Sessile droplet evaporation

Evaporation of a Sessile Droplet of Solvent at Room Temperature... 

The influence of evaporation on the contact angle of a sessile droplet of solvent has been studied. There are three regimes that a solvent droplet on a smooth substrate undergoes during evaporation. In the first regime, the diameter of the droplet remains constant alternatively one can say that the droplet-substrate area... 

During the drying of a sessile droplet, the mass loss of liquid evaporating from the contact line is replenished by the migration of liquid from the interior, thereby causing a radial flow which carries solutes towards the contact line in the process. This is the reason for coffee-ring formation first noticed by Deegan in 1997 [1]. hi this work, the mass of solute accumulated at the contact line, M R, t) was found to follow the expression [1], where t is the time of evaporation. 

Cazabat, A.-M., Guena, G. Evaporation of macroscopic sessile droplets. Soft Matter 6, 2591-2612 (2010)... 

Hu, H., Larson, R.G. Analysis of the effects of Marangoni stresses on the microfiow in an evaporating sessile droplet Langmuir 21, 3972-3980 (2005)... 

Barmi, M.R., Meinhart, C.D. Convective flows in evaporating sessile droplets. J. Phys. Chem. B 118, 2414-2421 (2014)... 

Perdana, J. A., Fox, M. B., Schutyser, M. A. L, Boom, R. M. (2011). Single-droplet experimentation on spray drying evaporation of a sessile droplet Chemical Engineering Technology, 34(1), 1151-1158. 

Abstract Instantaneous distribution of mass and thermal fluxes inside and outside of an evaporating sessile droplet is considered using computer simulations. The latter distribution is calculated in a self consistent way by considering an intercoimected problem of vapour transfer in the vapour phase outside the droplet heat transfer in vapour, liquid and solid substrate and Marangoni convection inside the liquid droplet. The influence of thermal conductivity of the solid support on the evaporation process is evaluated. The deduced dependences of instantaneous fluxes can be applied for self-consistent calculations of time evolution of the evaporation processes of sessile droplets. 

Theoretical and computer simulation studies [5, 11-15] give the following equation for the evaporation rate of a sessile droplet ... 

Ristenpart et al. [19] investigated the influence of the substrate conductivity on reversal of Marangoni circulation within evaporating sessile droplet. Despite of plenty assumptions made by the authors, their quantitative criteria for the circulation direction is confirmed by experiments. 

The system under consideration is a pinned sessile droplet of a liquid (water in these simulations) oti a solid substrate open to ambient air. The problem is taken axisymmetric with a cylindrical system of coordinates r and z (Fig. 1). We focus attention on relatively small droplets, neglecting gravity force. Thus the droplet is a spherical cap. Diffusimi model of evaporation is taken to describe the quasi-steady process of droplet evaporation. The characteristic time scales of heat, ty gat 10s, and momentum, Is, transfer processes inside the droplet are smaller than the droplet evaporatimi time (lO s) at least by one order of magnitude. For this reason all those processes are taken as steady state processes. Convection in air caused by evaporation is neglected, because the experiments [21] did not reveal any difference in evaporation regimes with and without forced convection in the ambient air. 

Under a number of reasonable approximations the evaporation of small enough sessile droplets has been investigated in a self cOTisistent way by considering the interconnected problem of vapour transfer heat transfer in vapour, liquid and solid support and the Marangoni convection inside the liquid droplet The influence of the thermal conductivity of the solid support on the evaporation process has been analyzed. The calculated total evaporation flux has been compared with the result in the case of isothermal evaporation. It has been shown that the lower the thermal conductivity of the solid support the higher the deviations appear from the isothermal case. However, if the mean temperature of the droplet surface is used instead of the temperature of the surrounding air for the vapour concentration on the droplet surface flien the results found coincide with those known for the isothermal case. 

One important lesson is that measurement of the apparent contact angle of a sessile drop will not give a true measure of the wettability - or even surface free energy - of the fiber surface. The apparent contact angle will always be affected by microscopic surface property (i.e. fiber surface), macroscopic surface geometry (fabric), and capillary effects. In case of hydrophilic substrates the sessile droplet will penetrate the porous textile, typically in seconds, and thus effectively prohibit the measurement. On a hydrophobic surface, the apparent contact angle will always differ from the true contact angle on the fiber surface. Capillary effects occur even on hydrophobic substrates and compete with evaporation of the liquid. 

Volatile fluids cannot form stable droplets they either evaporate or grow until either the vapor reservoir is exhausted or their growth is limited by gravity or the finite size of the system. However, there are many fluids that exhibit a very low vapor pressure and a small evaporation rate such that on the time scale of a typical laboratory experiment, they behave as if they were nonvolatile. In this sense, such fluids—often polymeric liquids or liquid metals—form stable sessile droplets. 

J. Arcamone, E. Dujardin, G. Rius, F. Perez-Murano, and T. Ondarcuhu, Evaporation of femtoliter sessile droplets monitored with nanomechan-Ical mass sensors,/ Phys. Chem. B, 111, 13020-13027 [2007]. 

Semenov, S., Starov, V.M., Rubio, R.G., Velarde, M.G. Instantaneous distribution of fluxes in the course of evaporation of sessile liquid droplets computer simulations. Colloid Surf. A 372, 127-134 (2010)... 

A typical static sessile drop is created by a microsyringe with an automated plunger to place a tiny drop of water on the polymer surface. Ideally, the polymer sample should be in a humidity chamber to minimize the water evaporation that would change the shape, and thus the C A of the droplet. The shape of the droplet is captured by a camera and CA is measured by an image analysis software [30]. 

Computer Simulations of Quasi-Steady Evaporation of Sessile Liquid Droplets... 

W. Xu, R. Leeladhar, Y. T. Kang, and C.-H. Choi, Evaporation kinetics of sessile water droplets on micropillared superhydrophobic surfaces, Langmuir, 29,... 

---