Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L11: Drops: Superhydrophobic Surfaces |
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Chair: Simon Dai, University of Texas, Dallas Room: Georgia World Congress Center B216 |
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Monday, November 19, 2018 4:05PM - 4:18PM |
L11.00001: Delaying Ice formation using Passive De-Icing Materials Rukmava Chatterjee, Daniel Beysens, Sushant Anand The menace of ice formation on functional surfaces is ubiquitous in our daily life, having a deleterious effect on many industries entailing gargantuan economic damage yearly. Over the years, despite remarkable progress in the fields of microfabrication and surface chemistry, majority of the engineered surfaces have been futile in passively curbing icing under extreme environs of high humidity and freezing temperatures; with the modern industry still relying primarily on energy and cost intensive active de-icing techniques. Motivated by this and conducting research on material characterization and surface engineering, we have developed a novel class of passive icephobic materials. On testing these materials in a high humidity and low substrate temperature atmosphere, significantly delayed ice formation was demonstrated as compared to superhydrophobic surfaces; with some of these materials exhibiting sustained ice-free operation for more than 90 hours. These materials on being infused into microtextured hydrophilic surfaces and tested in a humid and frigid atmosphere, outperformed the control superhydrophobic surfaces by 2-4 times. The encouraging ice mitigation results of these novel materials have the potential for the design-fabrication of durable industrial de-icing coatings. |
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Monday, November 19, 2018 4:18PM - 4:31PM |
L11.00002: Triple line depinning criteria of planar and axisymmetric drops. José Graña-Otero, Ignacio E. Parra Fabian Precise quantitative criteria predicting triple lines depinning are scarce. In order to find the, we have studied planar and axisymmetric layers and drops pinned at a sharp edges with apparent contact angle θ0. Equilibrium is compatible with a fan of angles θ0 bracketed by the equilibrium contact angles of the both flanks of the edge, so the triple line could remain pinned as long as θ0 is within this fan. We find, however, critical depinning advancing θ0a and receding θ0r contact angles, which occur as subcritical saddle-node bifurcations with no equilibrium solutions beyond a critical suitably defined Bond number. |
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Monday, November 19, 2018 4:31PM - 4:44PM |
L11.00003: Designing a Bioinspired Hydrophobic Surface with honeycomb-like microtextures Rajneesh Bhardwaj, Manish Kumar We report the non-wetting characteristics of a hydrophobic leaf Colocasia and design a bioinspired microtexture surface based on the morphology of the leaf. SEM of the leaf surface shows a two-tier honeycomb-like microtextures. The static, advancing and receding contact angle of water droplet on the surface of the leaf were measured as 149, 153 and 144 degrees, respectively. One-tier SU-8 hexagonal cavities on a silicon wafer were manufactured in our lab using photolithography techniques. The measured static contact angle on the engineered surface was found to be function of the ratio of the radius of inner and outer circle, circumscribing the hexagonal cavity. The angle increases with the ratio and a maximum angle of 149 degrees was measured. The variation in the angle in Cassie state was consistent with an existing model (Patankar, Langmuir, 2005). We report evaporation characteristic of a sessile droplet and impact dynamics with different impact velocities on this surface. The evaporation characteristics were compared with a diffusion-limited evaporation model for a sessile droplet (Bhardwaj, Colloidal and Interface Science Communications, Vol. 24, 2018). |
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Monday, November 19, 2018 4:44PM - 4:57PM |
L11.00004: In-between full levitation and stable Cassie-Baxter: A range of interesting wetting states enabled by gas perfusion through porous media Nikolaos Vourdas, Vassilis Stathopoulos Actuation of droplets and manipulation of their mobility on surfaces is very crucial for a wide range of applications related to interfacial phenomena. In treating such challenges various methods have been proposed and demonstrated. For porous hydrophobic surfaces in particular droplet actuation may be enabled also by gas perfusion through the porous body. This was mainly achieved by applying the adequate gas flow rate in order to depin the initially quiescent droplet from the porous surface resting on the solid faction (partially wetting, Cassie-Baxter state), to a fully levitated state on which the droplet move frictionless (non-wetting, Leidenfrost-like state). This actuation required high flow rates and therefore high amount of energy. In this work we explore the states in-between these two extremes and prove that actuation and mobility manipulation may be delivered at ultra-low gas flow rates and accordingly to ultra-low energy consumption. The actuation mechanism was followed employing the continuity equation and the equations of momentum transfer that are coupled with the Volume of Fluid method, to track the shape of the droplets. Applications for water droplets on plane surfaces, confined surfaces as well as for viscous fluids will be provided. |
