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 L10: Drops: Heat Transfer, Evaporation and Buoyancy Effects II |
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Chair: Christian Diddens, University of Twente, Department of Physics of Fluids Room: Ballroom J |
Monday, November 25, 2024 8:00AM - 8:13AM |
L10.00001: Local flow structure and microdroplet trajectories near the boundary of a dry patch. Md Nayan Dhali, Vladimir S. Ajaev For the past few decades, there have been many studies of evaporation near contact lines where solid substrates and liquid-gas interfaces meet. The focus of our investigation is on local solutions near a contact line, which is the boundary of a dry patch. We consider a single, circular dry patch that has been formed in an evaporating liquid layer on a flat heated substrate. Flow is induced by evaporation and thus depends on the distribution of vapor concentration in moist air. We use Tranter’s method to solve the diffusion equation for vapor concentration in the domain above the liquid layer. Evaporative flux at the interface near the contact line and the corresponding local flow structure are determined based on this solution. The trajectories of microscale liquid droplets in vicinity to the contact line are assessed using different approximations for the aerodynamic drag force. Furthermore, the comparison of experimental and numerical findings is highlighted. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L10.00002: Viscosity influence on heat transfer in drop-laden turbulence Francesca Mangani, Alfredo Soldati This work numerically investigates the effect of viscosity differences on the heat transfer in a drop-laden turbulent channel flow, where warm drops are dispersed in a cold carrier fluid. This problem is studied by coupling direct numerical simulation (DNS) of turbulence with a phase field method (PFM) for the interface description. We consider warm water drops in cold oil and warm oil drops in cold water. The oil viscosity is four times larger than the water viscosity, while density and thermal diffusivity are equal in the two fluids. Thus, in the first case we set a viscosity ratio (drops over carrier viscosity) of μr=1/4 and a Prandtl number (momentum over thermal diffusivity) of Pr=4 in the water carrier phase, which determines a local Prandtl of Pr=16 in the oil drops. Similarly, in the second case we set μr=4 and Pr=16 in the oil carrier phase, giving Pr=4 in the water drops. To maintain a constant Péclet number in the systems, a shear Reynolds of Reτ=600 and Reτ=150 are imposed in the water and oil carrier phases, respectively. It is observed that the average temperature of each fluid shows an almost coincident evolution in time in the two cases. Since the thermal diffusivity is constant, this result can be attributed to the impact of viscosity on the convective heat transport. Indeed, in the more viscous oil phase the heat transfer is slowed down, while in the less viscous water phase the heat transfer is accelerated. These effects balance each other in the two cases, which therefore result in having an equal heat transfer rate. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L10.00003: Shape matters: Geometry-driven Marangoni instability in evaporating binary droplets Duarte F Rocha, Detlef Lohse, Christian Diddens When sessile glycerol-water droplets evaporate, the solutal Marangoni flow remains stable and axisymmetric, whereas e.g. ethanol-water droplets usually exhibit highly non-axisymmetric chaotic flow scenarios. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L10.00004: Thermocapillary migration of a slightly deformed spherical fluid drop in an immiscible fluid Jai Prakash, Huan J. Keh In this work, we report on theoretical investigation of thermocapillary migration of a slightly deformed spherical fluid drop in a viscous incompressible fluid with a uniformly prescribed but arbitrarily oriented temperature gradient in the steady limit of vanishing Péclet and Reynolds numbers. The flow fields in the exterior and interior of the drop are governed by the Stokes equations whereas the temperature fields in both the regions are governed by Laplace's equation. The governing equations are solved asymptotically using a method of perturbed expansions under suitable boundary conditions. The deformation from spherical shape is characterized by a small parameter, and we have solved the problem up to the second order of this deformation parameter. The thermocapillary migration velocity of the slightly deformed spherical drop is obtained by applying force free condition. The explicit expressions for the migration velocity of the drop are obtained for the special cases of prolate and oblate spheroidal drops for various values of the internal-to-external viscosity ratio, axial-to-radial aspect ratio and relative thermal conductivity of the drop. