Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session T10: Drops: Superhydrophobic Surfaces |
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Chair: ABHIJIT KUMAR KUSHWAHA, King Abdullah Univ of Sci & Tech (KAUST) Room: Ballroom J |
Monday, November 25, 2024 4:45PM - 4:58PM |
T10.00001: Droplet wetting dynamics on nanostructured surfaces induced by magnetic field Rupresha Deb, Amaresh Dalal The wetting transition of magnetic droplets from Cassie-Baxter to Wenzel states caused by an external magnetic field is investigated experimentally in the present work. We fabricated square-pillared surfaces of varying dimensions using photolithography. Droplet wetting experiments were performed for various configurations of the pillared substrates. The surface roughness was altered by varying the size of the square pillar, the gap between two pillars and the height of the pillars. The droplet deposited on the substrate experiences deformation and a wetting state transition upon the introduction of an external magnetic field. The externally applied magnetic field deforms the droplet shape and the pinning force due to pillars resists the deformation. Hence, the magnitudes of input magnetic energy required to break this energy-barrier is crucial in the understanding of the transition behaviour. In the present study an attempt has been made to obtain a regime map for roughness ratio vs magnetic Bond number for different volumes of ferrofluid droplet in which the deformation of ferrofluid droplet becomes irreversible and it undergoes Cassie-Baxter state to Wenzel state transition. Theoretical modelling of the droplet spreading dynamics in presence of an external magnetic field can better comprehend the wetting transition from energy minimization point of view. |
Monday, November 25, 2024 4:58PM - 5:11PM |
T10.00002: Drop Distribution Tuning and Dynamics during Flow Condensation on Superhydrophobic Surfaces Shaur Humayun, Daniel Maynes, Julie Crockett, Brian D Iverson Condensate removal is known to enhance heat transfer in dropwise condensation scenarios. Drop departure size in the presence of an applied shear flow (flow condensation) varies depending on surface conditions and wetting characteristics, in addition to the applied forces from the shear flow. This work explores the effect of varying surface solid fraction and surface feature size on drop mobility and departure in a condensing shear flow environment. Imaging of condensation experiments for humid airflow in a channel with a condensing surface captured the drop size distribution and maximum drop departure size for several superhydrophobic surface types (microstructured, nanostructured, and two-tiered) comprised of carbon nanotubes grown on etched Si surfaces. Particle image velocimetry was used to appropriately determine the applied forces acting on the drop in shear flow. The introduction of microscale surface features to a nanostructured surface (resulting in a two-tiered surface) decreases the maximum observed drop departure diameters by approximately 50%. Further, drop mobility increases by 3-4 times on two-tiered surfaces as compared to nanostructured surfaces. Results highlight a possible path to tuning drop size distributions which, in turn, alters the heat transfer rate at the condensing surface. |
Monday, November 25, 2024 5:11PM - 5:24PM |
T10.00003: Hot Liquid Marbles Pritam K Roy, Yui Takai, Rui Matsubara, Mizuki Tenjimbayashi, Timothée MOUTERDE Droplets in non-wetting states have minimal adhesion and exceptional mobility due to the presence of a gas layer between the liquid and its substrate. On superhydrophobic surfaces, the lubricating gas layer is maintained by hydrophobic texture at the solid surface. Here, the liquid only touches the tops of the texture and primarily contacts air [1]. Alternatively, the hydrophobic texture can be directly embedded at the liquid surface by covering the drop with hydrophobic particles. Such coated drops, known as liquid marbles [2], have been extensively studied for their low adhesion and high mobility [3]. In this presentation, we discuss the stability and adhesion of marbles containing hot liquids depending on their temperature, the substrate wettability, and the liquid volatility. |
Monday, November 25, 2024 5:24PM - 5:37PM |
T10.00004: Oblique impact of droplets on heated supherydrophobic surfaces Brigham C Ostergaard, Julie Crockett, Daniel Maynes, Brian D Iverson Superhydrophobic surfaces have been a topic of much recent interest in fluid dynamics and heat transfer applications. Many studies addressing droplet impact dynamics have focused on the fluid flow physics and thermal transport behavior for drops impacting the surface in the normal direction. However, in practice, the impact of fluid droplets on surfaces occurs across a full range of incident angles leading to different impact dynamics and heat transfer. Experiments were conducted that utilized high-speed IR imaging to measure the average temperature difference between pre- and post-impact droplets impinging on superhydrophobic surfaces. The total heat transfer to an impacting droplet is a function of the surface temperature, surface micro-structure cavity fraction, drop impact Weber number, and the angle at which the droplet impacts. There has been little prior work addressing the impact angle on the overall heat transfer and this is the primary focus of this work. Results show a dramatic reduction in heat transfer as the impact angle increases and as the level of superhydrophobicity of the surface increases. Furthermore, the formation of coherent Worthington jets was observed during the impact process in a narrow range of the influencing parameters. |
