Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session S4: Drops XI: Interaction with Superhydrophobic Surfaces |
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Chair: Jonathan Rothstein, University of Massachusetts, Amherst Room: 307 |
Tuesday, November 22, 2011 3:05PM - 3:18PM |
S4.00001: Liquid-encapsulating surfaces: overcoming the limitations of superhydrophobic surfaces for robust non-wetting and anti-icing surfaces J. David Smith, Rajeev Dhiman, Kripa Varanasi In this work we address fundamental limitations of superhydrophobic surfaces for non-wetting and anti-icing applications by impregnating them with a hydrophobic liquid. The encapsulated liquid serves as a barrier to the penetration of impinging water droplets and forces preferential condensation and frost formation on texture tops. We conducted droplet impact and roll-off experiments to assess the robustness of liquid-encapsulating micro- and nano-scale textured surfaces and found that their ability to shed droplets was improved dramatically. Furthermore, environmental scanning electron microscope experiments demonstrated that frost formation as well as condensation occurs preferentially on these surfaces thereby limiting ice contact to texture tops only. Ice adhesion strength was quantified using a custom-built adhesion testing apparatus to demonstrate greatly enhanced anti-icing performance of the liquid-encapsulating surfaces compared to superhydrophobic surfaces. [Preview Abstract] |
Tuesday, November 22, 2011 3:18PM - 3:31PM |
S4.00002: Two dimensional microfluidic devices for sorting, mixing, and analyzing drops using superhydrophobic surfaces Michael Nilsson, Hristo Goumnerov, Jonathan Rothstein The effect of sharp transitions in contact angle hysteresis of a superhydrophobic surface on water droplet motion is investigated. The surfaces are created by sanding naturally hydrophobic Teflon, which results in a surface possessing microscale roughness. With careful sanding and masking of the surface, regions with similar advancing contact angles near 150 degrees but varying contact angle hysteresis can be created. This talk presents detailed results on how these sharp transitions can deflect drops when the transition is at an angle to the droplet trajectory. The physics of the droplet-transition interaction is discussed and the sensitivity of the alignment of the angle of transition and Weber number is presented. Deflection using an angled transition in contact angle hysteresis is selective, demonstrating the ability of this fabrication method to sort drops based on size, speed, and wettability. Results of surfaces with stripes of different contact angle hysteresis are presented, showing the ability to more effectively deflect. Finally, a two-dimensional droplet reactor is presented that encompasses droplet sorting, coalescence, mixing, and trapping of two droplets. [Preview Abstract] |
Tuesday, November 22, 2011 3:31PM - 3:44PM |
S4.00003: Hierarchical roughness of sticky and non-sticky superhydrophobic surfaces Muhammad Akram Raza, Stefan Kooij, Arend van Silfhout, Harold Zandvliet, Bene Poelsema The importance of superhydrophobic substrates (contact angle $>$150\r{ } with sliding angle $<$10\r{ }) in modern technology is undeniable. We present a simple colloidal route to manufacture superstructured arrays with single- and multi-length-scaled roughness to obtain sticky and non-sticky superhydrophobic surfaces. The largest length scale is provided by (multi-)layers of silica spheres (1$\mu $m, 500nm and 150nm diameter). Decoration with gold nanoparticles (14nm, 26nm and 47nm) gives rise to a second length scale. To lower the surface energy, gold nanoparticles are functionalized with dodecanethiol and the silica spheres by perfluorooctyltriethoxysilane. The morphology was examined by helium ion microscopy (HIM), while wettability measurements were performed by using the sessile drop method. We conclude that wettability can be controlled by changing the surface chemistry and/or length scales of the structures. To achieve truly non-sticky superhydrophobic surfaces, hierarchical roughness plays a vital role. [Preview Abstract] |
Tuesday, November 22, 2011 3:44PM - 3:57PM |
S4.00004: Reducing contact time of drops on superhydrophobic surfaces James Bird, Rajeev Dhiman, Hyuk-Min Kwon, Kripa Varanasi When water drops impact on to a superhydrophobic surface, the drops can recoil to such an extent that they completely bounce off the solid material. The time it takes for the drop to spread and recoil -- the contact time -- scales with the hydrodynamic inertial-capillary timescale. However, there is evidence that the coefficient of this scaling depends on surface-structure interactions, such as pinning. Here we investigate how surface interactions can influence droplet contact time, and we compare our results to existing models. We highlight an assumption in the current theory that imposes a lower-bound on the contact time. By designing around this constraint, we demonstrate novel superhydrophobic surfaces on which water droplets impact with shorter contact times than previously thought possible. [Preview Abstract] |
Tuesday, November 22, 2011 3:57PM - 4:10PM |
S4.00005: Fog interaction with a textured hydrophobic surface: deposition, growth, and wetting Tony S. Yu, Joonsik Park, Hyuneui Lim, Kenneth S. Breuer A hydrophobic surface patterned with micro-pillars can exhibit ``superhydrophobic'' behavior, wherein drops only touch the tops of pillars (the so-called Cassie-Baxter state). This state, however, requires drops much larger than the pillar size/spacing. In contrast, drops smaller than the microtexture can adsorb both on top of and in- between these pillars and readily stick to the solid. In this study, we image the deposition of drops from fog (i.e. micron-sized droplets suspended in air) flowing over a substrate textured with square pillars, which are tens-of-micron in size. After texturing, the surface is coated with a hydrophobic monolayer. While drop growth at early time is dominated by deposition, late-time growth is dominated by coarsening due to the coalescence of adjacent drops. In this talk, we quantify the evolution of the size of these adsorbed drops. In addition, we show how closely-spaced pillars limit the size distribution of growing drops and enhance their spatial ordering. [Preview Abstract] |