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L10.00005: Violation of D2- Law in Droplet Combustion under Gravity: New Shrinking Kinetic Law Due to Flame-Driven Buoyant Convection Hsien-Hung Wei, You-An Chen, Shou-Yin Yang D2-law is a long-established and widely used relationship for describing the shrinking behavior of a vaporizing droplet in fuel spray and combustion processes, stating that square of the droplet instantaneous diameter D decreases linearly with time t. Here we show both experimentally and theoretically that this classical law is not the true law for a burning droplet under the influence of gravity. Experimentally, we find that such a droplet actually obeys a Dn-law with the exponent n =2.53±0.30-2.69±0.19 for a variety of common liquid fuels such as alkanes and alcohols. On the theoretical side, we are able to identify mechanisms involved to show n=8/3 well capturing the experimental values. This new D8/3-law originates from the additional inherent buoyancy length determined solely by fuel properties. It is a consequence of simultaneous momentum, heat, and mass transfer arising from buoyant convection set up by the blazing flame around the droplet, insensitive to combustion chemistry and detailed reaction kinetics. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L10.00006: Evaporation-induced translation of Multiple Binary Droplets Debarshi Debnath, Anna Malachtari, George Karapetsas, Daniel Orejon, Khellil Sefiane, Alidad Amirfazli, Prashant Valluri Evaporation of multiple sessile droplets is a complex physical scenario due to their dynamic interaction through the vapour phase. In situations when such evaporating droplets comprise a binary mixture, their interaction becomes more complex due to the inclusion of solutal Marangoni. The physical understanding of such instances is extremely important for many crucial applications ranging from inkjet printing and spray cooling to disease diagnosis and DNA chip manufacturing. We propose a lubrication theory-based finite element model for two evaporating sessile drops comprising a binary mixture on a heated surface. We consider a diffusion-limited evaporation regime for thin droplets in the presence of a pre-cursor film. We consider that the liquid components are ideally mixed and that both components are volatile. The net surface tension is taken as a linear function of both temperature and concentration. Further, we also solve for the vapour concentration for both the volatile components in the presence of air in 2D using the standard diffusion equation. Our solutions for two adjacent droplets comprising the same binary mixture compositions demonstrate attraction, coalescence, and repulsion between the droplets as a function of thermal and solutal Marangoni. In addition, we also performed experiments with water-morpholine drops to demonstrate the predictions from our model physically. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L10.00007: Intricate Evaporation Dynamics in Different Multi-Droplet Configurations Hari Govindha A., Sayak Banerjee, Saravanan Balusamy, Kirti Sahu We experimentally investigate the evaporation dynamics of an array of sessile droplets arranged in different configurations. Utilizing a customized goniometer, we capture side and top view profiles to monitor the evolution of height, spread, contact angle, and volume of the droplets. Our results reveal that the lifetime of a droplet array surpasses that of an isolated droplet, attributed to the shielding effect induced by neighbouring droplets, which elevates the local vapour concentration, thereby reducing the evaporation rate. We found that lifetime increases as droplet separation distance decreases at a fixed configuration and substrate temperature. It is observed that the lifetimes increase with the number of droplets. We observe a decrease in lifetimes, following a power law trend with increasing substrate temperature, with the shielding effect diminishing at higher substrate temperatures due to natural convective effects. We also observe a generalized behavior for the centrally placed droplet across various separation distances and substrate temperatures. Different droplet configurations and substrate temperatures modify the local vapour concentration around the droplets without significantly impacting the contact line dynamics. Additionally, the experimental results are compared with a diffusion-based theoretical model that incorporates the evaporative cooling effect to predict the lifetime of the central droplet within the array. We observe that the theoretical model satisfactorily predicts the lifetime of the droplet at room temperature. However, for high-temperature cases, the model slightly overpredicts the evaporative lifetimes. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L10.00008: Liquid redistribution and the Kelvin effect in evaporating breath figures Joseph Kilbride, Fouzia F Ouali, David Fairhurst Multiple droplets are abundant in nature and industry, for example: condensing from warm breath on a cold day, on the underside of a coffee cup lid and as raindrops on windows. |
