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 R33: Drops XV: Superhydrophobic Surfaces |
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Chair: Xin Yong, University of Pittsburgh Room: 404 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R33.00001: Slide, Sweep and Vanish: Droplet manipulation by wettability engineering Aritra Ghosh, Ranjan Ganguly, Thomas M. Schutzius, Constantine M. Megaridis Achieving controlled droplet transport on substrates is important for multiphase heat transfer, water harvesting and lab-on-chip applications. We use a facile, scalable surface wettability engineering approach to generate wettability patterned surfaces that comprise of superhydrophilic tracks of various geometrical patterns and length scales ($\mu m$-\textit{mm}) on superhydrophobic backgrounds. Liquid transport on such surfaces harnesses the force arising from the spatial contrast of surface energy on the substrate, providing rapid actuation for micro and nanoliter drops. Considering a variety of dimensions, shapes and strategic locations of the superhydrophilic patterns on the substrate, effective modes of droplet transport through hemiwicking and Laplace pressure-driven flow are analyzed. The work provides proof-of-concept for salient digital microfluidic tasks, e.g. droplet capture, transport, merging and dispensing on such patterned substrates. This droplet manipulation is pumpless and fast. With suitable patterns and wettability contrast, we demonstrate on-chip droplet transport speeds of O(10 cm/s). The study examines the geometric and surface wettability parameters for optimal substrate design for droplet manipulation. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R33.00002: Dynamic Wetting of a Droplet on a Hydrophobic Micro-patterned Surface Xian Wei, Huan Li, Cheng Wang, Xin Tang, Sascha Hilgenfeldt, K. Jimmy Hsia The dynamics of droplets moving over patterned surfaces of micro-pillar arrays is of great practical interest, but has lacked detailed study at the level of the micron-scale pattern. We develop an imaging method and a force measurement setup to study contact line (CL) evolution and contact angle hysteresis (CAH) induced resistant force for a water droplet sliding on PDMS micro-pillar arrays. The topography of the CL between droplet and surface is imaged using fluorescence microscopy in combination with high-speed video. To measure the CAH induced resistant force, a micro-force sensor is attached to the droplet and the substrate moved relative to the droplet with prescribed velocity. The resultant force-time curve displays an initial maximum and subsequently a dynamic steady state with a sawtooth-like shape. The ratio between the average force at maximum and that in the dynamic balance state is approximately constant and close to the ratio between pillar numbers at the trailing edge at both states. In steady state, the shape of the force curve can be correlated with events of pillar pinning and depinning at the CL to obtain a detailed understanding of attachment and detachment forces. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R33.00003: Simulations of Droplets on Micro-patterned Surfaces Banglin Liu, Michael Grigola, Huan Li, Sascha Hilgenfeldt, K. Jimmy Hsia The behavior of liquid droplets on micro-patterned surfaces made from arrays of micropillars is important for applications in self-cleaning surfaces, refrigeration, or pore filtration. Properties like droplet contact angles and their hysteresis have been described in macroscopic terms from coarse-grained variables like pillar density. However, for accurate modeling of the droplet shape and dynamical behavior, microscopic parameters like pillar positioning and the topography of the contact line are crucial. We have developed an energy-based model of a water droplet on a PDMS substrate in the Cassie-Baxter state using Surface Evolver. We assess the changes in droplet energy upon deformation and displacement, with particular attention to the pinning and depinning from individual pillars. The majority of shape distortion and energy change is found to occur in close proximity to the substrate, encouraging a simplified theoretical description using concepts of 2D contact-line pinning. The versatile simulation tool can be used to study the effects of pillar position pattern, pillar orientation, substrate symmetry, and many more general problems of contact-line statics and dynamics. