Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G35: Drops: Heat Transfer and Evaporation |
Hide Abstracts |
Chair: Julien Landel, University of Cambridge Room: Ballroom B |
Monday, November 23, 2015 8:00AM - 8:13AM |
G35.00001: Impact of internal transport on the convective mass transfer from a droplet into a submerging falling film Julien R. Landel, Amalia Thomas, Harry McEvoy, Stuart B. Dalziel We investigate the convective mass transfer of dilute passive tracers contained in small viscous drops into a submerging falling film. This problem has applications in industrial cleaning, domestic dishwashers, and decontamination of hazardous material. The film Peclet number is very high, whereas the drop Peclet number varies from 0.1 to 1. The characteristic transport time in the drop is much larger than in the film. We model the mass transfer using an analogy with Newton's law of cooling. This empirical model is supported by an analytical model solving the quasi-steady two-dimensional advection-diffusion equation in the film that is coupled with a time-dependent one-dimensional diffusion equation in the drop. We find excellent agreement between our experimental data and the two models, which predict an exponential decrease in time of the drop concentration. The transport characteristic time is related to the drop diffusion time scale, as diffusion within the drop is the limiting process. Our theoretical model not only predicts the well-known relationship between the Sherwood number and the external Reynolds number in the case of a well-mixed drop Sh $\sim$ Re$^{1/3}$, it also predicts a correction in the case of a non-uniform drop concentration. The correction depends on Re, the film Schmidt number, the drop aspect ratio and the diffusivity ratio between the two phases. This prediction is in good agreement with experimental data. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G35.00002: Droplet evaporation on a soluble substrate Alexandra Mailleur, Christophe Pirat, Jean Colombani Stains left by evaporated droplets are ubiquitous in everyday life as well as in industrial processes. Whatever the composition of the evaporating liquid (colloidal suspensions, biological fluids\textellipsis ), the stains are mostly constituted by a deposit at the periphery of the dried drop, similar to a coffee stain (Deegan, 1997). All these studies have been carried with non-reacting solids. In this presentation, we focus on the behavior of a pure-water droplet evaporating on a soluble substrate which is more complex, since three phenomena are strongly interacting: the dissolution of the substrate, the diffusion/convection of the dissolved species into the drop and the evaporation of the liquid. NaCl and KCl single crystals have been chosen for this experimental study as they are fast-dissolving solids. We have observed that the dissolution induces a pinning of the triple line from the beginning of the evaporation, leading to a decrease of the contact angle in time. At the end of the evaporation, a peripheral deposit is always formed, proof of an outward flow inside the drop (coffee-ring effect). [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G35.00003: Evaporation of multi-component mixtures and shell formation in spray dried droplets Pedro Valente, \'Iris Duarte, Tiago Porfirio, M\'arcio Temtem Drug particles where the active pharmaceutical ingredient (APIs) is dispersed in a polymer matrix forming an amorphous solid dispersion (ASD) is a commonly used strategy to increase the solubility and dissolution rate of poorly water soluble APIs. However, the formation and stability of an amorphous solid dispersion depends on the polymer/API combination and process conditions to generate it. The focus of the present work is to further develop a numerical tool to predict the formation of ASDs by spray drying solutions of different polymer/API combinations. Specifically, the evaporation of a multi-component droplet is coupled with a diffusion law within the droplet that minimizes the Gibbs free energy of the polymer/API/solvents system, following the Flory-Huggins model. Prior to the shell formation, the evaporation of the solvents is modelled following the simplified approach proposed by Abramzon {\&} Sirignano (1989) which accounts for the varying relative velocity between the droplet and the drying gas. After shell formation, the diffusion of the solvents across the porous shell starkly modifies the evaporative dynamics. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G35.00004: Numerical simulations of evaporative instabilities in sessile drops of ethanol on heated substrates Sergey SEMENOV, Florian CARLE, Marc MEDALE, David BRUTIN The work is focussed on numerical simulations of thermo-convective instabilities in evaporating pinned sessile droplets of ethanol on heated substrates. Computed evaporation rate of a droplet is validated against parabolic flight experiments and semi-empirical theory presented here. To the best authors' knowledge, this is the first study which combines theoretical, experimental and computational approaches in convective evaporation of sessile droplets. The influence of gravity level on evaporation rate and contributions of different mechanisms of vapor transport (diffusion, Stefan flow, natural convection) are shown. The qualitative difference (in terms of developing thermo-convective instabilities) between steady-state and unsteady numerical approaches is demonstrated. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G35.00005: Evaporation dynamics of femtoliter water capillary bridges Kun Cho, In Gyu Hwang, Yeseul Kim, Su Jin Lim, Jun Lim, Joon Heon Kim, Bopil Gim, Jung Gu Kim, Byung Mook Weon Capillary bridges are usually formed by a small liquid volume in confined space between two solid surfaces and particularly they have lower internal pressure than 1 atm at femtoliter scales. Femtoliter capillary bridges exhibit rapid evaporation rates. To quantify detailed evaporation kinetics of femtoliter bridges, we present a feasible protocol to directly visualize femtoliter water bridges that evaporate in still air between a microsphere and a flat substrate by utilizing transmission X-ray microscopy. Precise measurements of evaporation kinetics for water bridges indicate that lower water pressure than 1 atm can significantly decelerate evaporation by suppression of vapor diffusion. This finding would provide a consensus to understand evaporation of ultrasmall capillary bridges. \newline [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G35.00006: ABSTRACT WITHDRAWN |
