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Experimental Studies on Evaporation Kinetics and Wetting Dynamics of Nanofluid Droplets on Superhydrophobic Surfaces of Micro-post Patterns

This paper reports experimental studies on the evaporation kinetics and wetting dynamics of nanofluid sessile droplets of systematically varying colloidal nanoparticle concentrations on the superhydrophobic surfaces patterned with micro-post structures of varying lateral dimensions. The superhydrophobic surface patterns with regulated inter-post distances (5, 10, 20, and 50 μm) were fabricated on a silicon substrate by photolithography and deep reactive-ion etching (DRIE) followed by a Teflon coating. The colloidal gold (Au) nanoparticles of 250 nm in size and varying concentrations (0.001, 0.01, and 0.1 wt%) in water were tested as nanofluids. With a goniometer system, the evaporation kinetics and wetting dynamics of the nanofluid droplets were measured by analyzing the evolution of droplet profiles evaporating on the superhydrophobic surfaces in ambient conditions, including changes in contact angle and contact diameter over time. The results show that there exist four distinct kinetic phases during the evaporation of nanofluid droplets on the superhydrophobic surfaces of micro-post patterns: (I) initial pinning mode, (II) interim receding mode with a constant contact angle, (III) second pinning mode, and (IV) final mixed mode. The evaporation kinetics and wetting dynamics in each phase were significantly affected by the inter-post distance and the nanoparticle concentration, including the duration and extent of spreading, pinning and receding modes, dynamic contact angles, and wetting transition from de-wetting (Cassie–Baxter) to wetting (Wenzel) states.