Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session E17: Drops: Superhydrophobic Surfaces |
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Chair: Brian Iverson, Brigham Young University Room: North 131 AB |
Sunday, November 21, 2021 2:45PM - 2:58PM |
E17.00001: 'Cool' Coatings: Turning the Sticky to Slippery Rukmava Chatterjee, Hassan Bararnia, Sushant Anand Nature inspired synthetic materials engineered to repel solid foulants on functional surfaces are lucrative for widespread real-world usage. To address the ‘sticky’ problems of our quotidian lives, we have explored novel material design strategies aimed at curbing the accretion of different types of solid contaminants, ranging from common hard foulants like ice to soft foulants including bacteria. The feat is achieved through the incorporation of a cryoprotectant phase switching liquid in a polymeric matrix, resulting in the formation of a family of hygro-responsive coatings capable of generating a slippery surface hydration layer which inhibits direct contact of the foulants with the underlying substrate. This empowers the coatings with fascinating functionalities like optical transparency, anti-icing/frosting, ice-shedding properties, in situ interfacial deicing, potential for ambient water vapor harvesting, prevent biofilm growth and ability to self-repair upon being physically damaged. Additionally, this bottom-up design approach involves use of only bio-friendly materials which have a low environmental footprint. It is envisioned that with this versatile portfolio of high impact applications these designer surfaces will find commercial usage in a broad spectrum of modern industries. |
Sunday, November 21, 2021 2:58PM - 3:11PM |
E17.00002: Condensation Optimization on Biphilic Patterned Surfaces Ethan M Doupe, Huy Tran, Min Y Pack Dropwise condensation plays a large role in heat transfer, both in nature and in industry. Other experiments have explored ideas regarding altering surface characteristics to change effectiveness on condensation. However, due to the complex surface-droplet interactions, condensed droplets continue to coalesce randomly across a substrate. Droplets must reach a certain size before gravitational effects have enough force to strip the droplets from the surface. Hence, heat transfer and condensation are still inhibited by condensation areas that inhabit un-shed droplets. In order to circumvent these obstacles, we propose patterned microstructures on the condensation surface to order the condensation and coalescence. Through optical imaging, we show the improved effect on condensation of patterned aluminum surfaces with biphilic coatings, to promote drop shedding events leading to more efficient heat transfer through condensation. Our work provides framework to predict and improve both condensation and drop shedding on patterned surfaces. |
Sunday, November 21, 2021 3:11PM - 3:24PM |
E17.00003: Rolling Drops on Heated Superhydrophobic Surfaces Joseph Furner, Julie Crockett, Brian D Iverson, Daniel Maynes Superhydrophobic surfaces are excellent for promoting drop mobility and could have important applications in highly efficient condensers and water desalination processes. Condensed drops are highly mobile and here we experimentally investigate droplets rolling across heated hydrophobic and post-patterned superhydrophobic surfaces to characterize thermal transport as a function of the surface cavity fraction, rolling speed, and droplet size. Experiments were performed with droplets with volume ranging from 10 mL to 40 mL and smooth and superhydrophobic surfaces with cavity fractions, ranging from 0% to 93%. All experiments were performed at subcritical surface temperatures and the surfaces were inclined to achieve various droplet rolling speeds. The droplet temperature was determined using a high-speed infrared camera, from which an instantaneous bulk-averaged temperature was calculated. The instantaneous velocity of the droplet was also determined by the infrared images as it translates across the surfaces. As the cavity fraction increases, heat transfer to the drop decreases significantly and the drop speed increases and the internal drop convection is notably altered. |
Sunday, November 21, 2021 3:24PM - 3:37PM |
