Capillary rising of self-rewetting solutions in laser engraved micro-channels

The article deals with capillary effects in laser-engraved grooves filled with ordinary liquids and self-rewetting fluids, i.e. dilute aqueous solutions of high carbon number alcohols that show surface tension minima versus temperature. Two different geometries of grooves have been realized and investigated, with triangular and trapezoidal sections. The research activity has been focused on the experimental study of capillary rising in both types of grooves, using pure and self-rewetting liquids with well characterized surface tension and contact angle properties. In particular, self-rewetting fluids exhibit improved wettability, especially on aluminium surfaces, in comparison to water; this is a very important property to guarantee inflow into micro-channels. Using theoretical models available in the recent literature capillary rising has been predicted in the capillary structure for of the different fluids, including Ethanol and self-rewetting solutions of Water-Butanol at several concentrations. The experiments carried out using Ethanol show a good agreement with the theoretical predictions; on the contrary, in the case of self-rewetting solutions, higher values of experimental capillary rising eights are obtained. In the latter case, when evaporation occurs, the concentration of the more volatile component is reduced and the surface tension increases. This effect may be the main responsible of the higher capillary rising of the self rewetting solutions. In addiction, the difference in surface tension, caused by evaporation of the more volatile component, causes a Marangoni flow towards the region with a lower alcohol concentration. The coupled effect of increasing capillary pressure at liquid-vapour interface and the inverse Marangoni flow could justify the experimental results. Numerical simulations are carried out to explain the different phenomena