Micro/nanoscale friction behavior has drawn a great deal of attention because it can reveal friction behavior at the interface level and ultimately lead to the discovery of the origin or real mechanism of friction. Surface wettability is an important property of solid surfaces and has been widely used in fundamental material research for surface characterization. Significant progress has been made in both research areas owing to the development of modern fabrication and surface characterization techniques.
One part of the work reported in this dissertation experimentally investigates the micro/nanoscale friction behavior of Ag-Bi alloys at elevated temperatures using nanoindentation-scratching techniques. Friction measurements have been conduced in both steady-state and transient thermal environments. The steady-state friction results are correlated with the material hardness obtained at each corresponding temperature. The transient friction measurements depict distinct friction transition behavior at the melting point due to different alloy compositions. A critical bismuth composition is experimentally identified.
Surface texture effects on friction at the macro- and microscales are studied using nanoindentation-scratching techniques on a polymer surface. It is found that surface textures significantly reduce friction due to reduced contact area at the macroscale, while this effect is less profound at the microscale. Friction on substrates with anisotropic textures is investigated and correlated with numerical simulation results.
This dissertation research also develops theoretical models of the contact angle on rough hydrophobic surfaces, and conducts matching experiments. Three major aspects have been covered: multiple equilibrium energy states, contact angle hysteresis, and contact angle anisotropy. The matching experiments agree with the theoretical predictions and lead to a design criterion for a robust superhydrophobic surface. A novel MEMS device, a roughness switchable membrane device, is then designed based on the theoretical analysis. It consists of a thin poly(dimethylsiloxane) (PDMS) membrane bonded on the top of a rough PDMS substrate. Contact angle measurements have been conducted on the device surface, which demonstrates that the surface wettability can be switched from medium hydrophobic to superhydrophobic through pneumatic actuation.