E17.00004: Condensation on CICNT Structured Superhydrophobic Surfaces Clint M Hubbard, Julie Crockett, Brian D Iverson, Daniel Maynes This study investigates subcooling, surface feature size (CNT diameter), and time duration on dropwise condensation and condensate removal on superhydrophobic surfaces. Experiments were conducted in a low-pressure chamber with a vertically oriented test CNT surface. The CNT diameter was varied between 20-120 nm, and the surface subcooling ranged from 0.5-9 C. Three types of condensation were observed: drop jumping, complete wetting of the CNTs, and surface flooding. The results reveal that drops retain their mobility at CNT diameters smaller than ~60 nm and at subcooling temperatures less than ~7C. Here drops are mobile and self-remove from the surface by the mechanism of jumping. The results also show that as the CNT diameter is increased to 70-75 nm, the drops lose mobility and begin to be more pinned. Condensation transitions from droplet jumping to surface flooding over this region. The same transition occurs with an increase in subcooling magnitude to 9-10 C. The amount of time the condensate resides on the surface also impacts mobility and the transition from jumping to wetting occurs at a smaller CNT diameters or lower surface subcooling. |
Sunday, November 21, 2021 3:37PM - 3:50PM |
E17.00005: Droplet Adhesion Forces on Microstructured Superhydrophobic Surfaces Shaur Humayun, Brian D Iverson, Daniel Maynes, Julie Crockett We present a method to determine the adhesion force for drops on microstructured superhydrophobic (SH) surfaces in the presence of a gravitational force. Base area shape and circumferential contact angle distribution of drops on micropillar-structured SH surfaces were determined experimentally providing new understanding of the contact angle distribution and base area shape of drops on inclined surfaces. The microstructured surfaces were comprised of an array of circular posts etched in silicon and coated with Teflon. Water droplets were placed on surfaces that span a wide range of solid fractions (1, 0.4, 0.22, 0.13, 0.07). Our results indicate that the droplet base on a post micro-structured SH surface can be best approximated by a combination of two ellipses at the advancing front and receding end of a drop. The adhesion force was also calculated using the experimentally obtained contact angle distribution and the base area of the drops resulting in an improved result for adhesion force values, as compared to previous models. |
Sunday, November 21, 2021 3:50PM - 4:03PM Not Participating |
E17.00006: Gliding Leidenfrost droplets: shear-thinning vs Newtonian Rafsan Rabbi, Addison Litton, Akihito Kiyama, Tadd T Truscott It is well-known that the droplet sliding velocity along an inclined hydrophobic surface would depend on the droplet rheological properties. For instance, the larger the viscosity, the slower a droplet would slide under the influence of gravity. Yet, if we mimic this experiment by substituting the hydrophobic surface with a Leidenfrost one where the surface temperature exceeds the so-called Leidenfrost point, we observe that the viscous influence is nullified. Here, we conduct a controlled experimental study where we investigate the droplet sliding velocity on a heated copper plate (length L ~ 60 cm, temperature T > 300 C, beyond the Leidenfrost point for copper to water) inclined to the ground at varying angles (2 to 8 degrees). We use a needle to produce different-sized droplets of a shear-thinning Xanthan gum aqueous solution with varying viscosity and show that the viscosity of the liquid doesn’t affect the final terminal velocity of the droplet. This non-intuitive result hints at deeper physics involving droplet motion on Leidenfrost surfaces. |
Sunday, November 21, 2021 4:03PM - 4:16PM |
E17.00007: Rolling of non-wetting droplets down a gently inclined plane Ory Schnitzer, Anthony M. J. Davis, Ehud Yariv In their pioneering 1999 paper, Mahadevan & Pomeau argued that small non-wetting drops roll, rather than slide, down gently inclined surfaces. They showed that the rolling speed possesses an anomalous scaling with an inverse dependence upon drop size, in contrast with conventional modes of drop mobility. The Mahadevan–Pomeau scaling was corroborated by the experiments of Quéré and co-workers using superhydrophobic surfaces (1999) and liquid marbles (2001). We here go beyond scaling arguments, carrying out an asymptotic analysis of the well-posed hydrodynamic problem governing a perfectly non-wetting drop moving down an inclined plane under gravity, in the rolling regime where the drop size and the inclination angle are suitably small. The analysis reveals that the Mahadevan–Pomeau scaling should be multiplied by the prefactor (3π/16)√(3/2)≈0.72, in good agreement with the experiments. The analysis also illuminates intriguing characteristics of the flow field inside the drop and a unique "peeling" mechanism by which the contact line propagates along the surface. |