Tuesday, November 22, 2011 4:10PM - 4:23PM |
S4.00006: Multiscale computation of wetting transitions on micro/nanoscale solid structures Carlos Colosqui, Michail Kavousanakis, Athanasios Papathanasiou, Ioannis Kevrekidis Modern microfabrication techniques allow the construction of solid surfaces with micro or nanoscale structure. The artificial micro/nanostructure, when properly designed, produces smart surfaces capable of exhibiting surperhydrophobic or superhydrophilic wetting properties in an addressable manner The design and control of transitions between superhydrophobic and superhydrophilic states using minimal external actuation (e.g. via electro-wetting, UV radiation, etc.) is an active research area with enormous impact in microfluidics and materials science. In this work, we demonstrate a computational methodology that allows microscopic or mesoscopic models to perform numerical continuation and stability analysis at macroscopic level. The method's ability to compute stable and unstable wetting states via accurate dynamic simulations is extremely valuable for the optimal engineering design of micro/nanostructures. The computational methodology is demonstrated by employing a novel lattice Boltzmann model that captures critical interfacial phenomena such as disjoining pressure and vapor adsorption at the solid-fluid surface. [Preview Abstract] |
Tuesday, November 22, 2011 4:23PM - 4:36PM |
S4.00007: 3D aspects of droplet coalescence during dropwise condensation on superhydrophobic surfaces Konrad Rykaczewski, John Henry J. Scott, Sukumar Rajauria, Jeff Chinn, Amy M. Chinn, Wanda Jones Only a few selected natural and artificial surfaces with water contact angles above 150 degrees retain their superhydrophobic characteristics during water condensation. On these robust superhydrophobic surfaces, individual droplets emerge out of a few micrometer wetted area between the nanostructures and initially grow mainly by increasing their contact angle. After reaching a nearly spherical shape with a diameter between 4 um and 6 um, the droplets grow in a near constant contact angle mode while remaining in an immobile Wenzel state. Microdroplets can depart the surface by coalescing with another large drop and forming a new drop in the mobile Cassie-Baxter state. Here we report that high contact angle primary drops can project over growing nano-to-microscale satellite droplets. We show that the large primary drops can sweep up the small satellite droplets without wetting their nucleation site, promoting rapid condensation of multiple satellite droplets from the same nucleation site. We discuss the effect of this coalescence mechanism on the drop size distribution and heat transfer during the dropwise condensation process. [Preview Abstract] |
Tuesday, November 22, 2011 4:36PM - 4:49PM |
S4.00008: Impact of Molten Metal Droplets on Textured Surfaces Rajeev Dhiman, Kripa Varanasi Most studies of droplet impact on micro/nano-textured surfaces have been done with water droplets. In this work, we studied impingement of molten metal droplets on micro and nanotextured surfaces which is relevant to many industrial applications such as thermal spray coating, spray forming, and solder jet bumping. The phenomenon is complicated by the fact that droplet spreading and freezing occur simultaneously. We used molten tin droplets (2.7 mm diameter) and deposited them on 10 um square posts surfaces made of silicon by standard photolithography. The impact process was photographed with a high-speed video camera and droplet deformations were analyzed. Surface temperature (25-240$^{\circ}$ C), impact velocity (0.5-3 m/s), and micropost spacing (5-50 $\mu$m) were the key parameters varied. We found that the maximum spreading diameter of the droplet increased with surface temperature and impact velocity, which could be predicted with the help of a simple mathematical model quite well. Droplets splashed through formation of satellite droplets at higher velocities. [Preview Abstract] |
Tuesday, November 22, 2011 4:49PM - 5:02PM |
S4.00009: Numerical simulations of drop impact on superhydrophobic structured surfaces Davide Guzzetti, Stefano Larentis, Nicola Pugno During the last decade drop impact dynamics on superhydrophobic surfaces has been intensively investigated because of the incredible properties of water repellency exhibited by this kind of surfaces, mostly inspired by biological examples such as Lotus leave. Thanks to the recent progress in micro-fabrication technology is possible to tailor surfaces wettability defining specific pillar-like structured surfaces. In this work, the behavior of impinging drops on these pillar-like surfaces is simulated, characterizing temporal evolution of droplets contact radius and drop maximal deformation dependence on Weber number. Numerical simulations results are compared with theoretical and experimental results guaranteeing simulation reliability. Fingering patterns obtained from drop impact has been studied obtaining a correlation between number of fingers and Weber number. Drop fragmentation pattern obtained from simulations supports the proposed correlation. Different drop impact outcomes (e.g. rebound, fragmentation) on structured superhydrophobic surfaces are simulated, focusing on the influence of micro-structured surface geometrical pattern. This investigation is relevant in order to define design rules for possible reliable non wettable surfaces. [Preview Abstract] |
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