Inkjet printers, diesel fuel injectors, and paint sprays. Very recently liquid jets have been shown to be useful for small volume transportation (\textit{Clestini et al. Soft Matter}, \textit{2010}), where a micro-scale liquid jet on superhydrophobic surface was investigated. Although the instability of the liquid jet for some circumstances was shown, the disintegration of the liquid jet was not discussed. In the present study, we aim to analyze the breakup of a micro liquid jet due to inclined impact to a superhydrophobic surface. A range of Weber and Reynolds numbers have been explored experimentally. Water-glycerin solution as the working fluid. Generally, it is shown that the liquid jet forms a disc-like film over the surface and further rebounds (``bouncing jet''). A simple energy balance method is provided to estimate the diameter of the disc-like film. It is shown, for the case of low viscosity (large Re), this parameter is logarithmically proportional to the normal Weber number. Additionally, linear stability analysis for viscous jets provides a good estimate of droplet size. From an application point of view, using superhydrophobic surfaces 1) enables rebound of the liquid jet 2) advances the breakup point (shorten the breakup length). [Preview Abstract] |
Tuesday, November 26, 2013 9:57AM - 10:10AM |
M33.00010: Drop impacts on electrospun nanofiber membranes Rakesh P. Sahu, Suman Sinha-Ray, Alexander Yarin, Behnam Pourdeyhimi This work reports a study of drop impacts of polar and non-polar liquids onto electrospun nanofiber membranes (of 8--10 mm thickness and pore sizes of 3--6 nm) with an increasing degree of hydrophobicity. The nanofibers used were electrospun from polyacrylonitrile (PAN), nylon 6/6, polycaprolactone (PCL) and Teflon. It was found that for any liquid/fiber pair there exists a threshold impact velocity (1.5 to 3 m/s) above which water penetrates membranes irrespective of their wettability. The low surface tension liquid left the rear side of sufficiently thin membranes as a millipede-like system of tiny jets protruding through a number of pores. For such a high surface tension liquid as water, jets immediately merged into a single bigger jet, which formed secondary drops due to capillary instability. An especially non-trivial result is that superhydrophobicity of the porous nano-textured Teflon skeleton with the interconnected pores is incapable of preventing water penetration due to drop impact, even at relatively low impact velocities close to 3.46 m/s. A theoretical estimate of the critical membrane thickness sufficient for complete viscous dissipation of the kinetic energy of penetrating liquid corroborates with the experimental data. [Preview Abstract] |
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