Sunday, November 21, 2021 4:16PM - 4:29PM Not Participating |
E17.00008: Two recipes for repelling hot water Timothée MOUTERDE, Pierre Lecointre, Gaëlle Lehoucq, Antonio Checco, Christophe Clanet, David Quéré While a hydrophobic microtexture at a solid surface most often reflects rain owing to the presence of entrapped air within the texture, it is much more challenging to repel hot water. As it contacts a colder material, hot water generates condensation within the cavities at the solid surface, which eventually builds bridges between the substrate and the water, and thus destroys repellency. In this talk, we will explore impacts of hot water drops on cold superhydrophobic materials. While both "small" (~100 nm) and "large" (~10 µm) model features do reflect hot drops at any drop temperature and in the whole range of explored impact velocities, surprisingly water does not always bounce on intermediate (~ 1 µm) textures. Hence, we can define two structural recipes for repelling hot water: drops on nanometric features hardly stick owing to the miniaturization of water bridges, while kinetics of condensation in large features is too slow to connect the liquid to the solid at impact. |
Sunday, November 21, 2021 4:29PM - 4:42PM |
E17.00009: Serial coalescence of multiple condensed droplets on superhydrophobic surfaces at the microscale Cheuk Wing Edmond Lam, Abinash Tripathy, Athanasios Milionis, Chander Shekhar Sharma, Dimos Poulikakos High contact angles (CA) and low contact angle hysteresis (CAH) are characteristic of structured superhydrophobic surfaces on which droplets are known to be very mobile. When two or more droplets coalesce, the excess surface energy could be converted to kinetic energy for spontaneous droplet motion. Most of the literature focus on the resulting motion normal to the surfaces (jumping), which originates from their low wettability and high CA. On the other hand, low CAH leads to tangential motion of coalesced droplets which has not received much attention. We study the mechanics of such motion for condensate droplets, which often manifests in a gravity-independent serial coalescence of multiple droplets across the surface. This, in turn, leaves a trail of fresh surface for nucleation. At the end of such events the droplet may become stationary or depart by jumping. We further discuss the possible implications for condensation heat transfer since a tangential serial coalescence sequence may clear a much larger area for fresh nucleation compared to normal jumping departure arising from the coalescence of droplets confined at a location. |
Sunday, November 21, 2021 4:42PM - 4:55PM |
E17.00010: Wetting transitions on superhydrophobic surfaces during recalescence Henry Lambley, Thomas M Schutzius, Dimos Poulikakos Supercooled droplet wetting and freezing on surfaces is important in nature and technology. The seemingly superb advantages of rapid supercooled liquid shedding and reduced ice adhesion are wholly reliant on the preservation of the fragile Cassie-Baxter composite wetting state on textured surfaces. Whilst condensation frosting and ice deposition are well documented failure modes for superhydrophobic solutions on the microscale, comparatively little research has investigated how the freezing of macroscopic droplets affect the stability of the non-wetting state. Here, we explore the recalescent freezing of a droplet, induced by rapid evacuation of the atmosphere, on a micropillar textured surface. Through this, we are able to determine the substrate characteristics necessary to induce so-called ice levitation behaviour and, in parallel, identify two distinct mechanisms through which repellency breaks down. These outcomes can then be rationalised through a theoretical model balancing the wetting properties of the surface with those resulting from the recalescence process. Finally, we present the complimentary ambient pressure freezing case, at reduced temperature, and link the two sets of findings together to facilitate the design of icephobic surfaces across the phase diagram. |
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