Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session E4: Drops III |
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Chair: Julie Vanderhoff, Brigham Young University Room: 23C |
Sunday, November 18, 2012 4:45PM - 4:58PM |
E4.00001: Influence of Flow on Longevity of Superhydrophobic Coatings Mohamed A. Samaha, Hooman Vahedi Tafreshi, Mohamed Gad-el-Hak Previous studies have demonstrated the capability of superhydrophobic surfaces to produce slip flow and drag reduction, which properties hold considerable promise for a broad range of applications. However, in order to implement such surfaces for practical utilizations, environmental factors such as water movement over the surface must be observed and understood. In this work, experiments were carried out to present a proof-of-concept study on the impact of flow on longevity of polystyrene fibrous coatings. The time-dependent hydrophobicity of a submerged coating in a pressure vessel was determined while exposing the coating to a rudimentary wall-jet flow. Rheological studies were also performed to determine the effect of the flow on drag reduction. The results show that the longevity of the surface deteriorates by increasing the flowrate. The flow appears to enhance the dissolution of air into water, which leads to a loss of drag-reduction. [Preview Abstract] |
Sunday, November 18, 2012 4:58PM - 5:11PM |
E4.00002: Use of the string method to find minimal energy paths of droplets on superhydrophobic surfaces Kellen Petersen Interest in superhydrophobic surfaces has increased due to interesting advances in science and engineering. Here we use a diffuse interface model for droplets on topographically and chemically patterned surfaces. We then apply the constrained string method to examine the transition of droplets between different metastable/stable states. The string method finds the minimal energy paths (MEPs) which correspond to the most probable transition pathways between the metastable/stable states in the configuration space. In the case of a hydrophobic surface we determine the MEP corresponding to the transition between the Cassie-Baxter and Wenzel states. Additionally, we realize critical droplet morphologies along the MEP associated with saddle points of the free-energy potential and the energy barrier of the free energy. We analyze and compare the MEPs and free-energy barriers for a variety of surface geometries, droplets sizes, and static contact angles ranging. We demonstrate the string method as a useful tool in the study of droplets on superhydrophobic surfaces by presenting a numerical study that finds MEPs in configuration space, critical droplet morphologies and free-energy barriers which in turn give us a greater understanding of the free-energy landscape. [Preview Abstract] |
Sunday, November 18, 2012 5:11PM - 5:24PM |
E4.00003: Characterization of heat transfer from superhydrophobic substrates to water droplets Robb Hays, Julie Vanderhoff, Daniel Maynes We report on measurements of thermal transport to solitary sessile water droplets placed on heated superhydrophobic substrates at constant temperature. Data was obtained by heating the substrates to specified constant temperatures and gently placing a single water droplet on the surface. The droplet was allowed to evaporate completely while two video cameras and one infrared camera captured images of the droplet. The images were post-processed to yield transient geometric and thermal information, including droplet volume, droplet-substrate contact area, and droplet temperature. The total evaporation time and transient and average convective heat transfer coefficients were determined from the measurements as a function of the substrate surface temperature and the superhydrophobic topography. Four different superhydrophobic surfaces were investigated: rib-patterned surfaces of 50{\%}, 80{\%}, and 93{\%} cavity fraction and a post-patterned surface of 97{\%} cavity fraction. Ribs and posts ranged in width from 3 to 30 $\mu $m and in height from 15 to 20 $\mu $m. Cavities between ribs or posts ranged in width from 30 to 37 $\mu $m. Results were also obtained for hydrophilic, smooth hydrophobic, and superhydrophilic substrates for comparison purposes. In general, the evaporation time and difference between the droplet and substrate temperatures are both much greater for the superhydrophobic surfaces compared to smooth surfaces. [Preview Abstract] |
Sunday, November 18, 2012 5:24PM - 5:37PM |
E4.00004: Two-Pronged Jet Formation Caused by Droplet Impact on Anisotropic Superhydrophobic Surfaces John Pearson, Daniel Maynes, David Bilodeau, Brent Webb When a liquid droplet impacts a horizontal superhydrophobic surface with anisotropic surface patterning in the form of alternating ribs and cavities, the rebounding droplet can exhibit a unique two-pronged jet emission under certain conditions. This behavior occurs as a result of the unequal shear stresses and dynamic contact angles that exist along the two major axes: parallel and perpendicular to the ribs/cavities. Due to these unequal conditions in the two major directions, the droplet spread and collapse occurs more rapidly in the direction parallel to the ribs/cavities than the transverse direction. Droplet impact experiments with eleven different fluids of viscosity that varied by more than three orders of magnitude were conducted, and the ranges of Capillary number, \textit{Ca}, and Ohnesorge number, \textit{Oh}, over which the two-pronged phenomenon occurs have been quantified. Further, the probability of the two-pronged jet emission has been quantified as a function of \textit{Oh} and Weber number, \textit{We}. For \textit{Oh $>$} 0.0154, the behavior was never observed, while at lower values of \textit{Oh}, the behavior occurs for an intermediate range of \textit{Ca} that depends on \textit{Oh}. [Preview Abstract] |
Sunday, November 18, 2012 5:37PM - 5:50PM |
E4.00005: Spreading of liquid drop on superhydrophilic micropillar array Seong Jin Kim, Myoung-Woon Moon, Kwang-Ryeol Lee, Ho-Young Kim When a drop is deposited on a superhydrophilic micropillar array, the upper part of the drop (referred to as the bulk) collapses while the bottom part penetrates into the gaps of the array, forming a fringe film. Here we quantify the dynamics of this process using a combination of experiment and theory. In the early stages when the fringe extension is negligible compared to the bulk radius, both the spreading of the bulk and the entire drop footprint follow the same power law ($t^{1/4}$), $t$ being time. The bulk shrinks toward the end of the spreading process due to the drainage of liquid into the fringe film. The film spreads like $t^{1/2}$ until the end of the process. A remarkable finding is that the entire footprint grows like $t^{1/4}$ despite the diffusive growth of the fringe film, implying that the shrinkage of the bulk compensates for the outward spreading of the film. We rationalize some of these results with scaling analyses based on the balance of capillary forces that drive the flow and viscous shear forces. [Preview Abstract] |
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