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Monday, November 19, 2018 4:57PM - 5:10PM |
L11.00005: Inducing droplet motion on SLIPS (Slippery Liquid Infused Porous Surfaces) Gaby Launay, Gary Wells, Rodrigo Ledesma Aguilar, Halim Kusumaatmaja, Glen McHale Slippery Liquid-Infused Porous Surfaces (SLIPS) are super-hydrophobic surfaces inspired by the nepenthes pitcher plant. These surfaces allow a very low contact angle hysteresis, and consequently, a high droplet mobility. Based on this property, we have developed SLIPS capable of inducing and controlling the motion of droplets. This has many practical applications in microfluidic, including: (i) sorting droplets depending on their size and/or fluid type, (ii) accurately positioning droplets and (iii) merging and splitting droplets. |
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Monday, November 19, 2018 5:10PM - 5:23PM |
L11.00006: Superhydrophobic surfaces that can selectively trap a drop based on temperature Samira Shiri, Armela Murrizi, James C Bird A water drop will bounce on a surface if the surface is sufficiently superhydrophobic. The degree of superhydrophobicity can be tuned by modulating the chemistry and microstructure of the surface, thus enabling external control of whether a particular drop bounces or sticks. A challenge in these approaches is that they require separate sensing, processing, and actuating steps. Here we explore how one might design a smart superhydrophobic surface in which the surface can sense a property of the drop, here its temperature, and, if above a critical threshold, passively adjust its functionality so that it will capture the drop in the absence of external control. Specifically we model two potential mechanisms in which a superhydrophobic surface could trap a sufficiently hot drop within milliseconds: melting of microtextured wax and condensation of the vapor within the superhydrophobic texture. We then test these mechanisms through systematic drop impact experiments in which we independently vary the substrate and drop temperatures on a waxy superhydrophobic Nasturtium leaf. In this regime a critical temperature threshold for bouncing can be controlled by considering the relative timescales between condensation growth and drop residence time. |
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Monday, November 19, 2018 5:23PM - 5:36PM |
L11.00007: Deposition of micrometric water droplets on rough hydrophobic surfaces Jeong-Hyun Kim, Daniel M Harris In this talk, we present new experimental observations of a droplet sliding down an inclined, rough hydrophobic surface. Periodic micrometer-scale grooves were fabricated on smooth PDMS surfaces using a commercial laser cutter. A millimetric water droplet was then deposited on the inclined surface and the resulting contact-line dynamics were visualized using high-speed imaging. We found that a micro-capillary bridge was formed at each structure, which ultimately ruptured at the trailing edge of the sliding water droplet. This detachment mechanism resulted in the deposition of a trail of uniform micrometric droplets on top of the periodic substrate. The size of the isolated droplets was sensitive to both the droplet sliding velocity and the dimensions of the grooved structures; in particular, the size of the droplet increases with increasing droplet velocity as well as the spacing between the structures. Potential applications and future directions will be discussed. |
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Monday, November 19, 2018 5:36PM - 5:49PM |
L11.00008: Ripening droplet: spontaneous movement of non-contact micro/nano droplets Zongqi Guo, Gaurav Kumar Sirohia, Xianming (Simon) Dai The rapid removal of condensates is highly desirable in condensation. Existing technologies are centered on jumping droplets, which can passively remove coalesced droplets on superhydrophobic surfaces. However, the jumping effect suffers from significant weaknesses. First, it is inefficient to jump off by waiting for the contact of multiple droplets. Second, once droplets jump off, they can not contribute to the removal of other droplets. Third, the jumping effect relies on the existence of air under droplets, but the air can be easily displaced by condensates at an elevated subcooling. To overcome those bottlenecks, we present the newly observed ripening effect: non-contact tiny droplets are propelled to coalesce with bigger droplets on a pinning-free liquid infused surface. Moreover, large droplets can easily form and sweep the remaining droplets on the surface. The ripening process can passively remove micro/nanoscale droplets by gradient vapor pressure and rapidly generate water-free area for further condensation. Our ripening droplet demonstrates the sustainable passive droplet removal of non-contact tiny droplets, outperforming the state-of-the-art jumping droplet condensation. |
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Monday, November 19, 2018 5:49PM - 6:02PM |
