Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D13: Drops: Superhydrophobic Surfaces |
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Chair: Julie Crockett, Brigham Young University Room: 3020 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D13.00001: Thermocapillary Dewetting of Superhydrophobic Surfaces Cristian Clavijo, Daniel Maynes, Julie Crockett One of the challenges in preserving the Cassie-Baxter state during liquid flow over micro-structured superhydrophobic surfaces occurs when the force due to pressure in the liquid exceeds that due to the surface tension above the gas cavities thereby forcing liquid in between the micro-structures. This commonly occurs at the impingement point of a jet or droplet where the stagnation pressure is significant. In this work, we present a novel and simple experimental analysis to show that such a wetting state (i.e. Wenzel state) can be reversed for an impinging liquid droplet through a temperature gradient normal to the solid surface. The temperature gradient acts to alter the surface tension along the structures normal to the surface resulting in possible de-wetting. The experiments consisted of 2 mm water and glycerol droplets held at room temperature impinging on heated micro-post superhydrophobic surfaces. The surface temperature was varied between 50 and 90 $^{\circ}$C and the height of the micro-posts between 8 and 18 $\mu $m. The results show that hotter surface temperatures and taller posts allow for a nearly complete Cassie to Wenzel transition on the order of 1 ms, thus droplets are able to rebound without sticking to the surface. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D13.00002: Direct measurement of the interaction forces during the sliding of a water droplet on a pillar-typed superhydrophobic surface Thanh-Vinh Nguyen, Hidetoshi Takahashi, Kiyoshi Matsumoto, Isao Shimoyama We directly measure the interaction forces between a sliding droplet and a single microstructure of a pillar-typed superhydrophobic surface. The measurement is realized using a MEMS (Micro Electro Mechanical Systems)-based two-axis force sensor fabricated under the micropillar. We find that, during the sliding of a water droplet on the micropillar array, the pillar is pulled upward when it meets the advancing and receding edges of the droplet as a result of the normal component of surface tension. The maximum value of the pulling force at the receding edge is larger than that at the advancing edge due to the difference in dynamic contact angles at the receding and advancing edges. Meanwhile, over the inner region of the contact area, the pillar is pushed down by the liquid pressure. Moreover, measured shear force shows that the friction during the droplet sliding is dominated by the adhesion of the droplet at the receding. Finally, we demonstrate that as the volume of the droplet increases, the normal force over the inner region of the contact area decreases while the forces at the receding edges do not change significantly. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D13.00003: Superhydrophobic frictions Timothee Mouterde, Pascal Raux, Christophe Clanet, David Quere We discuss the nature of the friction opposing the motion of drops running on superhydrophobic materials. Despite the high mobility of the drops (which are typically 100 times faster than on usual solids), friction is always found to have a viscous origin. For highly viscous liquids, we recover a Mahadevan-Pomeau regime for which the drop speed is inversely proportional to the viscosity. In the opposite limit (e.g. water), friction depends on the material texture and it is interpreted as resulting either from the development of a viscous boundary layer at the solid/liquid interface, or from the shear inside the subjacent air cushion. In the latter case, air is not only responsible for the existence of superhydrophobic states, but also for the resistance limiting speed of water in these states. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D13.00004: Droplet Train Impingement on Superhydrophobic Surfaces Jonathan Stoddard, Julie Crockett, Daniel Maynes The dynamics of a droplet train impinging on a superhydrophobic (SH) surface is investigated. The surfaces are patterned with either post or ribbed microfeatures and coated with Teflon to render them hydrophobic. The height of the features are nominally 15 microns and the spacing for the post and ribbed surfaces are approximately 16.5 microns and 32 microns respectively. Droplets at the induced frequencies of 600-4600 Hz with sizes varying from 0.7 to 1.5 mm in diameter are forced to impinge on these SH surfaces. When each individual droplet impinges on the surface, a crown forms which spreads out radially until reaching a semi-stable or regularly oscillating crater radius. At this point the water either builds up or breaks up into droplets. In some cases the crown may breakup before reaching the crater, causing splashing. We characterize the occurrence of these dynamics and quantify the crater radius over a Weber number range of 70-1450. In addition we compare the crater radius to the breakup radius of a uniform jet with equivalent momentum and describe the transition dynamics from droplet train impingement to uniform laminar jet impingement on a SH surface. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D13.00005: Puddle Jumping Andrew Wollman, Trevor Snyder, Mark Weislogel Rebounding droplets from superhydrophobic surfaces have attracted significant public and scientific attention because they are both enjoyable as well as industrially relevant. Demonstrations of bouncing droplets with volumes between 0.003 and 0.03 ml are common in the literature and limited primarily by gravity. In this presentation we demonstrate large droplet ``rebounds'' made possible by low-gravity testing in a drop tower. The up to 300 ml drops are best described as puddles that launch in a nearly identical manner to rebounding drops 4 orders of magnitude smaller in volume. A variety of jumping liquid and gas puddles are shown including puddles of highly specified and unusual initial geometry. The large length sales of the capillary fluidic surfaces $\sim\mathcal{O}$(10 cm) enable 3D printing of all superhydrophobic surface topologies demonstrated. In addition, we demonstrate such puddle jumping as a passive drop-on-demand technique for large low-gravity drop dynamics investigations; such as collisions, rebounds, heat and mass transfer, and containerless possessing. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D13.00006: Robust and Drain Resistant Lubricated Omniphobic Fabrics Cassidee Kido, Viraj Damle, Xiaoda Sun, Ajay Roopesh, Kyle Doudrick, Konrad Rykaczewski The implications of omniphobic fabrics range from stainproof clothing to civilian and military protection from chemical weapons. The challenge comes in developing a product that remains effective in repelling droplets of liquids with a wide range of surface tensions even after being subjected to various stimuli imposed by human use. Omniphobic fabrics can be made by infusing hydrophobic nanoparticle coated fibers with a low surface energy lubricant [1]. These types of lubricant impregnated surfaces can shed large deposited droplets as well as condensed microdroplets of variety of low surface tension liquids [2]. However, here we show that lubricated omniphobic fabrics can easily lose their properties due to degradation of the nanostructure coating or drainage of the lubricant upon contact with a porous surface. We also demonstrate that this issue can be resolved with use of cross-linked polymer coated fibers that are swollen with the lubricant. Use of flexible polymers avoids structure degradation due to fabric deformation, while swelling of the polymer with lubricant minimizes lubricant drainage upon contact maintaining the omniphobic characteristics of the fabric. [1] Shillingford, C. et al. Nanotechnol. 2014, 25, 014019. [2] Rykaczewski, K et al. Sci. Rep 2014, 4. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D13.00007: Behavior of severely supercooled water drops impacting on superhydrophobic surfaces Tanmoy Maitra, Carlo Antonini, Manish K. Tiwari, Adrian Mularczyk, Zulkufli Imeri, Philippe Schoch, Dimos Poulikakos Surface icing, commonplace in nature and technology, has broad implications to daily life. To prevent surface icing, superhydrophobic surfaces/coatings with rationally controlled roughness features (both at micro and nano-scale) are considered to be a promising candidate. However, to fabricate/synthesize a high performance icephobic surface or coating, understanding the dynamic interaction between water and the surface during water drop impact in supercooled state is necessary. In this work, we investigate the water/substrate interaction using drop impact experiments down to -17$^{\circ}$C. It is found that the resulting increased viscous effect of water at low temperature significantly affects all stages of drop dynamics such as maximum spreading, contact time and meniscus penetration into the superhydrophobic texture. Most interestingly, the viscous effect on the meniscus penetration into roughness feature leads to clear change in the velocity threshold for rebounding to sticking transition by 25{\%} of supercooled drops. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D13.00008: Delayed frost formation on hybrid nanostructured surfaces with patterned high wetting contrast Youmin Hou, Peng Zhou, Shuhuai Yao Engineering icephobic surfaces that can retard the frost formation and accumulation are important to vehicles, wind turbines, power lines, and HVAC systems. For condensation frosting, superhydrophobic surfaces promote self-removal of condensed droplets before freezing and consequently delay the frost growth. However, a small thermal fluctuation may lead to a Cassie-to-Wenzel transition, and thus dramatically enhance the frost formation and adhesion. In this work, we investigated the heterogeneous ice nucleation on hybrid nanostructured surfaces with patterned high wetting contrast. By judiciously introducing hydrophilic micro-patches into superhydrophobic nanostructured surface, we demonstrated that such a novel hybrid structure can efficiently defer the ice nucleation as compared to a superhydrophobic surface with nanostructures only. We observed efficient droplet jumping and higher coverage of droplets with diameter smaller than 10 $\mu $m, both of which suppress frost formation. The hybrid surface avoids the formation of liquid-bridges for Cassie-to-Wenzel transition, therefore eliminating the `bottom-up' droplet freezing from the cold substrate. These findings provide new insights to improve anti-frosting and anti-icing by using heterogeneous wettability in multiscale structures [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D13.00009: Bioinspired Antifreeze Secreting Frost-Responsive Pagophobic Coatings Xiaoda Sun, Viraj Damle, Konrad Rykaczewski Prevention of ice and frost accumulation is of interest to transportation, power generation, and agriculture industries. Superhydrophobic and lubricant impregnated pagophobic coatings have been proposed, however, they both fail in frosting conditions [1, 2]. Inspired by functional liquid secretion in natural systems, such as toxin secretion by poison dart frost in response to predator presence, we developed frost-responsive antifreeze secreting pagophobic coatings. These are bi-layered coatings with an inner superhydrophilic ``dermis'' infused with antifreeze and an outer permeable superhydrophobic ``epidermis.'' The superhydrophobic epidermis separates the antifreeze from the environment and prevents ice accumulation by repelling impinging water droplets. In frosting conditions, the antifreeze is secreted from the dermis through pores in the epidermis either due to contact with condensed droplets or temporary switch of the epidermis wettability from hydrophobic to hydrophilic caused by surface icing. Here we demonstrate superior performance of this multifunctional coating in simulated frosting, freezing mist/fog, and freezing spray/rain conditions. \\[4pt] [1] Varanasi et al., App. Phys. Lett., 97, 2010.\\[0pt] [2] Rykaczewski et al., Langmuir, 29, 2013. [Preview Abstract] |
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