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R33.00004: Effect of Vapor Flow on Jumping Droplets during Condensation on Superhydrophobic Surfaces Daniel J. Preston, Nenad Miljkovic, Ryan Enright, Alexander Limia, Evelyn N. Wang Upon coalescence of droplets on a superhydrophobic surface, the net reduction in droplet surface area results in a release of surface energy that can cause the coalesced droplet to ``jump'' away from the surface. Jumping condensing surfaces have been shown to enhance condensation heat transfer by up to 30{\%} compared to state-of-the-art dropwise condensing surfaces. While the heat transfer enhancement of jumping condensation is well documented, droplet behavior after departure from the surface has not been considered. Vapor flows to the condensing surface due to mass conservation. This flow can increase drag on departing droplets, resulting in complete droplet reversal and return to the surface. Upon return, these larger droplets impede heat transfer until they jump again or finally shed due to gravity. By characterizing individual droplet trajectories during condensation on hydrophobic nanostructured copper oxide surfaces for a variety of heat fluxes ($q$'' $=$ 0.1 -- 2 W/cm2), we showed that vapor flow entrainment dominates droplet motion for droplets smaller than $R \quad \approx $ 30 um at high heat fluxes ($q$'' \textgreater 2 W/cm2). Furthermore, we developed an analytical model of droplet motion based on first principles and the Reynolds drag equation which agreed well with the experimental data. We considered condensation on both flat and tubular geometries with our model, and we suggest avenues to further enhance heat transfer which minimize droplet return due to entrainment. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R33.00005: Cold-induced Spreading of Water Drops on Hydrophobic Surfaces Faryar Tavakoli, Pirouz Kavehpour Superhydrophobic surfaces received tremendous attention in recent years mainly due to their self--cleaning properties. Wenzel and Cassie--Baxter models for relating stable equilibrium contact angle to physical parameters of liquid and solid ignore tangible factors such as temperature and humidity. Here, we show a peculiar behavior of equilibrium contact angle on cold hydrophobic surfaces. Water drops were cooled by a peltier element to temperatures below the melting point of water and, surprisingly, substantial change in static contact angle and base diameter were observed during the cooling process. Physical variables such as substrate temperature, humidity, drop volume, and even fabrication type of hydrophobic surfaces are found to be detrimental to post--spreading shape. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R33.00006: Direct observation of self-similar contact line depinning from superhydrophobic surfaces Adam Paxson, Kripa Varanasi The adhesion of a drop to a superhydrophobic surface, although very low, is never altogether eliminated. As the drop moves along the surface, the advancing portion of the contact line simply lies down onto the upcoming roughness features, contributing negligibly to adhesion. Instead, the pinning and contact angle hysteresis are governed by the depinning of capillary bridges formed at the receding portion of the contact line. We use environmental scanning electron microscopy to observe these depinning events at the microscale. After measuring the local receding contact angle of capillary bridges formed on a micropillar array, we find that these depinning events follow the Gibbs depinning criterion. We further extend this technique to two-scale hierarchical structures to reveal a self-similar depinning mechanism in which the adhesion of the entire drop depends only on the pinning at the very smallest level of roughness hierarchy. With this self-similar depinning mechanism we develop a model to predict the adhesion of drops to superhydrophobic surfaces that explains both the low adhesion on sparsely structured surfaces and the surprisingly high adhesion on surfaces whose features are densely spaced or tortuously shaped. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R33.00007: Technique for needle-free drop deposition: Pathway for precise characterization of superhydrophobic surfaces Prashant R. Waghmare, Siddhartha Das, Sushanta K. Mitra The most important step for characterizing the wettability of a surface is to deposit a water drop on the surface and measure the contact angle made by the drop on the surface. This innocuously simple process relies on bringing a needle holding the water drop in close proximity to the surface, with a ``desire'' that the drop would spontaneously detach from the needle and get deposited on the surface. Problem occurs when the surface is superhydrophobic, expressing an ``unwillingness'' to ``see'' the water drop in preference to a much more ``water-loving'' needle surface. There exists no solution to this problem, and surfaces are invariably characterized where the drop-needle assembly contacts the superhydrophobic surface. Such a configuration will always lead to an incorrect estimation of the contact angle, as there is no certainty of the existence of the drop-surface contact. Here we shall discuss our recently invented technique, where we solve this long-standing problem--we indeed ensure a needle-free drop in contact with the superhydrophobic surface, thereby ascertaining precise determination of the contact angle. The successful application of the technique will address a major headache of the big research community interested in science and technology of superhydrophobic surfaces. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R33.00008: New drop deposition technique for wettability characterization of under-liquid superoleophobic surfaces Sushanta Mitra, Prashant Waghmare, Siddhartha Das From understanding the remarkable self-cleaning behavior of fish scales to the preparation of surfaces that will counter the destructive effects of oil-spills, there has been a remarkable interest in understanding the wettability of a solid in an ``under-liquid" configuration. Like surfaces in air, here too, the main focus remain in designing surfaces (such as fish scales) that exhibit repelling behavior to a multiple other liquids in this ``under-liquid" state. Problem occurs, just as with surfaces in air, when this ``under-liquid" surface is too repelling to a given liquid. In that case, the standard drop deposition technique is unable to deposit a drop that is not ``interfered" by the needle holding the drop. Here we shall discuss a unique technique that ensures that we achieve a ``needle-free" deposited drop on the under-liquid surface. A drop is produced at the end of the needle, with the needle placed inside the liquid bath. Then the needle holding the drop is moved away from the concerned surface, and the moment this drop-needle assembly hits the liquid-air or liquid-another-liquid (a layer of this another liquid is intentionally created at the location where the liquid bath is exhausted), the surface tension effects will ensure that the drop is detached from the needle. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R33.00009: Liquid transfer between two solid surfaces with the effect of contact angle hysteresis Huanchen Chen, Tian Tang, Alidad Amirfazli Drop transfer from one solid surface to another (e.g. due to the approach of a surface from top to a sessile drop resting on a lower surface) is widely observed in many industrial areas, e.g. offset printing. This process is governed by many factors such as the contact angle (CA) and contact angle hysteresis (CAH) of surfaces, viscosity of the liquid and the rate at which the donor and acceptor surfaces are separated. In this work, an experimental apparatus is developed to study the transfer of liquid drop between surfaces, with the particular focus on addressing the effect of the surfaces' CAH when the loading speed is low (transfer is quasi-static). In the experiment, a liquid bridge between the two surfaces is first formed by compression; then stretched to the point of breakage. By using surfaces that have similar CA but dissimilar CAH, the liquid transfer ratio (the amount of liquid transferred to the acceptor surface over the total amount of liquid) is found to be significantly influenced by CAH. In addition, as a result of CAH, the maximum compression of the liquid bridge is found to play an important role in determining the transfer ratio. These findings can be very helpful for the design of surfaces and loading conditions to achieve desired transfer ratios in practice. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R33.00010: Passing of a drop from one surface to another: a simulation and analytical study Alidad Amirfazli, Tian Tang, Huanchen Chen Due to its high throughput and cost-effectiveness, offset printing is a widely used printing technique in which the ink is transferred from a substrate to the target surface. A thorough understanding of liquid transfer between two solid surfaces can significantly improve the working efficiency of offset printing. Depending on the approaching and separation speeds of the surfaces, their wettability and liquid viscosity, the liquid transfer can be categorized into two regimes: quasi-static regime where the surface tension force dominates, and dynamic regime where contributions from viscous and inertia forces are not negligible; however, the delineating conditions for these two regimes are not understood. In this work, a numerical model based on the volume of fluid method was developed to study the liquid transfer under different approaching and separation speeds. The effect of dynamic contact angle is included. With this model, the liquid transfer ratio (the amount of liquid transferred to the acceptor surface over the total amount of liquid) is calculated and used to determine the boundary between the quasi-static and the dynamic regimes. A systematic study is conducted on how the transfer ratio is affected by the speed of the surfaces, viscosity of liquid and surface's wettability in the dynamic regime. [Preview Abstract] |
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