Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N39: Drops, Bubbles, and Interfacial Fluid Mechanics II |
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Sponsoring Units: DFD Chair: Patricia McGuiggan, Johns Hopkins University Room: 348 |
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N39.00001: Multimode Multidrop Serial Coalescence Effects during Condensation on Two-Tier Superhydrophobic Surfaces Konrad Rykaczewski, Adam T. Paxton, Sushant Anand, Xuemei Chen, Zuankai Wang, Kripa K. Varanasi Mobile coalescence leading to spontaneous drop motion was initially reported to occur only during water condensation on two-tier superhydrophobic surfaces (SHS), consisting of both nanoscale and microscale topological features. However, subsequent studies have shown that mobile coalescence also occurs on solely nanostructured SHS. Thus, recent focus has been on understanding the condensation process on just nanostructured surfaces rather than on two-tier SHS. Here, we investigate the impact of microscale topography of two-tier SHS on the droplet coalescence dynamics and wetting states during the condensation process. We identify new droplet shedding modes, which consist of serial coalescence events that lead to merging of multiple droplets. The formed drops either depart or remain anchored to the surface. We explain the observed post-merging drop adhesion trends through direct correlation to formation of drops in nanoscale as well as microscale Wenzel and Cassie-Baxter wetting states. We find that optimally designed two-tier SHS, which promote the highest number of departing microdrops, consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state, yet at the same time provide sufficient area in-between the features for occurrence of mobile coalescence. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N39.00002: Elasticity of the contact line for droplets on anisotropic superhydrophobic surfaces Marco Rivetti, Anais Gauthier, Jeremie Teisseire, Etienne Barthel We present an experimental and numerical investigation on the receding of contact line for water droplets on glass superhydrophobic surfaces. In particular, we focus our attention on surfaces textured with anisotropic lattice posts. We measure that the receding contact angle is not affected by the anisotropy of the lattice. This surprising behavior is closely related to the elastic deformations of the contact line which can be by studied by direct observation. We interpret this phenomenon in term of propagation of kink defects along the lattice. We detail the influence of the morphology of the lattice on the propagation of kinks, as well as the importance of the shape of the posts. Three dimensional numerical simulations confirm that kinks are the key ingredient for the comprehension of the receding contact angle. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N39.00003: Dynamics of condensation on lubricant impregnated surfaces Sushant Anand, Adam Paxson, Konrad Rykaczewski, Daniel Beysens, Kripa Varanasi Replacing the filmwise condensation mode with dropwise condensation promises large improvements in heat transfer that will lead to large cost savings in material, water consumption and decreased size of the systems. In this regards, use of superhydrophobic surfaces fabricated by texturing surfaces with nano/microstructures has been shown to lead decrease in contact line pinning of millimetric drops resulting in fast shedding. However, these useful properties are lost during condensation where droplets that nucleate within texture grow by virtue of condensation to large sized droplets while still adhering to the surface. Recently we have shown that liquid impregnated surfaces can overcome many limitations of conventional superhydrophobic surfaces during condensation. Here we discuss aspects related to condensation on lubricant surfaces, such as behavior of growing droplets. We compare the characteristics of droplets condensing on these surfaces with their behavior on conventional un-impregnated superhydrophobic surfaces and show how use of lubricant impregnated surfaces may lead to large enhancement in heat transfer and energy efficiencies. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N39.00004: Contact Angle Hysteresis of Photo-Responsive Materials Samuel Rosenthal, Patricia McGuiggan An atomic force microscope (AFM) is used to measure the meniscus force on individual microspheres coated with photo-responsive materials such as anatase and rutile TiO$_{2}$, azobenzene, and other doped oxides as they contact and are retracted from an air/water interface. By exposing the coated microspheres to UV light, the contact angle change. The change can be detected by measuring the increase in the meniscus force. Exposure to visible, infrared, or far infrared light -- as the specific material requires - reverses the contact angle change. The measured force-distance curves are fitted to macroscopic wetting theory. From these measurements, the contact angle, the contact angle hysteresis, and the position of the contact line pinning were simultaneously determined. This allowed for a quantification of the contact angle changes from photo-switching. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N39.00005: Thermodynamic Model for Contact Angle Hysteresis on Rough Surfaces Rishi Raj, Ryan Enright, Solomon Adera, Evelyn Wang Wettability of solid surfaces can be tuned by introducing roughness. This effect has been explained by Wenzel, whereby texturing increases the degree of hydrophilicity (hydrophobicity) of an intrinsic hydrophilic (hydrophobic) flat surface. However, experimentally observed dynamic contact angles deviate significantly from those predicted by Wenzel equation. In this work, we demonstrate that local contact line distortion and pinning on structured surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions and pinning were visualized and analyzed to develop a thermodynamic model for contact angle hysteresis on rough surfaces. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. The understanding developed in this study offers new insight extending the fundamental understanding of solid-liquid interactions required for the design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N39.00006: Wetting Transition of Water Serah Friedman, Matt Khalil, Peter Taborek Pure liquid water does not wet most solid surfaces. Liquid water on these surfaces beads up and forms droplets with a finite contact angle. General thermodynamic principles suggest that as the temperature approaches the critical point, the contact angle should go to zero, marking the wetting transition. We have made an optical cell which can operate near the critical point of water (Tc$=$373C, Pc$=$217 atm) to study this phenomenon on sapphire, graphite and silicon. We have used two methods to measure the wetting temperature of water on these surfaces. Firstly, we studied a single droplet on a horizontal surface and optically measured the change in contact angle as a function of increasing temperature. Second, we studied the condensation of droplets on a vertical plate as a function of temperature. As the temperature approached the wetting temperature in both cases, the droplets spread and eventually form a smooth film along the surface of the plate. The wetting temperature on sapphire is near 240C and is considerably higher on graphite. Our observed values of Tw are significantly higher than the predictions made by the sharp-kink approximation and recent molecular dynamics simulations. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N39.00007: Moving Water Droplets on Aluminum and Copper Surfaces Using Surface Tension Gradients Muidh Alheshibri, Nathaniel Rogers, Andrew Sommers, Khalid Eid The behavior of water droplets on metal surfaces is very important for many applications, especially in heat exchangers in air conditioning and refrigeration. We use photolithography and/or shadow masks to create alternating hydrophobic/hydrophilic Cu micro-channels on an aluminum surface and to move water droplets on the surface. The contact angle that is formed between water droplets and the surface is clearly asymmetrical due to the different surface properties at the contact line between the droplets and the patterned surface. An HDFT self-assembled mono-layer allows for a large change in the water droplet contact angle on the copper, but seems to have no effect on the aluminum surface. We will show our results on the effect of the surface patterning and surface roughness on water droplet behavior. We also demonstrate that the engineered surface gradients cause water droplets to travel more than 1'' on a horizontal or upward tilted surface. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N39.00008: Atomistic simulations of surfactant adsorption kinetics Eugeniya Iskrenova, Soumya Patnaik Enhancing heat transfer is an important and challenging problem in a variety of industrial and technological applications including aircraft thermal management. Nucleate pool boiling is recognized as one of the most efficient methods to enhance heat transfer. Describing the plethora of multi-physics phenomena involved in nucleate pool boiling requires developing a multiscale model aimed at not only advancing our understanding but also at providing insights into the mechanisms for control and prediction of heat transfer in boiling. Adding surfactants to boiling water has been experimentally observed to enhance or inhibit the heat transfer depending on the surfactant concentration and chemistry. On a molecular level, addition of surfactants leads to the development of dynamic surface tension and changes in interfacial and transfer properties, thus contributing to the complexity of the multiscale model. We present an atomistic modeling study of the interfacial adsorption kinetics of aqueous surfactant systems at a range of concentrations at room and boiling temperature. Large scale classical molecular dynamics simulations were used to study the surfactant kinetics and estimate the adsorption and desorption rates at liquid-solid and liquid-vacuum interfaces. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N39.00009: Small Scale Evaporation Kinetics of a Binary Fluid Mixture Carl Basdeo, Dezhuang Ye, Devendra Kalonia, Tai-Hsi Fan Evaporation induces a concentrating effect in liquid mixtures. The transient process has significant influence on the dynamic behaviors of a complex fluid. To simultaneously investigate the fluid properties and small-scale evaporation kinetics during the transient process, the quartz crystal microbalance is applied to a binary mixture droplet of light alcohols including both a single volatile component (a fast evaporation followed by a slow evaporation) and a mixture of two volatile components with comparable evaporation rates. The density and viscosity stratification are evaluated by the shear wave, and the evaporation kinetics is measured by the resonant signature of the acoustic p-wave. The evaporation flux can be precisely determined by the resonant frequency spikes and the complex impedance. To predict the concentration field, the moving interface, and the precision evaporation kinetics of the mixture, a multiphase model is developed to interpret the complex impedance signals based on the underlying mass and momentum transport phenomena. The experimental method and theoretical model are developed for better characterizing and understanding of the drying process involving liquid mixtures of protein pharmaceuticals. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N39.00010: Red blood cell in simple shear flow Wei Chien, Yayu Hew, Yeng-Long Chen The dynamics of red blood cells (RBC) in blood flow is critical for oxygen transport, and it also influences inflammation (white blood cells), thrombosis (platelets), and circulatory tumor migration. The physical properties of a RBC can be captured by modeling RBC as lipid membrane linked to a cytoskeletal spectrin network that encapsulates cytoplasm rich in hemoglobin, with bi-concave equilibrium shape. Depending on the shear force, RBC elasticity, membrane viscosity, and cytoplasm viscosity, RBC can undergo tumbling, tank-treading, or oscillatory motion. We investigate the dynamic state diagram of RBC in shear and pressure-driven flow using a combined immersed boundary-lattice Boltzmann method with a multi-scale RBC model that accurately captures the experimentally established RBC force-deformation relation. It is found that the tumbling (TU) to tank-treading (TT) transition occurs as shear rate increases for cytoplasm/outer fluid viscosity ratio smaller than 0.67. The TU frequency is found to be half of the TT frequency, in agreement with experiment observations. Larger viscosity ratios lead to the disappearance of stable TT phase and unstable complex dynamics, including the oscillation of the symmetry axis of the bi-concave shape perpendicular to the flow direction. The dependence on RBC bending rigidity, shear modulus, the order of membrane spectrin network and fluid field in the unstable region will also be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N39.00011: Non-laminar motion of biological suspension: an illustration for blood cell passing a 3-micrometer capillary Iat Neng Chan Discovering in video images of blood cell motion, a new concept is developed for cell passing a tight capillary that has a large difference compared to the published simulation results. In video image the deformation of moving blood cell shows abnormal pressure from cell membrane under highly contacted condition with capillary wall. Moreover, when the cell struggles through the narrow capillary the appearance of additional force to assist the cell motion is necessary. In more detail analysis, the flow motion in capillary displaying a non-laminar pattern which is obviously different to that shows in a nearby larger capillary on the same image, can be explained as a non-regular flow described by an equivalent flow companied with sink and source. Using this illustration with the calculated volumes for normal and deformed cells, the flow speed and pressure are derived to compare with the best known results and also to the calculated flow speed from the images. After compared to diffusion effect, the exchange rate of materials in the flow and the efficiency factor to the circulatory system can be estimated. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N39.00012: Stability of a falling viscous sheet Claude Perdigou, Gilles Pfingstag, Basile Audoly, Arezki Boudaoud Falling films can be found in various processes of the food, glass and polymer industry. We study thin viscous films flowing vertically under the action of gravity, when poured from a slit. The lateral sides are unconstrained and the stretching effect of gravity induces a narrowing of the film in the horizontal direction, by Poisson's effect. This leads to compressive stress for some range of parameters, and we study the associated viscous buckling instabilities. A local stability analysis is used to characterized the flow parameters leading to potential instabilities. A global stability analysis is carried out, and an eigenvalue problem is solved numerically. This is implemented using the finite-element method with high order derivatives. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N39.00013: A Different Cone: Bursting Drops in Solids Xuanhe Zhao Drops in fluids tend to be spheres---a shape that minimizes surface energy. In thunderstorm clouds, drops can become unstable and emit thin jets when charged beyond certain limits. The instability of electrified drops in gases and liquids has been widely studied and used in applications including ink-jet printing, electrospinning nano-fibers, microfluidics and electrospray ionization. Here we report a different scenario: drops in solids become unstable and burst under sufficiently high electric fields. We find the instability of drops in solids morphologically resembles that in liquids, but the critical electric field for the instability follows a different scaling due to elasticity of solids. Our observations and theoretical models not only advance the fundamental understanding of electrified drops but also suggest a new failure mechanism of high-energy-density dielectric polymers, which have diverse applications ranging from capacitors for power grids and electric vehicles to muscle-like transducers for soft robots and energy harvesting. [Preview Abstract] |
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