S. David

The strong influence of substrate conductivity on droplet evaporation

We report the results of physical experiments that demonstrate the strong influence of the thermal conductivity of the substrate on the evaporation of a pinned droplet. We show that this behaviour can be captured by a mathematical model including the variation of the saturation concentration with temperature, and hence coupling the problems for the vapour concentration in the atmosphere and the temperature in the liquid and the substrate. Furthermore, we show that including two ad hoc improvements to the model, namely a Newtons law of cooling on the unwetted surface of the substrate and the buoyancy of water vapour in the atmosphere, give excellent quantitative agreement for all of the combinations of liquid and substrate considered.

On the effect of the atmosphere on the evaporation of sessile droplets of water

An experimental and theoretical study of the effect of the atmosphere on the evaporation of pinned sessile droplets of water is described. The experimental work investigated the evaporation rates of sessile droplets in atmospheres of three different ambient gases (namely, helium, nitrogen, and carbon dioxide) at reduced pressure (from 40 to 1000 mbars) using four different substrates (namely, aluminum, titanium, Macor, and polytetrafluoroethylene) with a wide range of thermal conductivities. Reducing the atmospheric pressure increases the diffusion coefficient of water vapor in the atmosphere and hence increases the evaporation rate. Changing the ambient gas also alters the diffusion coefficient and hence also affects the evaporation rate. A mathematical model that takes into account the effect of the atmospheric pressure and the nature of the ambient gas on the diffusion of water vapor in the atmosphere and the thermal conductivity of the substrate is developed, and its predictions are found to be in enc...

Wetting and Evaporation of Binary Mixture Drops

Experimental results on the wetting behavior of water, methanol, and binary mixture sessile drops on a smooth, polymer-coated substrate are reported. The wetting behavior of evaporating water/methanol drops was also studied in a water-saturated environment. Drop parameters (contact angle, shape, and volume) were monitored in time. The effects of the initial relative concentrations on subsequent evaporation and wetting dynamics were investigated. Physical mechanisms responsible for the various types of wetting behavior during different stages are proposed and discussed. Competition between evaporation and hydrodynamic flow are evoked. Using an environment saturated with water vapor allowed further exploration of the controlling mechanisms and underlying processes. Wetting stages attributed to differential evaporation of methanol were identified. Methanol, the more volatile component, evaporates predominantly in the initial stage. The data, however, suggest that a small proportion of methanol remained in the drop after the first stage of evaporation. This residual methanol within the drop seems to influence subsequent wetting behavior strongly.

On the Effect of an Atmosphere of Nitrogen on the Evaporation of Sessile Droplets of Water

The effect of an atmosphere of nitrogen on the evaporation of pinned sessile droplets of water is investigated both experimentally and theoretically.

The effect of the thermal conductivity of the substrate on droplet evaporation

The evaporation of liquid droplets is of fundamental importance to industry, with a vast number of applications including ink-jet printing, spray cooling and DNA mapping, and has been the subject of considerable theoretical and experimental research in recent years. Significant recent papers include those by Deegan [1], Deegan et al. [2], Hu and Larson [3], Poulard et al. [4], Sultan et al. [5], and Shahidzadeh-Bonn et al. [6].

On the Dynamics of Wetting of Water-Methanol Volatile Sessile Drops on Smooth Substrates

The wetting and evaporation behaviour of methanol-water droplets deposited on a smooth silicon substrate were investigated experimentally. Contact angle and droplet shape kinetics were studied using an optical technique. Drops were deposited onto a silicon substrate and enclosed in a cell with nitrogen as the ambient gas. Besides the case of pure water and pure methanol, three different volume fractions of methanol in water were investigated: 10%, 50% and 80%. Using a Kruss DSA100 contact angle analyser, the behaviour of the contact angle, droplet volume, and base width was determined as a function of time. Results show that evaporation of the droplet takes place in successive stages for mixtures. The more volatile component seems to evaporate principally in the first stage, during which the contact angle of the binary drop is closer to that of pure methanol. Because the wetting behaviour is partly dictated by the surface tension of the liquid-vapour interface, methanol is believed to be concentrated at the interface during this first stage. After complete evaporation of the methanol, the wetting behaviour of the droplet tends towards that of pure water. The mechanisms that dictate the evaporation and wetting behaviour of such binary droplets include many effects: diffusion of methanol in water in the liquid phase; accumulation of one of the component near the interface and preferential evaporation followed by diffusion of one component in another in the vapour phase. In order to model the phenomenon, the above effects must be taken into account. Solutal Marangoni stress as well as interfacial instabilities may also play an important role in the behaviour of theses systems.

Experimental Investigation of the Effect of the Ambient Gas on Evaporating Sessile Drops

This paper presents the results of an experimental study of evaporating sessile drops in a controlled environment. The experimental setup allowed the investigation of the evaporation rate of sessile drops under reduced pressure (40 to 1000 mbar) and various ambient gases. Sessile drops of initial volume 2.5μL are deposited on substrates and left to evaporate in a controlled atmosphere. The effect of reducing pressure on the evaporation rate as well as changing the ambient gas is studied. Three different gases are used; namely Helium, Nitrogen and Carbon Dioxide. The role of vapour diffusion as a limiting mechanism for evaporation is studied. It is found that in all cases the evaporation rate is limited by the mass diffusion in the ambient gas provided that interfacial conditions are properly accounted for. This includes important evaporative cooling observed at higher evaporation rates and lower substrate thermal conductivity.