Photo-responsive titania (TiO2) surfaces have demonstrated wettability change upon exposure to ultraviolet light. However, their applications are often limited either by the inability to respond to visible light or the need for special treatment (that is, storage in dark or heat) to recover the original wetting state. Sensitizing TiO2 surfaces with visible light-absorbing materials (e.g., organic dyes) has been utilized in photovoltaic applications to efficiently absorb solar radiation and convert it to electrical energy. Here we demonstrate that a dye-sensitized TiO2 surface can change its wetting state upon visible light illumination. We show that this wettability change arises due to a photo-induced voltage established between the liquid and the underlying surface upon incident 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 photo-induced wettability change of our surfaces, 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 microfluidic devices with customizable wettability, solar-driven oil clean-up and demulsification techniques.

Visible light guided manipulation of liquid wettability on photoresponsive surfaces

254th National Meeting (2017)

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