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 H21: Drops: Impacts with Solids I |
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Chair: Jose Gordillo, Universidad de Sevilla Room: 603 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H21.00001: Droplet Dynamics Post Oblique Surface Impact Vanessa Kern, Cathy Jin, Paul Steen Oblique droplet impact onto dry surfaces is ubiquitous in industry and nature, yet scarce in the literature. Here we investigate the post-impact dynamics of capillary water droplets obliquely impacting homogeneous chemically-prepared hydrophilic and hydrophobic surfaces. We analyze high-speed images of the impact event. Pre-impact Weber numbers range from 0-16. Impingement angles range from 0-53$^{\circ}$. After impact, the droplet spreads and after spreading the droplet pins. Post-pinning, there is a slow decay to the rest state. During this underdamped decay, the droplet's remaining kinetic energy partitions into a linear combination of axisymmetric and non-axisymmetric vibrational modes corresponding to those predicted by theory for the pinned horizontally-aligned (non-oblique) sessile droplet. The influence of droplet volume, impacting angle, and contact angle on these post-impact dynamics will be discussed. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H21.00002: Contact-line Fluctuations and the Dynamics of Wetting. Joel De Coninck Using MD simulations, we have shown previously that the fluctuations about the mean position of the contact line may be interpreted in terms of an overdamped one dimensional Langevin harmonic oscillator of stiffness$ k$ and demonstrated a relationship between the variance of the fluctuations of the contact line, the time decay of the oscillations and the contact-line friction . Here, we extend this work to study the fluctuations when the contact lines are moving, yielding dynamic advancing and receding contact angles that differ from their equilibrium values. A steady dynamic state is achieved by moving the plates in opposite directions at constant velocities . Under these conditions, we obtain an identical Langevin expression to that found at equilibrium, but now with the harmonic term centered about the mean location of the \textit{dynamic} contact line and a fluctuating capillary force arising from the fluctuations of the \textit{dynamic} contact angle around its mean value. .During the talk, we will show that the contact-line fluctuations are the same irrespective of whether the contact line is at equilibrium or moving and contain all the information necessary to predict the dynamics of wetting. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H21.00003: Contact time of water drop on cylindrical superhydrophobic surfaces Choongyeop Lee, Jeonghoon Han, Wonjung Kim, Changwoo Bae, Dong Woo Lee, Seungwon Shin, Youngsuk Nam When the water drop is impinged upon a cylindrical superhydrophobic surface with varying diameter, its contact time with the surface decreases over that on the flat superhydrophobic surface. However, the prediction of the contact time remains challenging due to the complex drop spreading dynamics on cylindrical superhydrophobic surface after impact. Here, based on systematic experimental and numerical studies, we develop a scaling relationship for the contact time reduction on the cylindrical superhydrophobic surface. We show that non-dimensionalized contact time can be expressed as a function of a single dimensionless parameter, which is expressed as the combination of the Weber number and the ratio of the cylinder diameter to the drop diameter. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H21.00004: Role of the Dynamic Contact Angle on Splashing Miguel A. Quetzeri-Santiago, Kensuke Yokoi, Alfonso A. Castrejón-Pita, J. Rafael Castrejón-Pita A drop impacting onto a solid dry substrate can, among other several results, splash or spread over the solid surface. The result depends not only on the droplet properties and speed, but on a wide range of parameters. Although many studies have aimed at finding scaling arguments to characterise splashing, the exact combination of parameters and their influence have remained elusive. In this work we perform a systematic study of liquid droplets impacting onto various solid substrates ranging from completely wetting to superhydrophobic. The experimental approach uses high-speed imaging and image analysis to recover the contact angle as a function of the spreading velocity. We show that, under our experimental conditions, liquids spread with a maximum advancing contact angle greater than 87 degrees, regardless of the liquid or substrate properties. Our results also show that existing dimensionless groups, i.e. the splashing parameter (K) and the capillary number (Ca), are not appropriate to characterise the splashing behaviour. Finally, we show that the splashing ratio $\beta$ and the maximum dynamic advancing contact angle, appropriately divides the splashing and no-splashing behaviour. