Surface tension gradient driven autonomous fatty acid-tetrahydrofuran liquid moving drops: Spreading to pinning

Abstract

In this work, we have studied the drop dynamics of binary tetrahydrofuran (THF) solutions containing fatty acids (stearic acid and myristic acid) or surfactants (N-Cetyl-N, N, N trimethyl ammonium bromide, and sodium dodecyl sulfate). Highly volatile non-aqueous pure THF drop shows dynamic wetting behaviour i.e., droplet expansion and contraction behaviour·THF spreads continuously on fabricated total wetting surface without pinning, however addition of fatty acids causes formation of drops (instead of thin films). Such drops show autonomous motility which can be tuned by maintaining the concentration of fatty acids. The addition of fatty acids provides evaporation driven surface tension gradient (Marangoni stress) which initialize the autonomous drop motion and reduces the innate contact line pinning. It is found that addition of fatty acids showed drop dynamics which was concentration dependent. At low concentrations drop moves, while contact line pinning and subsequent evaporation was noticed at higher concentrations. The study of drop area with respect to time conveyed about the slowdown of evaporation timing of THF. This work proclaims the novel combination of highly evaporating THF with solutes (fatty acids/surfactants) showing its potential applications in self-propelled motion of non-aqueous drops.