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 G33: Drops VII: Wetting and Spreading |
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Chair: Alban Sauret, Princeton University Room: 404 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G33.00001: Shapes of non-circular drops on inclined hysteretic surfaces Nachiketa Janardan, Mahesh Panchagnula The shapes of non-circular drops on inclined hysteretic surfaces have been studied experimentally. The drops have an initially elliptical triple line and are formed by allowing two circular drops to coalesce. The drop is initially at rest on a horizontal substrate at different orientations to the tilt axis. This substrate is then tilted and the drop is allowed to move down the inclined substrate. The moving and sliding angles are measured as a function of the initial triple line topology and surface characteristics. The moving angle is the first critical inclination angle at which the triple line is on the verge of being deformed. The sliding angle is the second critical inclination angle at which the entire drop is in a state of impending motion. It is seen that the drop shape, sliding and moving angle are dependent on the initial conditions and the initial orientation of the drop. The moving angle, which is an indication of the hysteresis force attempting to keep the drop from sliding, is shown to scale with the triple line length, as well as the contact angle hysteresis. In addition, the configurations of the drops during the evolution process are studied to establish the mechanism for the sliding process. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G33.00002: Liquid spreading in the partial wetting regime Amir A. Pahlavan, Michael Chen, Luis Cueto-Felgueroso, Gareth H. McKinley, Ruben Juanes The flow of thin films over flat surfaces has been the subject of much theoretical, experimental and computational research [D. Bonn et al., Rev. Mod. Phys., 2009]. Using the lubrication approximation, the classical mathematical model for these flows takes the form of a nonlinear fourth-order PDE, where the fourth-order term models the effect of surface tension [e.g. H. E. Huppert, Nature, 1982]. This classical model effectively assumes that the film is perfectly wetting to the substrate, whereas partial wetting is responsible for stopping the spread of a liquid puddle. Here, we present experiments of (large-volume) liquid spreading over a flat horizontal substrate in the partial wetting regime, and characterize the four spreading regimes that we observe. We develop a macroscopic phase-field model of thin-film flows that naturally accounts for the dynamic contact angle. Our model therefore permits describing thin-film flows without invoking a precursor film, leading to compactly-supported solutions that reproduce the spreading dynamics and the static equilibrium configuration observed in the experiments. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G33.00003: Drag on Sessile Drops Andrew J.B. Milne, Brian Fleck, David Nobes, Debjyoti Sen, Alidad Amirfazli We present the first ever direct measurements of the coefficient of drag on sessile drops at Reynolds numbers from the creeping flow regime up to the point of incipient motion, made using a newly developed floating element differential drag sensor. Surfaces of different wettabilities (PMMA, Teflon, and a superhydrophobic surface (SHS)), wet by water, hexadecane, and various silicone oils, are used to study the effects of drop shape, and fluid properties on drag. The relation between drag coefficient and Reynolds number (scaled by drop height) varies slightly with liquid-solid system and drop volume with results suggesting the drop experiences increased drag compared to similar shaped solid bodies due to drop oscillation influencing the otherwise laminar flow. Drops adopting more spherical shapes are seen to experience the greatest force at any given airspeed. This indicates that the relative exposed areas of drops is an important consideration in terms of force, with implications for the shedding of drops in applications such as airfoil icing and fuel cell flooding. The measurement technique used in this work can be adapted to measure drag force on other deformable, lightly adhered objects such as dust, sand, snow, vesicles, foams, and biofilms. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G33.00004: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 8:52AM - 9:05AM |
G33.00005: Static and Dynamic Contact Angles of Immersed Ferrofluid Droplets Souvick Chatterjee, Dipanwita Bhowmik, Achintya Mukhopadhyay, Ranjan Ganguly Ferrofluid plug driven micro-pumps are useful for manipulating micro-volume of liquids by providing remote actuation using a localized magnetic field gradient. Inside a microchannel, the ferrofluid experiences combined actions of different relevant body forces. While the pressure, viscous and magnetic forces can be estimated using established techniques, the surface tension force requires information about the contact angle between the ferrofluid and glass capillary wall. We address this phenomenon through experimental characterization of static and dynamic contact angles of oil based ferrofluid (EFH3) droplets on glass surface immersed in pure or surfacted distilled water. The equilibrium static contact angle is found to significantly reduce in presence of a magnetic field. Dynamic contact angles are measured through high-speed imaging as the ferrofluid droplets slide along an inclined glass surface. Variation of contact angle hysteresis, which falls outside the Hoffmann Tanner equation for this case, is also investigated as a function of contact line velocity. A strong dependence is found between the contact angle hysteresis and the wetting time. Findings of the work is useful for designing ferrofluid plug-driven microfluidic plugs for different lab-on-a-chip applications. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G33.00006: Volume-filtered Surface Forces for the Simulation of Contact Lines Gerry Della Rocca, Guillaume Blanquart Although contact lines are present in many industrial processes, there is no universal agreement how to implement these singularities in numerical simulations. Often a dynamic contact-angle law is applied with a slip boundary condition at the contact line. However, most slip boundary conditions require realistic slip lengths much smaller than the largest length scale. At a sufficient numerical grid resolution, this multi-scale problem have a prohibitive computational expense. In this study, volume-averaged source terms are constructed for both a contact-angle restoring surface force and the viscous dissipation in the vicinity of the contact line. Closure terms are proposed for the unresolved slip length scales. Unlike previous studies, no a-priori angle relation is applied and the contact angle evolves naturally from the explicitly represented physics. Simulations of fluid displacement in a channel and droplet spreading are demonstrated with this new framework. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G33.00007: Relaxation of contact-line singularities solely by the Kelvin effect and apparent contact angles for isothermal volatile liquids in contact with air Alexey Rednikov, Pierre Colinet The contact (triple) line of a volatile liquid on a flat solid is studied theoretically. Like with a pure-vapor atmosphere [Phys. Rev. E 87, 010401, 2013], but here for isothermal diffusion-limited evaporation/condensation in the presence of an inert gas, we rigorously show that the notorious contact-line singularities (related to motion or phase change itself) can be regularized solely on account of the Kelvin effect (curvature dependence of the saturation conditions). No disjoining pressure, precursor films or Navier slip are in fact needed to this purpose, and nor are they taken into consideration here (``minimalist'' approach). The model applies to both perfect (zero Young's angle) and partial wetting, and is in particular used to study the related issue of evaporation-induced contact angles. Their modification by the contact-line motion (either advancing or receding) is assessed. The formulation is posed for a distinguished immediate vicinity of the contact line (the ``microregion''), the corresponding problem decoupling to leading order, here up to one unknown coefficient, from what actually happens at the macroscale. The lubrication approximation (implying sufficiently small contact angles) is used in the liquid, coupled with the diffusion equation in the gaz phase. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G33.00008: X-ray imaging technique for studying contact-line statics and dynamics of drops on soft substrates Su Ji Park, Ji San Lee, Jun Ho Lee, Jinkyung Kim, Byung Mook Weon, Jung Ho Je When a drop sits on a soft surface, its surface tension deforms the soft material and creates a wetting ridge. X-ray microscopy is useful to measure the shape of the ridge with high spatial and temporal resolutions. This technique allows us to directly image ridge-growth dynamics in real time. We find that the ridge-tip formation is actually asymmetric and independent of substrate stiffness and growth dynamics. From this situation, we directly measure the solid surface stresses. Our approach is a general technique that can be used to measure surface stresses for soft materials within a wide stiffness range. Finally, we suggest a general framework of the wetting behaviors on a soft solid with the combination of Young's and Neumann's laws in macroscopic and microscopic scales, respectively. X-ray microscopy would be useful for further understanding of contact-line dynamics on soft materials. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G33.00009: Multiscale approach to studying super-spreading: molecular dynamics and continuum-level models Panayiotis Theodorakis, Erich Muller, Richard Craster, Omar Matar We study surfactant-assisted spreading of fluid droplets on solid substrates. The ``super-spreading'' problem is a prime example of complex behavior exhibited by such systems, which is not fully-understood. Continuum-level models disregard molecular architecture effects or the specific interactions between the building blocks of the system; hence, they are unable to provide a physical reasoning for the super-spreading mechanism at the molecular level. Molecular dynamics (MD) simulations are restricted to small systems, and are, therefore, unable to provide a continuum description of spreading. Hence we employ a multi-scale modeling approach to study the problem. We use the Statistical Associating Fluid Theory to estimate the intermolecular potentials between the solvent and the surfactant particles using a top-down coarse-grained approach. As a result, we have achieved quantitative matching of our simulations to experimental macroscopic thermophysical properties. Based on these interactions, we perform MD simulations by taking into account, the molecular architecture of surfactants, and we estimate the relevant microscopic parameters and boundary conditions for use in our continuum description. Results from our multiscale approach will be presented at the meeting. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G33.00010: Effects of the size of the domain in the evolution of thin films Juan Manuel Gomba, Jonatan Raul Mac Intyre, Carlos Alberto Perazzo We investigate theoretically the possible final stationary configurations that can be reached by a laterally confined uniform film of liquid. The liquid is under the action of gravity, surface tension and the molecular interaction with the solid substrate. The governing parameters of the problem are the initial thickness of the fluid, the size of the recipient that contains the liquid, and a dimensionless number which quantifies the relative strength of gravity with respect to the molecular interaction. The uniform film is always a possible final state, and depending on the value of the parameters may exist up to 3 different additional final states, each one consisting in a drop with a thin precursor film. We derive analytical expressions for the energy of these possible final configurations, and from this we analyze which one is indeed reached. We conclude that the fluid may show three different behaviors after perturbation: the system recovers its initial shape for any perturbation, the system evolves towards a drop (if more than one is possible, the final state corresponds is the one with the thinnest precursor film) for any perturbation, or the system ends as a uniform film or a drop depending on the details of the perturbation. [Preview Abstract] |
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