Papers by Author: Khellil Sefiane

Abstract: We present the results of an experimental investigation of the evaporation of a liquid meniscus in a high aspect ratio micro-channel. The study investigates evaporation rates of a stationary liquid meniscus in a high aspect ratio microchannel, the wall of which is electrically heated using transparent resistive coating. Four different liquids are used as working fluids. We report on the dependence of the measured overall evaporation rate on the applied power. The results indicate, and consistently, that the evaporation rate increases with the applied power then peaks before declining. In order to gain insight into these results, we used thermographic infra red imaging to map the temperature field on the external wall of the microchannel. The measurements show that there is a good correlation between the maximum in the evaporative rate and the onset of instabilities of the interface. These instabilities, to our mind, are induced by an increasing temperature gradient along the microchannel wall around the three phase contact line region. These instabilities are revealed by a high speed camera used to record the behaviour of the interface during evaporation.

Abstract: Single vapour bubble growth and heat transfer mechanism during flow boiling in a rectangular horizontal mini-channel were experimentally investigated. The hydraulic diameter of the channel was 1454 μm, with an aspect ratio (Win/din) of 10. Degassed FC-72 was used as the working liquid. In this paper, bubble equivalent radius was found to increase linearly till a critical time, beyond which the growth turned into exponential. Bubble growth rate increases with increasing heat flux. Heat transfer mechanisms of the bubble growth at different heat fluxes and mass fluxes were discussed. In addition, the relation between thermal and flow conditions with bubble temporal geometry was explored.

Abstract: The excellent spreading and wetting behaviour of superspreader solutions has been known and extensively studied over recent years. However, explanations for spreading dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the spreading exponents, but none of the models so far was able successfully to describe the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated the partial wetting of Silwet L-77® superspreader solutions of high concentrations (well above CMC) on polymer coated substrates of varying hydrophobicity. Results obtained can be explained in terms of the Marangoni effect as the major driving force for trisiloxane enhanced spreading. A simple theory, which involves surface tension gradients governing the spreading process, was developed in order to explain the specific evolution of the drop radius and consequent decrease in the contact angle. The proposed model was found to be in excellent agreement with the experimental results. Determined equation coefficients were shown to be dependent on both surfactant concentration and the hydrophobicity of the substrate.

Abstract: An experimental investigation into the evaporation of sessile nanofluid droplets is reported in this paper. The effect of nano-particle addition on the evaporative behaviour is studied using ethanol and Titanium Oxide nano-particles. The results show that a distinct ‘stick-slip’ pinning behaviour is observed when nano-particles are added to the base liquid. Increasing the nano-particle concentration was found to enhance the ‘stick-slip’ behaviour. This behaviour is attributed to the effects of evaporatively driven particle accumulation near the contact line. This in turn leads to an increase in localised viscosity, and an enhancement of contact line pinning.

Abstract: Evaporation in restricted domains, e.g. in capillaries, is of industrial importance but is poorly understood. Where the evaporating liquid is a binary mixture, preferential evaporation of the more volatile component occurs initially and the evaporation rate is not constant, indeed it appears to occur in stages. Experiments of evaporation from the entrance of a capillary were performed for various binary mixtures of acetone and water and for pure liquids for comparison. Measurements of mass were taken over time for a range of capillary diameters from 0.6 mm to 2 mm. For simplicity, the experiments were performed with the meniscus “stationary” at the entrance of the tube, rather than allowing the meniscus to recede. The data were analysed and showed that, for the binary mixtures, the evaporation process had two distinct stages for the mixtures. The second stage always had a lower slope than the first, indicating a slower evaporation (similar multistage evaporation processes have been observed for sessile drops of binary mixtures). There are many phenomena at work in this process: surface evaporation; diffusion (or natural convective mass transfer) in the air beyond the capillary; diffusion in the binary mixture; circulation in the liquid; thermal effects of evaporative cooling. These are investigated, comparisons made and further studies are proposed.

Abstract: Many industrial and biological phenomena involve the evaporation of liquids in porous media. In drying processes the evaporation of a liquid meniscus from the solid is the key mechanism in the process and its efficiency. After a first steady stage of evaporation the meniscus becomes unsteady and recedes inside the pore. Diffusion of vapour becomes the controlling mechanism for evaporation in a later stage. In this work an experimental investigation is undertaken to study the various stages of evaporation of different liquids in capillary tubes (pores) of various sizes. The analysis of the data obtained from this investigation reveals some interesting behaviours and emphasizes the role played by vapour diffusion in the case of unsteady interface. The preliminary transient regime allowing the thermal field establishment, is followed by the first stage of evaporation is found to be dominated by thermocapillary effects associated with non-uniform evaporation and temperature gradients. The laste stage is a molecular diffusion-limited mode. The liquid volatility and the effect of the size of the tube (ranging from 200 to 900 μm) are also analysed to show the interaction between the various effects at different scales.

Abstract: 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.

Abstract: 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.