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H21.00005: Impact dynamics of ferrofluid drop on superhydrophobic surface under horizontal magnetic field Nilamani Sahoo, Devranjan Samanta, Purbarun Dhar In this study, we focus on impact of ellipsoidal or columnar droplet by horizontal magnetic field on a non-wetting substrate and investigates experimentally the effect of Weber number (We) and magnetic Bond number (Bo$_{\mathrm{m}})$. The orthogonal spreading depends on the magnitude of applied magnetic field (manifested through Bo$_{\mathrm{m}})$, since the applied field alters the spherical shape of pre-impact drop into either ellipsoidal or columnar drop. The orthogonal spreading induces non- axisymmetric distribution pre-impact inertial energy along the transverse and longitudinal axes of the applied magnetic field, promoting rebound suppression for a fixed We. With increase in Bo$_{\mathrm{m}}$, the nature of orthogonal spreading is more prominent to break up symmetry retraction as observed experimentally. In addition, at higher orthogonal spreading ratio, the shattering of liquid lamella occurs due to nucleation of holes at different locations after post impact during retraction phase. We can suggest that the shattering of liquid lamella is critically dependent on the thickness of the liquid film after post impact and the wettability of the surface. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H21.00006: A theory on the spreading of droplets Jose Gordillo, Guillaume Riboux, Enrique S. Quintero Here we provide a self-consistent analytical solution describing the unsteady flow in the slender thin film which is expelled radially outwards when a drop hits a dry solid wall. Thanks to the fact that the fluxes of mass and momentum entering into the toroidal rim bordering the expanding liquid sheet are calculated analytically, we show here that our theoretical results closely follow the experimentally measured time-varying position of the rim with independence of the wetting properties of the substrate. The particularization of the equations describing the rim dynamics at the instant the drop reaches its maximal extension which, in analogy with the case of Savart sheets, is characterized by a value of the local Weber number equal to one, provides an algebraic equation for the maximum spreading radius also in excellent agreement with all the experimental data available in the literature. The self-consistent theory presented here provides us with the time evolution of the thickness and of the velocity of the rim bordering the expanding sheet. We also test these predictions and show that our theory is also able to predict the splash threshold velocity when the substrate is superhydrophobic and also the velocities and the diameters of the droplets ejected. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H21.00007: Axisymmetric Lattice Boltzmann Simulation of Droplet Impact on Spherical Surfaces Xin Yong, Hussein Dalgamoni Droplet impact on solid surfaces plays important roles in many engineering applications, in which the ability to exert control over the detailed dynamics is critical. While past studies have established a complete understanding of droplet impact on flat substrates, what we know about the impact dynamics on curved surfaces is limited. This work simulated the normal impact of droplets on spherical surfaces with physical density and viscosity contrasts in the low Weber number regime, in which droplet deformation is assumed to be axisymmetric. We extended our recently developed axisymmetric free-energy lattice Boltzmann method (LBM) to capture droplet wetting and contact line motion on a curved surface. The conventional staircase approximation of curved boundaries was used to reduce computational cost. Its accuracy was validated by the static wetting simulations. The impact simulations show that surface curvature and wettability significantly affected the spreading and recoiling of droplet. Five impact outcomes were observed, which ranges from deposition to total rebound. An impact phase diagram was constructed and correlated with the total contact time to provide guidelines for surface design for anti-icing applications [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H21.00008: Experimental and numerical study of the effect of surrounding gas on splashing at high Weber and Reynolds numbers David A. Burzynski, Stephan E. Bansmer We investigated the influence of the surrounding gas on a droplet impacting a smooth dry glass surface at high Weber and Reynolds numbers. We analyzed experimentally the splashing outcome by measuring the size, velocity, and angle of the secondary droplets and by calculating the total volume ejected. Numerical simulations complement our study by providing detailed information about the flow in the liquid lamella and the surrounding gas. The results show that gas entrapment is not the mechanism responsible for splashing and demonstrate that splashing is influenced first by the density, second by the viscosity, and lastly by the mean free path of the surrounding gas. The simulations are used to estimate the forces acting on the ejected lamella and to compare it with the theory of Riboux \& Gordillo [Phys. Rev. Lett. 113, 024507 (2014)], who proposed that these aerodynamic forces are the responsible for splashing. