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

Dash, Susmita. Ph.D., Purdue University, December 2014. Droplet Behavior on Superhydrophobic Surfaces: Interfaces, Interactions, and Transport. Major Professor: Suresh V. Garimella, School of Mechanical Engineering. 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 antiwetting characteristics, and lower contact angle hysteresis compared to single-roughness