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 A09: Drops: Heat Transfer, Evaporation and Buoyancy Effects I |
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Chair: James Bird, Boston University Room: Ballroom I |
Sunday, November 24, 2024 8:00AM - 8:13AM |
A09.00001: ABSTRACT WITHDRAWN
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Sunday, November 24, 2024 8:13AM - 8:26AM |
A09.00002: Abnormal evaporative flux on binary mixture droplets Minhyeok Kuk, Hyoungsoo Kim An evaporating binary mixture droplet has been substantially investigated for its industrial applications, such as spray cooling, inkjet printing, uniform coating, and evaporation-induced self-assembly (EISA), as well as the intriguing physical phenomena it presents. However, a complete understanding of the dynamic flow transition remains elusive due to changing initial concentration conditions caused by the relative evaporation rates and density stratification between two different liquid components. This gap in understanding is mainly due to the reliance on indirect investigation methods, such as Particle Image Velocimetry (PIV) to study local effects. To address this issue, we recently conducted a direct visualization of the evaporated vapor distribution in the gas phase. Using this approach, we successfully measured the evaporative flux of a single component of the droplet. In this talk, we will present the direct evaporative flux measurement results of the binary mixture droplet using laser interferometry. Our findings show that the location of the maximum evaporative flux shifts from the contact line to the top apex of the binary mixture droplet. These results are consistent with those obtained from PIV measurements. |
Sunday, November 24, 2024 8:26AM - 8:39AM |
A09.00003: Complex morphology on the underside of a Leidenfrost-levitated hydrogel sphere. Vicente Luis Luis Diaz Melian, Scott R Waitukaitis, Isaac Lenton, Jack Binysh, Anton Souslov When a liquid droplet approaches a hot surface, vaporization can become sufficient to cause the drop to levitate—this is the Leidenfrost effect. Vaporizable soft solids, e.g., hydrogels, can also exhibit levitation or, additionally, a sustained bouncing effect. In the case of floating liquids, vapor pressure and surface tension balance create an inversion of curvature on the droplet underbelly. Naively, one might expect that with a levitating soft solid, vapor pressure and elasticity should create a similar equilibrium with a similar curvature inversion. We use high-speed interferometric imaging to measure the 2D height profile underneath a floating hydrogel sphere and discover that the presence of such curvature inversion critically depends on the approach speed and total duration in the levitation state. Poking the system from a variety of experimental angles, we find that this curious behavior is due to permanent morphological changes caused by mass loss during vaporization. |
Sunday, November 24, 2024 8:39AM - 8:52AM |
A09.00004: Damped bouncing of Leidenfrost drops Pawan Indolia, Gopal c Pal, NARENDRA BAHADUR, Devranjan Samanta, Chander Shekhar Sharma When a liquid drop comes in contact with a very hot surface, a continuous vapor layer is formed between the drop and the solid surface, thereby hindering direct liquid-solid contact. This well-known phenomenon is commonly referred to as the Leidenfrost effect. A drop in the Leidenfrost state is highly mobile, and recent studies have shown that pressure perturbations in the vapor layer, arising from ripples at the bottom of the drop, can cause the drop to spontaneously bounce on the substrate as its size reduces to below capillary length due to evaporation. In this work, we show that this bouncing of the Leidenfrost droplets can be damped if the surface is textured with interconnected cavities of length scale comparable to or smaller than the ripple wavelength. We systematically investigate this effect by observing gently deposited Leidenfrost drops on a range of textures of length scale spanning from a few tens to a few hundred micrometers. We show that gently deposited Leidenfrost drops can traverse such surfaces with minimal bouncing and quantify the dynamics of such drops in terms of drop motion and ripples on the drop surface. We also show that disconnected cavities do not lead to suppression of bouncing, thus providing a design criterion for such surfaces. |
Sunday, November 24, 2024 8:52AM - 9:05AM |
