Leidenfrost Self-Rewetting Drops.

As discovered by Leidenfrost, liquids placed on very hot solids levitate on a cushion of their own vapor. This is also called the calefaction phenomenon, a dynamical and transient effect, as vapor is injected below the liquid and pressed by the drop weight. To account for the film vapor, we consider the surface tension magnitude as well as the Marangoni effect (in particular the thermal one) which arise with imbalance of surface tension forces. For standard liquids, these forces contribute to amplify the thickness of the film layer and the levitation of the droplet. Our findings imply the ability of recent binary mixture liquids, called self-rewetting fluids, to reduce the vapor film thickness and demonstrate the powerful influence exerted by different binary mixtures to enhance the heat transfer at high temperature. Such self-rewetting fluids are presenting a high value of surface tension at high temperature, and in which the Marangoni forces are inversed as from critical temperature. We consider our assay to be a way for improvement in the high temperature mass cooling applications.

Marangoni Flow Induced Evaporation Enhancement on Binary Sessile Drops.

The evaporation processes of pure water, pure 1-butanol, and 5% 1-butanol aqueous solution drops on heated hydrophobic substrates are investigated to determine the effect of temperature on the drop evaporation behavior. The evolution of the parameters (contact angle, diameter, and volume) during evaporation measured using a drop shape analyzer and the infrared thermal mapping of the drop surface recorded by an infrared camera were used in investigating the evaporation process. The pure 1-butanol drop does not show any thermal instability at different substrate temperatures, while the convection cells created by the thermal Marangoni effect appear on the surface of the pure water drop from 50 °C. Because 1-butanol and water have different surface tensions, the infrared video of the 5% 1-butanol aqueous solution drop shows that the convection cells are generated by the solutal Marangoni effect at any substrate temperature. Furthermore, when the substrate temperature exceeds 50 °C, coexistence of the thermal and solutal Marangoni flows is observed. By analyzing the relation between the ratio of the evaporation rate of pure water and 1-butanol aqueous solution drops and the Marangoni number, a series of empirical equations for predicting the evaporation rates of pure water and 1-butanol aqueous solution drops at the initial time as well as the equations for the evaporation rate of 1-butanol aqueous solution drop before the depletion of alcohol are derived. The results of these equations correspond fairly well to the experimental data.

Experimental Droplet Study of Inverted Marangoni Effect of a Binary Liquid Mixture on a Nonuniform Heated Substrate.

We present an experimental study on the inversion of the Marangoni effect of a binary mixture droplet under a horizontal temperature gradient. In particular, we studied the dynamics and the evaporation behavior under these conditions. We show that a binary mixture (97% water-3% butanol) droplet has a tendency to migrate to warmer areas, as opposed to spreading in pure fluids. During the evaporation process, we distinguish three stages of evaporation that are correlated to the dynamics of the droplet.