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 S21: Drops: Spreading and Wetting II |
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Chair: Vladimir Alvarado, University of Wyoming Room: 603 |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S21.00001: Water Droplet Spreading On Stochastic Si Nanowires Damiano Auliano, Manuel Auliano, Maria Fernandino, Pietro Asinari, Carlos Dorao Wicking plays a key role in many industrial and biological applications such as passive capillary-driven cooling technology, oil recovery, inject printing and DNA chips. Nowadays, the attempt to design an appropriate surface to enhance the wicking capabilities is getting always more significant. In fact, the control of the wetting properties has increased enormously over the past few decades. These properties are strongly related to the spreading features of a surface. Due to the advances in nanotechnology, it has been possible to tune the wettability of a surface by modifying its topography. Different works have reported the effect of the height of micro/nano structures on the wicking transport, but they have shown contrasting results. In this work, the effect of the height on Si random nanowires has been investigated. In this regard, different nanowire arrays with different heights were fabricated. Therefore, a method to characterize accurately both the wettability and wicking of super-hydrophilic surfaces has been developed and validated. After a proper characterization of the samples, it has been observed that taller nanowires provide stronger wicking. [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S21.00002: The Spreading of Superfluid Drops Matthew Wallace, David Mallin, Michael Milgie, Kenneth Langley, Andres Aguirre-Pablo, Sigudur Thoroddsen, Peter Taborek We have used video microscopy and interferometry to investigate the spreading of normal and superfluid helium drops impacting on a sapphire substrate in a saturated atmosphere of its own vapor. We find that in spite of having zero viscosity, the short-term spreading of superfluid drops (time t less than 30 ms) is nearly identical to normal helium; in both cases, the drop spreads to a characteristic diameter of 5 mm and assumes a pancake-like shape. Both normal and superfluid drops shrink with time; the normal fluid drops last up to 15 minutes, but the superfluid drops last only 2-15 seconds. Superfluid drops shrink via superflow through a fluid film at the contact line, flowing at the Feynman critical velocity. Remarkably, the drops undergo a two-phase geometry-dependent retraction. During the first phase, the drop is toroidally-shaped and the radius shrinks linearly in time; in the second phase the drop assumes the shape of a spherical cap and shrinks with the square root of time. Superfluid outflow causes the drop edges to become ragged and frayed, and causes droplets of fluid that appear to form spontaneously outside the expanding drop (exodroplets.) We provide detailed maps of drop topography and contact angle. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S21.00003: Simulation of Droplet Spreading Dynamics by Particle Finite Element Method Based Model and Hydrodynamic Lubrication Elaf Mahrous, R.Valery Roy, Alex Jarauta, Marc Secanell, Pavel Ryzhakov Modeling droplet dynamics is an active area of research. Of particular interest is the prediction of spreading rates and spatio-temporal evolutions of droplets of varying physical properties such as surface tensions on substrates with different wettability. We adopt two distinct numerical approaches: Particle finite element method (PFEM) and hydrodynamic lubrication. PFEM is advantageous due to its ability to track evolving fluid domains and provide an accurate mesh-based boundary description which facilitates the computation of the surface tension in droplet problems. An alternative modeling approach is hydrodynamic lubrication which is based on a small slope approximation, thus requiring small contact angles. The key advantage of this approach is its low computational requirements. Using a no-slip prevents any contact line movement, and applying perfect slip leads to unrealistically large velocities. A particular challenge is to relieve this singularity while accounting for dynamic contact angles. We show how this can be resolved for both methods. Also, we compare their contact line velocities with hydrodynamic analytic model, as well as with experimental results of capillary driven water droplets on substrates with widely differing properties. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S21.00004: Oil drop spreading on a liquid substrate Varun Kulkarni, Suhas Tamvada, Sushant Anand Several systems in nature such as oil spills, food dressings, and pharmaceutical drugs demonstrate an interaction of oil and water. The process of merger of a single drop and the underlying bulk liquid represents one such scenario which has so far not received much attention in literature. In this study we use high speed imaging to experimentally investigate the dynamics of drop deformation and spreading during a gentle deposition of an oil drop onto an air-water interface. After contact with the bulk the merger of the oil drop and its spreading is found to either proceed in stages or instantly depending on the viscosity of the oil drop as characterized by its Ohnesorge number ($Oh=\mu_{p} \mbox{/}\sqrt {\rho_{p} \sigma D_{p} } )$. The topological features during transition scale as a function of the drop viscosity and are theoretically validated using appropriate force balances. It is found that the spreading behavior of the drop depends dominantly on the viscosity of the oil, ultimately determining the extent of film coverage over the liquid substrate. To validate the spreading behavior of the oils, a theoretical model based on the damped harmonic motion is also presented. [Preview Abstract] |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S21.00005: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S21.00006: Water-air dynamic contact angle hysteresis on rough metal substrates Marina Machado, Joseph Murphy, William Rice, Vladimir Alvarado Ice adhesion measurements for impact ice and statically frozen water are critical for solving important aerospace challenges, such as ice-resistant coatings. The surprisingly large spread of ice adhesion values obtained on seemingly very similar materials indicate that additional considerations, such as surface roughness and contact-line spreading, need to be incorporated into the analysis of ice adhesion. In this work, we focus on dynamic water-air contact angle on metal substrates of interest, such as stainless steel and aluminum, using a pendant-drop system over a frequency range 0-10 Hz. Surface roughness of metal substrates is measured using confocal microscope profilometry mapping. Dynamic contact angle hysteresis turns out to be a weak function of frequency in the range examined, but a strong function of the type, e.g., anisotropy and spatial correlation, and degree of surface roughness. These results might partially explain differences and large spread reported in the literature regarding impact ice adhesion. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S21.00007: Droplet Depinning under Forcing by Wind and Gravity Edward White, Alireza Hooshanginejad, Roger Simon, Eleazar Herrera Hernandez, Sungyon Lee Partially wetting drops are ubiquitous in nature and industry. In many cases such as rain drops on windows, droplets are subject to combined effects of gravity and wind forcing. Stability of water drops under forcing by wind and gravity is relevant to aircraft icing, heat exchangers, and fuel cells. Recent studies by Schmucker and White (2012) demonstrate a sharp transition in the trend of critical wind speed for droplet depinning from any inclined surface as a function of droplet size. To investigate what marks this self similar behavior, we conduct additional experiments of water drops on an inclined aluminum surface subject to forcing by wind. We find that the transition coincides with the onset of depinning for the advancing contact line. Inspired by this observation, we develop a mathematical model to rationalize the original experimental results of Schmucker and White. [Preview Abstract] |
Tuesday, November 26, 2019 12:02PM - 12:15PM |
S21.00008: Impulse-driven drop Hamed Habibi, Rouslan Krechetnikov Drop deformation and disintegration regimes have been studied in many contexts ranging from an impact on a solid surface or a liquid layer of varying thickness to an aerodynamic shock wave propagating in air and hitting a suspended liquid drop. As a counterpart, we study deformation and disintegration of a sessile drop on a stiff membrane in the context of an impulsive acceleration. The key objectives here are to elucidate the effects of viscosity, surface tension, and wetting on the drop deformation and disintegration as well as to convey the physical mechanisms behind the observed transient responses of the initially static drop of controlled shape and size. Hence, the significant amount of experimental data -- used to map the possible drop morphological changes along with the transitions between them: crown height, radius, and instability wavelength in the crown regimes; drop detachment and disintegration times as well as probability density functions of the secondary droplets in the disintegration regimes -- is interpreted with phenomenological models, scalings, and estimates highlighting the rich multiscale physics of the impulse-driven drop phenomena. [Preview Abstract] |
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