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H21.00009: Surrounding gas-independent splash at high-velocity drop impact with a projected smooth solid surface Masao Watanabe, Taku Ashida, Kazumichi Kobayashi, Hiroyuki Fujii, Toshiyuki Sanada We study a drop impact on a fast-moving smooth solid plate in a reduced-pressure condition. A water drop of radius $R$=1.1 mm was released from a rest needle in a stainless vacuum chamber; then, the free falling drop was brought into collision with a vertically upward flying solid impact plate which was projected by an iron bullet accelerated by a coilgun. The impact plate consisted of a cover glass with a static contact angle of 60$^\circ$ and a surface roughness $R_a$ of 2.1 nm, adhered to an acrylic plate. The surrounding gas pressure was varied between 1 and 100 kPa, and the impact velocity was varied between 4.2 and 33 m/s. The drop impact on a smooth solid plate with high impact velocity and the subsequent splash was recorded by a high-speed video camera at a frame rate of 1,000,000 fps at pixel resolution of 16.1 $\mu$m/px. In a reduced-pressure condition, preceding the occurrence of corona splash, fine daughter droplets, possibly smaller than those observed in corona splash, fly extremely fast along the solid surface only for a few microseconds. The experimental results show that this splashing is independent of both the surrounding gas pressure and gas species. We discuss the threshold of the occurrence of this splashing and identify the mechanism for this splashing. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H21.00010: Air entrapment under drop impacts on soft solids Kenneth R. Langley, Alfonso A. Castrejón-Pita, S. T. Thoroddsen Prior studies have found that soft solids delay the critical velocity at which drops begin to splash upon impact [1]. We investigate the effects of the surface compliance on the air cushioning at the bottom of a liquid drop impacting onto a soft solid and the resulting entrapment of a central bubble using high-speed interferometry at 5 million frames per second and spatial resolution of 1.05 $\mu$m/pixel. The soft solid delays the effects of gas compressibility. We also observe extended gliding of the drop as it initially avoids contact with the surface and spreads over a thin layer of air and investigate the threshold velocity for the onset of gliding. Such extended gliding layers have previously been seen for high viscosity drop impacts [2], but not for low viscosity drops. Additionally, we observe the dynamics as the drop spreads near the splashing threshold to observe effects of the compliance on the ejected lamellae. [1] Howland, C. J., Antkowiak, A., Castrej\'{o}n-Pita, J. R., Howison, S. D., Oliver, J. M., Style, R. W., {\&} Castrej\'{o}n-Pita, A. A. (2016). \textit{Phys Rev Lett},~\textbf{117} \quad (18), 184502. [2] Langley, K., Li, E. Q., {\&} Thoroddsen, S. T. (2017). \textit{J Fluid Mech},~\textbf{813}, 647-666. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H21.00011: Characterizing the wettability/non-wettability transition during drop impact Harish Dixit, Praveen Sharma Recent experiments have shown a new bouncing mechanism on smooth surfaces where the drop is supported on a thin cushion of gas beneath it. Using incompressible high-resolution computations, we study drop impact dynamics at moderate speeds of impact. We restrict our attention to parameters where drop undergoes complete rebound and show that five distinct regimes can be identified in a Reynolds-Weber number phase diagram. In a broad sense, a stable gas cushion can be formed for low Reynolds number but for a wide range of Weber numbers. With increasing Reynolds/Stokes number, the gas film undergoes rupture either in an annular ring near the drop periphery or near the center of the drop. Each of the five regimes differs in the shape of the gas film near the impact surface. The exact transition boundary between wettability-independent and wettability-dependent bouncing is difficult to determine, but the simulations are found to be in good agreement with known scaling laws for initial deformation height, maximum spreading radius and minimum thickness of the gas film attained. Access to velocity and surface energy data allows us to characterize the energy exchange during the impact process. The obtained coefficient of restitution is again found to be in good agreement with experiments. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H21.00012: Some additional considerations on the splashing of droplets Guillaume Riboux, Jose Gordillo When a drop of a low viscosity liquid impacts against a smooth solid substrate at a velocity $V$, a liquid sheet of thickness very small compare to the drop radius is expelled tangentially to the substrate at high velocity compare to $V$. If the impact velocity is such that $V > V^*$ with $V^*$ the critical velocity for splashing, the edge of the expanding liquid sheet lifts off from the wall as a consequence of the gas lubrication force at the wedge region created between the advancing liquid front and the substrate. In the present talk, we show that the magnitude of the gas lubrication force is limited by the values of the slip lengths at the gas-liquid interface and at the solid. We demonstrate that the splashing regime changes depending on the value of the ratio of the slip lengths, a fact explaining the spreading-splashing-spreading-splashing transition for a reduced value of the surrounding gas pressure as the drop impact velocity increases. We also provide an expression for $V^*$ as a function of the inclination angle of the substrate, the drop radius, the material properties of the liquid and the gas and the mean free path of gas molecules. [Preview Abstract] |
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