Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session H33: Drops IX: Evaporating Sessile Drops |
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Chair: Pirouz Kavehpour, University of California, Los Angeles Room: 404 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H33.00001: Dynamics of evaporating sessile droplets Pedro S\'{a}enz, Prashant Valluri, Khellil Sefiane, George Karapetsas, Jungho Kim, Omar Matar A sessile droplet laying on a horizontal substrate evaporates into its surrounding gas. The dynamics of this physical system are investigated by means of 3D Direct Numerical Simulations and experiments. A non-isothermal two-phase model is employed to compute the spatio-temporal evolution of the system under consideration. Transient species transport in the gas phase is also accounted for via the general advection-diffusion governing equation. The interface mass transfer is computed considering that the vapour diffusion is the rate-limiting mechanism. On this premise, it is assumed that the liquid and the gas maintain thermodynamic quasi-equilibrium at the interface. The same system is also experimentally investigated by simultaneously recording the droplet evaporation in a controlled environment with a CCD camera (side) and an IR camera (top). Comparisons between numerical and experimental data are presented along with a discussion of the role played by other singularities of the system, namely the triple line, the effect of thermocapillarity, etc. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H33.00002: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 10:56AM - 11:09AM |
H33.00003: On the lifetimes of evaporating droplets Stephen Wilson, Jutta Stauber, Brian Duffy, Khellil Sefiane The evaporation of a fluid droplet on a solid substrate is a practically important problem which has been the subject of considerable research in recent years, much of it motivated by a range of technological applications, such as the application of pesticides to plants, DNA microarray analysis, inkjet printing, micro-fabrication, and spray cooling. In particular, the lifetime of a fluid droplet is not only of fundamental scientific interest, but is also important in a number of technological applications, such as inkjet printing and spray cooling applications (in which shorter droplet lifetimes are often needed) and the application of pesticides to plants (in which longer droplet lifetimes are often needed). In this talk we will analyse the lifetimes of fluid droplets evaporating in a variety of modes and, in particular, show that the widely believed folklore that the lifetime of a droplet is always longer than that of an identical droplet evaporating in the constant radius (i.e. pinned contact line) mode and shorter than that of an identical droplet evaporating in the constant angle mode is not, in general, true. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H33.00004: Contact line and bulk velocities in evaporating micron-scale droplets Yi Fan, Kenneth S. Breuer The famous ``coffee stain'' phenomena is well known during the evaporation of a liquid droplet seeded with colloidal particles. However, the different phases of evaporation on a hydrophobic surface have not yet been fully explored and explained. In this experiment, evaporating micro-drops (diameter $\sim$100 $\mu $m) are seeded with 300 nm red fluorescent particles and observed from below using epifluorescent microscopy. We observed four phases: (i) steady evaporation with the contact line retreating at a constant speed of $\sim$2 $\mu $m/s; (ii) stagnation of the contact line for several seconds; (iii) a sharp transition leading to fast evaporation with a rapidly retreating contact line, and (iv) final dry-out of the film leaving the particles immobilized on the substrate. These four phases of motion, as well as the pattern of the deposited nano-particles are strong functions of both the colloidal concentration and the static contact angle. Statistical Particle Tracking Velocimetry is used to quantify the velocity fields inside the micro-drop during the evaporation history. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H33.00005: Volumetric thermal measurements using thermo-liquid crystal (TLC) micro-particles in evaporating drops Rodrigo Segura, Alvaro Gomez Marin, Christian Kaehler Freely evaporating sessile droplets develop weak temperature gradients that can generate Marangoni flows at the drop's surface. Quantitative temperature measurements of small gradients at such scales are very difficult. In this work, a method to track the temperature of individual thermo-liquid crystal (TLC) particles is employed to extract the temperature field inside an evaporating droplet. TLC thermography has been investigated for several years but the low quality of individual TLC particles, as well as the methods used to extract temperature from their color appearance, has prevented the development of a reliable approach to track their temperature individually. In order to overcome these challenges, an emulsion of stable non-encapsulated TLC micro particles with a narrower size distribution than that of commercial encapsulated TLC solutions was used along with a multi-variable calibration approach, as opposed to the direct hue-temperature relationship usually implemented (Segura et al, Microfluid Nanofluid, 2012). In addition, an optimized color space was implemented as well as circular polarization filtering to remove background noise and improve signal-to-noise ratio. Using this technique, a 3D temperature-velocity field within a droplet could be simultaneously resolved. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H33.00006: Marangoni or not Marangoni? Thermal Marangoni flow measurements in evaporating drops Alvaro Gomez Marin, Robert Liepelt, Massimiliano Rossi, Christian Kaehler Sessile evaporating droplets fascinate for the rich and complex behavior that hides behind their apparent simplicity. Although the basic physics of the coffee-stain formation can be explained assuming thermal equilibrium (Deegan, 1997), thermal effects play an important role in the flow patterns within the droplet and in the deposits left on the substrate. Understanding such flows would give a chance to add a higher degree of control in these not-so-simple systems. For example, several studies have recently suggested that such thermal Marangoni flows can be strong enough to neutralize the coffee-stain effect. Experimental work in this sense has been scarce due to the difficulty of tracking particles at the surface of the droplet, where the flow is originated. In this study we perform fully three-dimensional and time resolved particle tracking measurements of particles suspended in sessile drops of liquids on substrates with different thermal conductivity ratios. The results are compared with some of the theoretical models and simulations available in the literature. Our final aim is to precisely quantify how important is the thermal Marangoni flow in an evaporating drop and if it can be used for practical applications. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H33.00007: Approximate analytical descriptions of the stationary single-vortex Marangoni convection inside an evaporating sessile droplet of capillary size Lev Barash Three versions of an approximate analytical description of the stationary single vortex Marangoni convection in an axially symmetrical sessile drop of capillary size are studied for arbitrary contact angle and compared with the results of numerical simulations. The first approach is heuristic extension of the well-known lubrication approximation. Two other new descriptions are developed for arbitrary contact angle and named $n\tau$- and $rz$-description. They are free from most of restrictive assumptions of the lubrication approach. For droplets with large contact angles they result in better accuracy compared to the heuristic extension of the lubrication approach, which still gives reasonable results within the accuracy 10-30 per cent. For droplets with small contact angles all three analytical descriptions well agree with the numerical data. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H33.00008: The effect of vapor diffusion on the evaporation of a sessile droplet on a heated substrate Mahnprit Jutley, Vladimir Ajaev The study of the physical behavior of sessile droplets on heated substrates is important for many applications, such as the coating of a solid substrate with another material or the spray cooling of electronics. In order to simulate the height evolution of the droplet and its effect on the temperature distribution in the substrate, a model that incorporates the effects of surface tension, gravity, evaporation, thermocapillarity, and disjoining pressure must be used. Due to the physical characteristics of a thin sessile droplet, a lubrication-type model that includes the aforementioned effects can be used. By solving the heat equation in the substrate, the lubrication-type equations in the droplet, and the quasi-steady diffusion equation in the gas phase, we simulate the effect of vapor diffusion on the evaporation of a sessile droplet and its coupling to the pattern of heating in the substrate. By using high-order numerical techniques for solving governing partial differential equations, the height evolution of the droplet, heat distribution in the substrate, and vapor diffusion over time are calculated. Connection of our predictions to recent experimental studies is discussed. [Preview Abstract] |
Monday, November 25, 2013 12:14PM - 12:27PM |
H33.00009: How gravity influences hydrothermal waves in alcohol sessile droplets Florian Carle, David Brutin This study, performed under several gravity levels, focuses on the formation and behaviour of hydrothermal waves (HTWs) that spontaneously develop on droplets surfaces when surface tension gradient are strong enough as a response to temperature gradients. HTWs have been found to form concentric torus around the apex rotating in the same direction from hot to cold area creating a shear phenomenon between the two torus where instability develops and get dragged in the flow. This leads to a detachment of the thermal plume, well visible on the top infrared visualisation. HTWs develop in a large number in methanol, half as much in ethanol and at this day, no instability have been experimentally observed in propanol. These behaviours are evidenced with the effective Marangoni number; its high value for the methanol indicates an intense turbulent flow. Gravity levels influence the atmospheric convective transport contribution to the droplets evaporation inducing diffusive evaporation under microgravity conditions and diffusive and convective evaporation under Moon, Mars and Earth gravity level; convection being directly linked to buoyancy. Evaporation dynamics is therefore modified and the temperature gradient between the contact line and the apex can be more or less important. [Preview Abstract] |
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