Prediction of water droplet behavior on aluminum alloy surfaces modified by nanosecond laser pulses

Abstract

Metal processing by laser radiation is shown to be a promising way to obtain surfaces with controllable wetting and evaporation characteristics. Such surfaces can be used in designing the modern cooling systems based on phase transitions. However, large-scale applications of metal processing by laser radiation are hampered by the lack of a theory that allows predicting the wetting and evaporation of liquid droplets on modified surfaces. In this work, we study a water droplet behavior during its wetting and evaporation on hydrophilic and hydrophobic aluminum alloy surfaces modified by single nanosecond laser pulses. To predict the wettability of the laser-processed surfaces, we proposed to use the empirical correlation of the contact angle on the surface free energy obtained in terms of dynamic contact angles. We analyzed the applicability of two well-known theoretical models to determine the evaporation rates of droplets on hydrophilic and hydrophobic laser-processed surfaces. The Spalding and diffusive evaporation models can be used for water droplet evaporating from the hydrophilic aluminum alloy surface at a temperature difference between the surface and air in the chamber of 0–6.5 K. At higher temperature difference from 6.5 K to 9 K, only the Spalding model is applicable. In the case of the hydrophobic laser-processed aluminum alloy surface, the mass loss rate of evaporating droplet at the temperature difference range of 0–9 K can be predicted by the diffusive model. In addition, this model was found to be appropriate for predicting the geometric dimensions of a droplet evaporating in a pinned contact line mode on hydrophilic surfaces without heating at any stage of its evaporation. The results of this study can be used to predict the wetting and evaporation characteristics of water droplets on aluminum alloy surfaces modified by single nanosecond laser pulses.