Monday, November 25, 2024 9:44AM - 9:57AM |
L10.00009: Controlling Droplet Shedding Size, Departure Time and Droplet Size Density during Dropwise Condensation on Silicone Oil Grafted Surfaces Anam Abbas, Zafar Iqbal, Gary George Wells, Glen McHale, Khellil Sefiane, Daniel Orejon Condensation plays a critical role in numerous industrial and engineering applications with dropwise condensation achieving 6-8 times better heat transfer than filmwise condensation. To date, several surface modification methods promoting dropwise condensation have been investigated. One of these methods involve uniformly applying low surface energy coatings with small contact angle hysteresis (CAH) that stimulate faster and smaller sized droplets shedding. Here, we use silicone oil grafting to create hydrophobic coatings on an otherwise hydrophilic silicon substrate. Based on fabrication parameters such as oil viscosity, volume and/or number of layers. the silicone oil grafting technique adopted achieves contact angles of ~108° and CAH ranging between 1° and 20°. Such surfaces are then subjected to condensation phase-change in a custom-built environmental chamber. We then use ImageJ and MATLAB for image processing where dropwise condensation is observed on all these grafted surfaces irrespective of the various fabrication parameters. However, droplet mobility, shedding time and droplet size distributions, can be actually controlled by changing the fabrication parameters. We observe faster shedding of smaller sized droplets on high viscosity oil grafted surfaces following their lower hysteresis when compared to low viscosity grafted ones. Fast droplet shedding on silicone oil grated surfaces creates space for new droplets to nucleate, grow, coalesce and shed, which is characteristic of high condensation heat transfer rates. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L10.00010: Fluid flow in an evaporating droplet on a solid surface VIKRANT CHANDRAKAR, Bibek Kumar, Mradul ojha, Rajneesh Bhardwaj We experimentally study internal fluid flow in evaporating droplets on a solid surface. Optical and fluorescent microscopy were used for the measurements. Water and isopropanol droplets of 0.2 microliter, glass and polydimethylsiloxane (PDMS) substrates, particle of 1.1 micrometer diameter, and 0.001% (v/v) particle concentration were considered. For a droplet of water on a glass surface, classical radially outward flow was seen in experiments. The droplet of water on PDMS shows two vortices in opposite directions without axisymmtericity in the flow. A droplet of isopropanol on PDMS shows Marangoni loops, as reported earlier in the literature. We also quantify the flow field using the two-dimensional Particle Image Velocimetry (PIV) technique. We also carry out theoretical analyses and compare the prediction with measurements. Fundamental insights into the internal convection will help to design application such as inkjet printing, bioassays, particle sorting, etc. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L10.00011: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 10:23AM - 10:36AM |
L10.00012: Evaporation of Drops on Sessile Oil Infused Surfaces Parth S Patil, Sushant Anand The evaporation of droplets is a crucial phenomenon with wide-ranging applications, from power generation to water purification, and even extends to myriad natural processes. Recently, the adoption of liquid-infused silicon surfaces (LIS) has increased, driven by their ability to reduce contact angle hysteresis, enhance droplet mobility, and modulate heat transfer in multiphase processes [1]. Surprisingly, few studies have explored the evaporation dynamics of drops on such surfaces. Herein, we study how water and organic liquids evaporate on oil-infused surfaces under identical environmental conditions. We study the role of infused oil’s physical properties in affecting drop evaporation. Moreover, theoretical models of mass diffusion have been developed and compared with the experimental study, which sheds light on the effect of oil properties on mass transfer behavior. Our study on droplet evaporation kinetics deepens the understanding of the evaporation phenomenon and could have significant implications for numerous applications, including distillation and evaporative cooling. |
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