One promising, new approach to actively control liquid flow in microfluidic systems is through changing the surface tension. In this dissertation, a new method based on visible and ultraviolet (UV) light to reversibly change the surface tension of photochromic azobenzene modified surfaces was developed and the dependence of the resulting surface wetting on the chemistry and morphology at the fluid-solid interface was investigated.

A photo-switchable azobenzene monolayer was formed on air oxidized Si surfaces through 3-aminopropylmethyldiethoxysilane linkages. The wettability of azobenzene-modified smooth surfaces was determined by measuring the advancing and receding contact angles with various liquids after visible and UV light illumination. Reversible light-induced advancing contact angle changes ranging from 8° to 16° were observe. The surface energy of the azobenzene modified surface after visible and UV illumination was calculated. Millimeter scale transport of 5 microliter droplets of various liquids on the photo-switchable azobenzene surface was demonstrated by using UV and visible illumination of spatially distinct areas of the droplet to form a surface tension gradient.

Azobenzene monolayers were also applied to surfaces with modified morphologies to investigate the effect of surface roughness on surface wettability and the light-induced contact angle change. The critical angle for liquids to penetrate pores on rough surfaces is typically below the 90° and this phenomenon is not accounted for in the Wenzel model which has been used for over fifty years to describe wetting. A modified wetting model based on the Wenzel approach is proposed with the observed critical angle for the onset of liquid penetration of pores included in addition to the measured surface roughness parameter. The wettability and light-induced contact angle changes were investigated on the smooth and rough surfaces with azobenzene monolayers by aqueous dimethylformamide solutions to cover the full range of wetting angles. The surface roughness parameter is systematically varied by the growth of silicon nanowires of different lengths. The modified model provides better agreement with our experimental data and with previously published results. The modified model is used to predict and optimize the amplification of light-induced contact angle changes on a rough surface.