Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session A30: Drops: Superhydrophobic Surfaces & Multiple Drop Interactions |
Hide Abstracts |
Chair: Glen McHale, The University of Edinburgh Room: 239 |
Sunday, November 20, 2022 8:00AM - 8:13AM |
A30.00001: Wetting of Auxetic Metamaterials Glen McHale, Steven Armstrong, Shruti Mandhani, Gary G Wells, Rodrigo Ledesma-Aguilar, Ciro Semprebon, Emma Carter, Andrew Alderson It is a common conception that when a material is stretched it becomes thinner. However, when an auxetic metamaterial is stretched it becomes wider. This is because the properties of an auxetic material are determined by its lattice arrangement and not by the materials properties of the individual solid elements. Because the expansion of the solid surface area is due to increased space between the solid elements, this allows new types of superhydrophobic, hemi-wicking and liquid-infused surfaces. To illustrate these ideas, we describe the theory for a hydrophobic bow-tie element based lattice constructed with joints that rotate under strain to create a hexagonal lattice. This creates an auxetic superhydrophobic material with a negative Poisson's ratio which can be converted by strain to a superhydrophobic material with a positive Poisson's ratio. We illustrate these ideas experimentally using surfaces designed with micro-scale bow-tie lattice structures. We show how the wetting and superhydrophobicity of these surfaces behave under strain for both positive and negative Poisson's ratio. |
Sunday, November 20, 2022 8:13AM - 8:26AM |
A30.00002: Condensation on Superhydrophobic Surfaces in Vapor Shear Flow Shaur Humayun, Daniel Maynes, Julie Crockett, Brian D Iverson In a condensing environment, the heat transfer through superhydrophobic (SH) surfaces is significantly affected by the size of the condensed drops, wetting state and resulting drop mobility on the surface. To maintain dropwise condensation and accompanying high heat transfer rates, condensed drops may be removed using an external force, such as gravity or vapor shear. Without removal, condensate remains on the surface eventually forming a film, dramatically inhibiting heat transfer. This experimental study focuses on predicting drop departure diameter and condensation heat transfer for drops exposed to vapor shear flow on SH surfaces. A facility that provides air with high moisture content at an elevated temperature is passed over a cooled SH surface for condensation to occur. The condensation process is imaged through time using multiple cameras and drop departure diameters are measured as a function of the vapor flow speed. Measured drop departure diameter is compared to theoretical calculations of departure by equating the drop adhesion force to the fluid drag force acting on the drop. Experiments were performed and the drop departure diameter was determined for shear flows ranging from 0.5 to 1.5 m/s. The influence of the surface solid fraction and pitch (spacing between micro- or nanoscale features) was also quantified over typical ranges of these parameters for realizable SH surfaces. The model results show good agreement with the experimental values and enable predictions that can be further integrated into an overall heat transfer model of condensing flows over SH surfaces. |
Sunday, November 20, 2022 8:26AM - 8:39AM |
A30.00003: Condensation on CICNT Structured Superhydrophobic Surfaces Clint M Hubbard, Daniel Maynes, Brian D Iverson, Julie Crockett This study experimentally investigates the impact of subcooling magnitude, CNT diameter, time duration of condensation, and amount of non-condensable gases on the nature of dropwise condensation and condensate removal. Hour long experiments took place in a vacuum chamber on a vertically oriented test surface with varying CNT diameter and subcooling magnitude. Thermocouples provided data to compute overall heat transfer magnitudes. Three types of condensation were observed: drop jumping, drop wetting of the CNTs, and surface flooding. Results reveal that drops retain 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. As CNT diameter increases to 70-75 nm, the drops lose mobility and become pinned to the surface; the nature of the condensation transitions from droplet jumping to surface flooding. This same transition occurs at subcooling magnitudes greater than 9-10 C, independent of CNT diameter. The amount of time the condensate interacts with the surface also affects drop mobility. After an hour exposure time, the condensation transition occurs at smaller CNT diameters and lower surface subcooling. Results show the influence of time is not due to a degradation of the surface structure. However, when non-condensable gases are introduced to the test chamber, the mechanism of drop jumping is completely suppressed for all conditions explored and only drop wetting or flooding is observed. |
Sunday, November 20, 2022 8:39AM - 8:52AM |
A30.00004: Rolling Drops on Heated Superhydrophobic Surfaces Joseph Furner, Julie Crockett, Brian D Iverson, Daniel Maynes Superhydrophobic (SH) surfaces could have important applications in highly efficient condensers and water desalination processes due to ideal heat transfer properties and increased drop mobility. Here we experimentally investigate drops rolling across heated subcritical hydrophobic and SH surfaces textured with both nanostructures and post and rib-patterned microstructures to characterize thermal transport as a function of the surface solid fraction, pitch (distance between structures), drop rolling speed, and droplet volume. Experiments were performed with drops ranging from 10 - 40 μL and smooth and SH surfaces with solid fractions ranging from 4 - 100%. Drop temperature was determined using a high-speed infrared camera, from which an instantaneous bulk-averaged temperature was calculated. An analytical model was also developed to quantify the heat transfer as a function of all influencing variables, while using a temperature jump length to account for SH surface characteristics. Good agreement between the model and experimental results is observed. As the solid fraction decreases or the pitch between neighboring micropillars increases, heat transfer to the drop decreases. |
Sunday, November 20, 2022 8:52AM - 9:05AM |