A09.00005: Deposition of Evaporating Microdroplets Near a Stagnation Point Md Shamser Ali Javed, Vladimir S Ajaev Respiratory droplets that can transmit infections were the subject of many recent studies which had a strong impact on understanding human respiratory diseases. In particular, trajectories and deposition locations of microdroplets in respiratory airways have been investigated. In the present study, we investigate the deposition of microdroplets near a stagnation point in the respiratory airway. Our model accounts for the presence of boundary layers in both vapor density distribution and moist air flow field. We consider evaporation on the droplet surface that reduces the size of the droplet as it travels through the region of varying vapor density and describe how evaporation changes the droplet trajectories and the deposition patterns at the wall of the respiratory airway. Droplets are found to be deposited further away from the stagnation point as their size is reduced by evaporation. |
Sunday, November 24, 2024 9:05AM - 9:18AM |
A09.00006: Bottom-up view of sweat droplet and film evaporation dynamics Konrad Rykaczewski, Ankush K Jaiswal, Cibin T Jose, Kambiz Sadeghi, Ankit Joshi, Krishna Kompally, Gokul Pathikonda, Stavros Kavouras, Konrad Rykaczewski Sweat evaporation is vital for human thermoregulation; however, it has yet to be studied on the length scale of single pores. Here, we discuss a wind tunnel-inspired ventilated capsule with a sapphire window that simultaneously measures sweat evaporation rate and enables mid-wave infrared imaging microscale sweating dynamics [1]. Besides the experimental and numerical characterization of the parabolic flow and evaporation rate on artificial surfaces, we also discuss the use of the device in human subject experiments. The experimentation highlights multiple microscale fluid processes occurring near or on the skin including out-of-duct evaporation, pulsating droplets, crevice filling, and hydration-induced droplet-to-film transitions. In addition, the results challenge the common assumption that sweat forms an isothermal film, revealing 3x higher mass transfer coefficients during cyclic dropwise than filmwise sweating. The research highlights significant differences between various stages and rates of evaporation, suggesting the need for further exploration of these mechanisms. |
Sunday, November 24, 2024 9:18AM - 9:31AM |
A09.00007: Experimental and numerical study of an evaporating capillary bridge of a binary liquid Mradul ojha, Lalit Kumar, Rajneesh Bhardwaj We numerically and experimentally study the evaporating capillary bridges of pure and binary liquids between poly-dimethylsiloxane (PDMS) coated surfaces. The temporal evolution of the bridge is captured using side visualization. The numerical model uses the Galerkin finite element method (FEM), for diffusion-limited species transport equation in cylindrical coordinates. Modified Roult's law is used for the binary liquid, and the ``Aerosol Inorganic-Organic Mixtures Functional Groups Activity Coefficient'' model approximates the activity coefficients. The model was validated using published experimental data for sessile droplets and in-house measurements for capillary bridges. Results reveal longer evaporation periods for capillary bridges than sessile droplets with identical initial parameters attributed to the confined space. Volume evolution for binary liquids is non-linear, dependent on the remaining alcohol percentage. The model accurately predicts observed non-linear evaporation rates for binary liquids. A reduced order model is developed for binary capillary bridges, which shows a good prediction for volume variation and evaporation time with the FEM model. The applications of this research are in rheometers, roller printing, and other areas. |
Sunday, November 24, 2024 9:31AM - 9:44AM |
A09.00008: Abstract Withdrawn
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Sunday, November 24, 2024 9:44AM - 9:57AM |
A09.00009: Optimizing Heat Exchanger Efficiency through Enhanced Surface Textures and Water Film Cooling Damien Thomas, Stephane L Zaleski, Stéphanie Lacour Water cooling is more effective at dissipating heat than air convection. However, the complex geometry of heat exchangers can hinder complete surface wetting, leading to dry areas and reduced cooling efficiency. To address this, enhanced fins with varying textures are necessary to maintain high heat transfer. This study focuses on the cooling effect of a water film. |
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