Monday, November 23, 2015 9:18AM - 9:31AM |
G35.00007: Droplet Impingement Boiling on Heated Superhydrophobic Surfaces Julie Crockett, Cristian Clavijo, Daniel Maynes When a droplet impinges on a solid surface at a temperature well above the saturation temperature, vaporization of the liquid begins immediately after contact. Different boiling regimes may result depending on the surface temperature and volatility of the liquid. The nucleate boiling regime is characterized by explosive atomization, which occurs when vapor bubbles burst causing an extravagant shower of small micro droplets as well as the well-known ``sizzling'' sound. In this work, we show that the vapor is surprisingly re-directed during impingement on a superhydrophobic surface such that atomization is completely suppressed. We hypothesize that this occurs because vapor escapes through the superhydrophobic interface such that the top of the droplet remains free of bursting vapor bubbles. We explore a wide range of surface patterning with feature spacing of 8 to 32 microns and solid area fractions of 10 to 50 percent; surface temperatures from 100 C to 400 C; and Weber numbers of 1 to 100. Atomization is found to decrease with increasing feature spacing and decreasing solid fraction, and vanishes completely for large spacing. It may be that large feature spacing promotes early transition to the Leidenfrost regime. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G35.00008: Enhanced condensation heat transfer with wettability patterning Pallab Sinha Mahapatra, Aritra Ghosh, Ranjan Ganguly, Constantine Megaridis Condensation of water vapor on metal surfaces is useful for many engineering applications. A facile and scalable method is proposed for removing condensate from a vertical plate during dropwise condensation (DWC) in the presence of non-condensable gases (NCG). We use wettability-patterned superhydrophilic tracks (filmwise condensing domains) on a mirror-finish (hydrophilic) aluminum surface that promotes DWC. Tapered, horizontal ``collection'' tracks are laid to create a Laplace pressure driven flow, which collects condensate from the mirror-finish domains and sends it to vertical ``drainage tracks'' for gravity-induced shedding. An optimal design is achieved by changing the fractional area of superhydrophilic tracks with respect to the overall plate surface, and augmenting capillary-driven condensate-drainage by adjusting the track spatial layout. The design facilitates pump-less condensate drainage and enhances DWC heat transfer on the mirror-finish regions. The study highlights the relative influences of the promoting and retarding effects of dropwise and filmwise condensation zones on the overall heat transfer improvement on the substrate. The study demonstrated $\sim$ 34{\%} heat transfer improvement on Aluminum surface for the optimized design. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G35.00009: Microgrooves improve dew collection Pierre-Brice BINTEIN, Henri LHUISSIER, Laurent ROYON, Claire MANGENEY, Anne MONGRUEL, Daniel BEYSENS We present a study about condensation of water drops on inclined surfaces textured with microgrooves and cooled under the dew point temperature. Usual microfabrication techniques are employed to produce substrates (silicium wafer) with grooves of 30-500 micrometers in spacing and 50-150 micrometers in depth. Such patterns induce a faster growth of the drops by coalescence, leading to earlier drainage and collection of water at the bottom of the plate. An additional grafting of hydrophilic polymer (Poegma) can even increase the efficiency of such condensation devices. [Preview Abstract] |
(Author Not Attending)
|
G35.00010: Directional droplet transport at high temperature mediated by structural topography Jing Li, Youmin Hou, Manoj Chaudhury, Shuhuai Yao, Zuankai Wang Controlling droplet dynamics on textured surfaces is of significant importance for a broad range of applications. Despite extensive advances, our ability to control droplet dynamics at high temperature remains limited, in part due to the emergence of complex wetting states complicated by the phase change process at the triple-phase interfaces. When the temperature of the surface is above a critical temperature, a continuous vapor layer separates the droplet from the hot surface, greatly reducing the heat transfer between the droplet and hot surface. In this work, we show that two concurrent wetting states (Leidenfrost and contact boiling) can be manifested in a single droplet by simply manipulating the structural topography. As a result, droplet vectors automatically towards the boiling region that is associated with a large heat transfer efficiency between the liquid and solid. Coupled with a dynamic Leidenfrost model, we show experimentally and analytically that the droplet directional motion depends on the interplay between surface structure and its imposed thermal state. Our basic understanding and ability to control the droplet dynamics at high temperature would find many potential applications in high temperature systems such as spray cooling and fuel injection. [Preview Abstract] |
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