Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session BG: Drops II |
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Chair: Pirouz Kavehpour, University of California, Los Angeles Room: 101A |
Sunday, November 23, 2008 10:30AM - 10:43AM |
BG.00001: Regimes of thermocapillary migration of droplets under partial wetting conditions Juan M. Gomba, George M. Homsy The actuation of very small droplets on rigid surfaces and inside channels is common to a large number of technological applications, such as cooling of integrated circuits, mixing of chemical products and bio-chemical analysis. In some of these, the control of drops can be achieved by thermocapillarity. Here, we study the effect of partial wetting on the thermocapillary migration of droplets along a constant temperature gradient imposed on the substrate. We solve for the temporal evolution of the droplet, taking into account the effect of wettability via a disjoining-conjoining pressure model. We find two flow regimes: one in which the droplet moves as a single entity, weakly distorted from its static shape and only very slowly losing mass into the thin film behind it; the other is analogous to the so-called ``Marangoni film,'' with a long film profile and a capillary ridge near the apparent contact line. While they depend on all parameters, these two flow regimes appear for relatively high and low contact angle, respectively. We also show some intriguing morphologies that appear in the transition between these two flow regimes. [Preview Abstract] |
Sunday, November 23, 2008 10:43AM - 10:56AM |
BG.00002: The short-time dynamics of partial wetting James Bird, Shreyas Mandre, Howard Stone When a liquid drop contacts a wettable surface, the liquid spreads over the solid to minimize the total surface energy. For perfectly wetting systems, the first moments of spreading are inertially dominated. In this work, we demonstrate that even in the presence of a contact line, the initial wetting is dominated by inertia rather than viscosity. We find that the spreading radius follows a power-law scaling in time where the exponent depends on the equilibrium contact angle. We propose a model, consistent with the experimental results, in which the surface spreading is regulated by the generation of capillary waves. [Preview Abstract] |
Sunday, November 23, 2008 10:56AM - 11:09AM |
BG.00003: Wetting of microspheres, rods and nanotubes using the AFM Jun Ma, Jay Wallace, Patricia McGuiggan An atomic force microscope (AFM) is used to measure the meniscus force on microspheres, rods and nanotubes as the material is pulled through an air/liquid interface. A fluid bridge forms between the liquid and the material as it is pulled out of the liquid. During retraction, the force reaches a maximum as the bridge necks down and finally detaches from the surface. The force distance curve can be used to determine the surface tension and contact angle. In particular, the values of the maximum force and zero force are used for the analysis. Contact angle hysteresis was observed as the material was pushed into the droplet and withdrawn from the droplet. [Preview Abstract] |
Sunday, November 23, 2008 11:09AM - 11:22AM |
BG.00004: Droplet spreading over topographical substrates Nikos Savva, Serafim Kalliadasis Consider contact line motion over topographical substrates by using a two-dimensional droplet of a partially wetting fluid spreading over such substrates as a model system. The spreading dynamics is modelled under the assumption of small contact angles where the long-wave expansion in the Stokes-flow regime can be employed to derive a single equation of the evolution type for the droplet thickness. Through a singular perturbation approach, the flow in the vicinity of the contact line is matched asymptotically with the flow in the bulk of the droplet to yield a set of two coupled differential equations for the spreading rates of the two droplet fronts. Analysis of these equations reveals a number of intriguing features that are not present when the substrate is flat. In particular, we demonstrate the existence of multiple equilibrium states which allows for a hysteresis-like effect on the apparent contact line. Further, we demonstrate a stick-slip- type behavior of the contact line as it moves along the local variations of the substrate shape and the interesting possibility of a relatively brief recession of one of the contact lines. [Preview Abstract] |
Sunday, November 23, 2008 11:22AM - 11:35AM |