L11.00009: Simulating drop-wise condensation using lattice Boltzmann method Yaroslav Vasyliv, Alexander Alexeev Drop-wise condensation (DWC) was first identified as an important heat transfer mode in the 1930s. Relative to film-wise condensation (FWC), DWC results in approximately an order of magnitude increase in heat transfer. Understanding the role microstructures serve in enhancing the DWC mode is critical to improve heat transfer in various applications. Here, we simulate drop-wise condensation of a single component multiphase (SCMP) fluid on 2D structured surfaces using the lattice Boltzmann method. A non-ideal equations of state is specified and the LBM fluid solver is coupled to a thermal solver. Compared to other SCMP models, the model does not require empirical correlations to describe phase transition, it enforces interface conditions in a diffuse manner with complex morphological transitions captured on an Eulerian grid, and it does not require an elliptic pressure solver. We validate the model by comparing with the Nusselt's falling film and the Stefan problem. We then quantify the non-dimensional heat transfer for selected microstructure geometries as a function of surface wettability. |
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Monday, November 19, 2018 6:02PM - 6:15PM |
L11.00010: Drop size distribution during condensation on superhydrophobic surfaces Kimberly Stevens, Julie Crockett, Daniel Maynes, Brian Iverson With recent interest in condensation on superhydrophobic surfaces, several models to predict the heat transfer rate have been proposed. The models typically consist of expressions for the heat transfer rate to an individual drop multiplied by the drop size distribution; the product is then integrated over the range of drop sizes found on the surface to obtain the total heat transfer rate. Population balance modeling is frequently combined with an empirical expression in a piece-wise fashion to obtain the drop size distribution. However, this approach assumes that droplet coalescence does not occur until a specified radius. The current work proposes a model where randomly distributed drops grow based on published models for heat transfer to an individual drop. When growing drops overlap, they coalesce and potentially jump. As drops become sufficiently large, gravity sweeps them along the vertical surface, removing all other drops in its path. Good agreement is found between current and previous models when employing conditions consistent with published expressions for drop size distribution. With the assumptions removed, the simulation predicts a size distribution more consistent with physical observations, thus improving prediction of the overall heat transfer rate. |
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Monday, November 19, 2018 6:15PM - 6:28PM |
L11.00011: Thermal Atomization during Droplet Impingement on Superhydrophobic Substrates Preston Emerson, Julie C Crockett, Daniel Maynes Water droplets impinging superheated substrates may be characterized by dynamic vapor bubbles rising to the surface and causing a spray of tiny droplets to erupt from the droplet. This spray, known as thermal atomization, is the focus of an experimental study of water droplets impinging superheated, superhydrophobic surfaces. In this study, atomization of impinging droplets is quantified for superhydrophobic substrates of varying microstructure shape and dimensions over a range of superheat temperatures, Weber numbers, and inclination angle. Each silicon microstructured substrate was placed on an aluminum heating block, and impingement events were captured with a high speed camera. The level of thermal atomization was quantified by estimating the volume of liquid spray present for each event using a new image processing technique. Leidenfrost temperature is estimated for each scenario. Additionally, the effect of varying parameters on maximum droplet diameter and atomization drop velocities is explored. We find thermal atomization is most significantly altered by microstructure spacing and design as well as Weber number. |
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Monday, November 19, 2018 6:28PM - 6:41PM |
L11.00012: Heat transfer to bouncing droplets on superhydrophobic surfaces Chunfang Guo, Daniel Maynes, Julie Crockett, Danyang Zhao This study experimentally and theoretically investigates the dynamics and heat transfer to impinging water droplets on superhydrophobic surfaces heated below the boiling temperature. Different from impingement on hydrophilic substrates, the droplets rebound from the surface after the spreading and retraction stages. Experiments are performed using simultaneous high speed video and infrared (IR) imaging to capture droplet dynamics and temperature variation during the transient event. Thermal images allow estimation of bulk droplet temperature change during contact such that the cooling effectiveness for an individual droplet can be estimated. A similarity solution is utilized to yield a model for the transient heat flux at the droplet-wall interface, where convection inside the droplet is accounted for. The experimental and theoretical results for the cooling effectiveness show good agreement. It is revealed that the cooling effectiveness increases with Weber number but decreases with droplet diameter and surface cavity fraction, defined as the ratio of cavity area on the surface. |
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