2, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
3, King Fahd University of Petroleum and Minerals, Dhahran, , Saudi Arabia
4, Korea Institute of Industrial Technology, Cheonan, , Korea (the Republic of)
Photoresponsive titania (TiO2) is well known for its photocatalytic properties and its ability to remove organic contaminants under UV light illumination. It is also known to switch its surface wetting characteristics from being hydrophilic to superhydrophilic under exposure to UV light in air. In this talk, we will first discuss the kinetics of oil contamination-induced hydrophobicity and photoinduced hydrophilicity of TiO2 using in situ contact angle measurements. A simple model that can relate the evolution in the measured contact angles to photocatalytically induced changes on the surface of nanoporous TiO2 by integrating the Langmuir–Hinshelwood adsorption isotherm with the Cassie–Baxter analysis of the effective contact angle of a drop sitting on a composite surface will be discussed. We will also introduce our recent work on the Janus membrane that can simultaneously separate oil-water mixtures and purify the water-rich permeate upon UV light illumination. Lastly, we will discuss a dye-sensitized TiO2 surface that can change its wetting state upon visible light illumination. A systematic study of the relationship between the energy levels of the dye and the liquid reveals that the highest occupied molecular orbital (HOMO) energy level of the dye and the reduction potential of the liquid govern the ensuing wetting behaviors. Utilizing the visible light-induced wettability change, we demonstrate light guided manipulation of liquid droplet motion along the surface. Further we show demulsification of surfactant-stabilized brine-in-oil emulsions via interfacial coalescence of brine droplets on our dye-sensitized TiO2 surface under visible light illumination. Such surfaces thus offer a wide range of potential applications including solar-driven oil clean-up and demulsification techniques.