Monday, November 25, 2024 5:37PM - 5:50PM |
T10.00005: Condensation control on biphilic jumping droplet surface Hiroki Yachida, Noémie Muquet, Sophia Laney, Martyna Michalska, Ioannis Papakonstantinou, Timothée MOUTERDE Controlling condensation is crucial for applications such as thermal management or to maintain optical surface transparency in humid conditions. Condensation behavior can be controlled by surface structure: on nanoscale hydrophobic structures, droplets can jump out of the surface upon merging [1]. Making the nanostructures conical further amplifies the anticondensation property, as the jumping probability rises close to 100% [2]. The constant condensation removal allowed by the jumping mechanism significantly increases the heat transfer performance of the surface [3] and can be further improved by promoting nucleation with micrometric hydrophilic patterns [4]. Condensation removal limits the droplet size to the distance to its nearest neighbor which helps maintaining a good optical transparency. However, this distance depends on the random nucleation process and droplets nucleating far from each other will cover a large surface area before jumping which is detrimental for optical properties. In this presentation, we spatially control nucleation with biphilic patterns on a jumping droplet surface and study how it affects the condensation properties such as surface coverage, volume, or lifetime. |
Monday, November 25, 2024 5:50PM - 6:03PM |
T10.00006: Abstract Withdrawn
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Monday, November 25, 2024 6:03PM - 6:16PM |
T10.00007: Dissipative anomaly in sliding drops Vatsal Sanjay, Jnandeep Talukdar, Uddalok Sen, Christian Diddens, Detlef Lohse We use volume of fluid-based direct numerical simulations to compare drop sliding on ideal superhydrophobic and lubricant-impregnated surfaces. Ideal superhydrophobic surfaces conceptualize a thin air cushion between drop and substrate, but maintaining this layer's stability proves challenging. Consequently, current approaches often employ lubricating oils, creating a thin film on the substrate using which the drops can slide--an approach that subsequently generates a meniscus where viscous dissipation can occur. Remarkably, even for vanishingly small lubricant viscosity, we find that the drop's terminal velocity is lower on the lubricated surface than on a dry superhydrophobic surface. This phenomenon is attributed to persistent viscous dissipation in the lubricant meniscus, despite negligible viscosity, mirroring the ``dissipative anomaly" observed in turbulent flows. Our findings reveal fundamental limitations in mimicking ideal superhydrophobic behavior with lubricated surfaces and provide insights for optimizing self-cleaning technologies. Additionally, we uncover intriguing parallels between microscale interfacial flows and macroscale turbulence, suggesting broader implications for understanding dissipation in multiscale fluid systems. |
Monday, November 25, 2024 6:16PM - 6:29PM |
T10.00008: Abstract Withdrawn
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Monday, November 25, 2024 6:29PM - 6:42PM |
T10.00009: An aerodynamic Leidenfrost droplet is faster than a cold Leidenfrost droplet ABHIJIT KUMAR KUSHWAHA, Tadd T Truscott Place your bets! Which surface do you think will win in a race between two droplets down an incline, a Leidenfrost, or a superhydrophobic surface? Come find out as we examine the friction forces encountered by droplets moving on non-wetting surfaces, with a specific focus on droplets of varying viscosity traversing a heated superhydrophobic surface. We highlight and contrast the differences by comparing the observed dynamics with that of a droplet moving on a Glaco-coated superhydrophobic surface at room temperature (T = 22 oC) and Leidenfrost temperature (T = 300 oC). Spoiler alert, the droplet moving on a heated Glaco-coated surface, which exhibits a cold Leidenfrost effect, moves slower than the droplet moving on non-heated superhydrophobic surface, which exhibits an aerodynamic Leidenfrost effect. High-speed interferometric visualization shows that the droplet on a heated Glaco-coated superhydrophobic surface can develop instabilities within the vapor layer, resembling a Taylor-Saffmann instability and indicating additional viscous dissipation. Furthermore, these results uncover previously unexplored droplet dynamics and offer valuable insights into the dissipative mechanism at play for a droplet traversing a heated superhydrophobic surface |
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