BG.00005: Impact and Spreading of a Compound Droplet on a Solid Wall Metin Muradoglu, Savas Tasoglu Impact and spreading of a compound viscous droplet are studied computationally using a finite-difference/front-tracking method. The problem is motivated by single cell epitaxy developed for printing biological cells on a solid substrate using ink-jet printer technology. In this study, the biological cell is modeled as a highly viscous Newtonian liquid encapsulated by a less viscous droplet. The substrate is partially wettable for the encapsulating droplet and non- wettable for the inner droplet. The contact angle is specified dynamically for the encapsulating droplet using the empirical correlation given by Kistler (1993). In addition, a precursive film model is also used especially for the highly wettable cases, i.e., the static contact angle is smaller than 30$^{\circ}$ due to numerical difficulty of resolving thin liquid later penetrating into surrounding gas near the solid surface. The numerical method is first applied to simple droplet spreading and the results are compared with experimental data of Sikalo et al. (2005). Then the impact and spreading dynamic of a compound droplet is studied in details. The effects of governing non-dimensional numbers on the spreading rate and apparent contact angle of the outer droplet as well as on the pressure force and deformation of the inner droplet (cell) are investigated. [Preview Abstract] |
Sunday, November 23, 2008 11:35AM - 11:48AM |
BG.00006: Liquid Drop Impingement on Superhydrophobic Surfaces John Pearson, Daniel Maynes, Brent Webb We report results of an experimental investigation of liquid drops impinging on structured superhydrophobic surfaces. The surfaces are fabricated in Silicon wafers with micro-ribs and cavities (grooves) that are subsequently coated with two different hydrophobic coatings. Liquid droplets of known size were dropped from heights ranging from 0.5 to 50 cm onto the surfaces and the resulting impact and droplet deformation was imaged at a rate of 6000 frames/second with a digital camera. Tests were conducted on structured and coated surfaces, structured and uncoated surfaces, and coated and uncoated smooth surfaces. The droplet impact speed, maximum droplet spread, horizontal spread speed, vertical speed of the issuing jet, and the time between impact and formation of the issuing jet were all characterized. The results show that the overall impact dynamics are influenced significantly by the different impinging surface conditions. In general the maximum droplet spread and the speed of the spread are greater in the direction aligned with the ribs/cavities than aligned in the transverse direction. Further, the results suggest a dependence on the relative rib/cavity size. Correlated results in terms of the governing dimensionless parameters provide insight into the underlying physics. [Preview Abstract] |
Sunday, November 23, 2008 11:48AM - 12:01PM |
BG.00007: Length scale effects in wetting of chemically heterogeneous surfaces Neeharika Anantharaju, Mahesh Panchagnula, Srikanth Vedantam Wetting of chemically heterogeneous surfaces is modeled using phase field theory. Contact angle hysteresis is incorporated by a modified kinetic parameter. Using this model, we study the effect of varying the heterogeneity length scale on the resulting sessile drop behavior during advancing and receding events. A chemically heterogeneous surface is said to be composed of a predetermined arrangement of two materials. The novelty in the current approach lies in the fact that a surface made from any of the two ``pure'' materials, itself exhibits contact angle hysteresis. Using this model, we demonstrate the effect of variation in the length scale of a chemically heterogeneous surface as deviation from Cassie theory for a surface with a finite length scale exceeding the diffuse interface thickness. In addition, we demonstrate that the shape of the advancing and receding contact line is sensitive to the specific arrangement of the two materials, leaving open the possibility to manipulate the drop using contact line kinetics. [Preview Abstract] |
Sunday, November 23, 2008 12:01PM - 12:14PM |
BG.00008: Technique for Measurement of Droplet Profiles for the Study of Droplet Dynamics on Rough Surfaces Jason Schmucker, Edward White A new technique for the measurement of droplet profiles on rough, opaque surfaces has been developed. This non-intrusive method provides full-field, instantaneous measurements of droplet interface heights. The technique consists of illuminating a rough surface by coherent laser light to form a speckle pattern and measuring the deformation of the speckle field by the presence of the droplet. The deformation of the speckle pattern depends upon the surface height and gradient. Computer algorithms identify the droplet contact line and analyze the control and droplet specklegram using an image correlation method to extract the speckle deformation field, providing the necessary information to reconstruct the droplet profile. Reconstruction is performed by a simulated annealing algorithm designed to minimize the error between the measured speckle shift vector field and that of the guessed reconstruction of the droplet profile. Tests of the technique show a satisfactory level of accuracy and spatial resolution, allowing future experimental validation of previously untested computational modeling results. [Preview Abstract] |
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