A30.00005: Driving flow through microfluidic channels: analytical solutions generalizing Philip (1972) Hiroyuki Miyoshi, Henry Rodriguez-Broadbent, Anna Curran, Darren G Crowdy Analytical solutions are presented describing pressure-driven flow in superhydrophobic channels whose top and bottom walls are decorated with periodic arrays of no-shear menisci spanning the interstitial regions between no-slip grooves. The case where the menisci have invaded the grooves is also treated. The solutions are natural extensions to well-known mixed no-shear/no-slip solutions due to Philip [ZAMP, 23, 1972]. From the solutions, useful diagnostic quantities such as the hydrodynamic slip length can be easily extracted. |
Sunday, November 20, 2022 9:05AM - 9:18AM |
A30.00006: Study of gas diffusion for underwater super-hydrophobic surfaces Ali Nosrati, Aleksey Bourgoun, Mehdi Raessi, Hangjian Ling Gas diffusion could cause a wetting transition for underwater super-hydrophobic surface (SHS). Here, we studied this problem through a combination of numerical and experimental approaches. In the numerical study, we solved the spatial-temporal evolution of the gas concentration in the liquid in COMSOL. We found that the profiles of gas concentration at different times are self-similar, and the mass flux reduces with time (t) at a rate of 1/t0.5. In addition, we examined the impact of texture parameters, including pitch, gas fraction, texture height, and advancing contact angle, on the diffusion process. We proposed simple analytical models that can predict the longevity for SHS with various texture geometries. Experimentally, we fabricated SHSs with regular patterns on transparent PDMS (Polydimethylsiloxane) surface and studied their longevity in a pressurized chamber. Reflection Interference Contrast Microscopy was used to investigate the shape of gas-liqiuid interface as well as the plastron longevity. We found that the patterned PDMS surfaces have much shorter longevities than the numerical predictions. This disagreement might be due to the presence of other wetting mechanisms in the experiments, such as thermodynamic energy minimization. |
Sunday, November 20, 2022 9:18AM - 9:31AM |
A30.00007: Atomization Prediction Model on Superhydrophobic Surfaces Eric Lee, Daniel Maynes, Julie Crockett When a drop impinges a superheated surface intense atomization can occur. Atomization intensity varies as a function of time, Weber number, and surface temperature; it is also a strong function of surface characteristics. This study considers dynamic atomization on both hydrophobic and superhydrophobic surfaces (SH). SH surfaces are coated with a hydrophobic coating and micro-scale pillars. This work presents an analytical model, based on thermal transport and impingement flow dynamics, that predicts the amount of vaporization that occurs when a drop impacts a SH surface. The model accounts for pitch and solid fraction, surface temperature, and We number. Vapor generation is calculated from the model, and the results correlate well with experimentally measured amounts of atomization on the same surfaces. The results show a variation of vapor production with surface superheat. Vapor production increases with increasing temperature until it reaches transition boiling and then decreases until the Leidenfrost point. The results also show vapor production varies strongly with solid fraction and pitch. A maximum in atomization intensity is observed on a SH surface with 8 µm pitch and solid fraction of 10% at a surface temperature of 280. |
Sunday, November 20, 2022 9:31AM - 9:44AM |
A30.00008: Drop Retention and Departure in Shear Flow on Structured Superhydrophobic Surfaces Blake Lyons, Brian D Iverson, Daniel Maynes, Julie Crockett Drops are retained on surfaces due to adhesion forces between the drop and the surface. The adhesion force depends on the surface tension of the liquid, drop geometry, and the contact angle between the drop and the surface. When gravitational or fluid dynamic forces exceed the adhesion force the drop begins to move. Various models have been proposed in previous works to relate when a droplet will depart from a flat plate due to an imposed air velocity. However, no validated model exists that can predict the onset of droplet motion over a large range of static contact angles ranging both hydrophobic and superhydrophobic regimes. In this study we explore the drop departure phenomena for hydrophobic and superhydrophobic (SH) surfaces that exhibit static contact angles from 115° to 170°. A steady air flow (ranging in speed from 1-5 m/s) is directed parallel to a flat plate and particle image velocimetry (PIV) has been utilized to characterize the near wall velocity distribution. The SH surfaces explored are fabricated with circular post microstructures of varying pitch (structure to structure spacing) and cavity fractions. Image analysis is utilized to quantify the instantaneous centroid position and contact angles of the drops as they oscillate and then eventually depart from the surface. Prior models have been evaluated and it has been shown that predictions deviate significantly from experimental results when the models are used outside of the regime (hydrophobic, superhydrophobic) they were developed for. |
Sunday, November 20, 2022 9:44AM - 9:57AM |
A30.00009: Drying dynamics of sessile-droplet arrays Azmaine Iqtidar, Joseph J Kilbride, Fouzia F Ouali, David J Fairhurst, Howard A Stone, Hassan Masoud We analyze the evaporation of multiple droplets placed near each other on a planar substrate. Specifically, we theoretically calculate the change in the volume of sessile droplets with various initial contact angles that are arranged in several different configurations. Our calculations are supplemented by experimental measurements for the case of initially hemispherical droplets deposited in four distinct arrangements. We find excellent agreements between the predictions based on the theory of Masoud et al. [Evaporation of multiple droplets, J. Fluid Mech. 927, R4 (2021)] and the data gathered experimentally using a technique that interprets the variable magnification of a pattern placed beneath the droplet array. Perhaps unexpectedly, we also find that, when comparing different arrays, the droplets with the same order of disappearance within their respective array, i.e., fastest evaporating, second-fastest evaporating, etc., follow similar drying dynamics. Overall, our study not only provides experimental validation of the theoretical framework introduced by Masoud et al., but also offers additional insights into the evolution of the volume of individual droplets when evaporating within closely-spaced arrays. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700