Droplet behavior on superhydrophobic surfaces: Interfaces, interactions, and transport

The primary objective of the present work is to study droplet dynamics on smooth hydrophobic and textured superhydrophobic surfaces, and to understand the dependence of interfacial interaction mechanisms on surface morphology. A detailed understanding of the dynamics of droplet response to an applied electric field is essential for implementation of electrowetting techniques in various devices. In the first part of the thesis, a systematic study of the transient response in terms of contact angle and contact radius of a sessile droplet on a smooth hydrophobic surface under electrical actuation is presented. A scaling analysis predicts the response time of a droplet during step actuation. It is shown that during time-varying electrical actuation of a droplet, in addition to the primary frequency response at the electrical forcing frequency, the droplet oscillation exhibits sub-harmonic oscillation at half the forcing frequency. The remaining part of the thesis focuses on the design, fabrication and characterization of superhydrophobic surfaces, and droplet behavior on such surfaces. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. The surfaces show enhanced anti-wetting characteristics, and lower contact angle hysteresis compared to single-roughness surfaces. A novel hybrid surface morphology incorporating communicating and non-communicating air gaps is proposed to enhance capillary pressure. The pressure balance during droplet impingement indicates that the effective water hammer is dependent on the surface morphology, and is significantly lower compared to that on smooth surfaces. The last part of the thesis includes evaporative phase change on flat and textured surfaces. An understanding of the evaporation characteristics of the droplet, and accompanying convection flow field on hydrophobic and superhydrophobic surfaces is important to several applications. In this dissertation, droplet evaporation characteristics on unheated and heated hydrophobic and superhydrophobic surfaces with negligible contact angle hysteresis are investigated systematically. A vapor-diffusion-only model is shown to overpredict the rate of evaporation on superhydrophobic surfaces, and the disparity increases with substrate heating. The evaporation characteristics are explained in terms of the evaporative cooling, and vapor buoyancy induced convection. Improved understanding of the convective flow mechanism inside an evaporating droplet can assist in non-intrusive particle manipulation inside a micro-droplet. The recirculating convective flow field inside a water droplet evaporating on hydrophobic and superhydrophobic surfaces is attributed to the thermal buoyancy induced convection. The flow pattern inside the droplet enables understanding of the dependence of flow behavior on the nature of the substrate. High recirculating flow velocity in droplets evaporating on superhydrophobic surfaces is proposed to enable `on-the-spot\u27 mixing in droplets for microfluidics application