Interfaces, Wetting and Photocatalysis
Divya Panchanathan

Superhydrophobic surfaces for drag reduction

When superhydrophobic surfaces are submerged underwater, a thin shiny layer of air trapped in the texture is advantageous for both anti-corrosion as well as frictional drag reduction in various applications ranging from microfluidic channels to marine vessels. However, during use, these airy textures are prone to impregnation by water due to pressure fluctuations and dissolution of plastron air into the water. In this work, we have designed superhydrophobic surfaces with a catalytic coating to produce gas underwater using a chemical reaction to replenish this air layer in-situ. We also provide a framework for designing superhydrophobic surfaces with optimal texture and chemistry for underwater plastron regeneration. The image on the right shows plastron regeneration in situ on a patterned surface by chemical gas generation.

Fouling remediation on membranes

The photocatalytic and hydrophilic nature of TiO₂ (titania) coatings can be exploited to ensure preferential wetting of water over oil under ultraviolet (UV) irradiation and this provides a mechanism for membrane surface recovery from fouling events. In this work, we studied the photoinduced cleaning ability of nanoporous titania coatings in oil-water environments for fouling recovery in oil/water applications. Fouled hydrophobic surfaces were irradiated with UV light in a fully-submerged oil-water environment to photocatalytically decompose the organic pollutant (oil phase) and restore hydrophilicity. The kinetics of this conversion from hydrophobicity to hydrophilicity were studied in situ under various UV intensities using goniometric measurements. A simple adsorption-photocatalysis model (LuCY – Langmuir-Hinshelwood Cassie-Baxter Young) was developed to quantitatively interrelate two surface phenomena – photocatalysis and wetting. In this way, we can quantify the kinetics of photocatalysis on nanoporous surfaces in situ using wetting data. We also demonstrate that UV irradiation can be used for in situ removal of oil fouling from a mesh coated with titania nanostructures. The calibrated kinetic model can subsequently be applied to optimally control wetting recovery in fouling remediation and droplet coalescence applications.

Figure 2.  A TiO₂ surface submerged under water and fouled by organic pollutant recovers hydrophilicity when irradiated with UV.

Measuring surface tension of sea water

Surface tension plays a key role in tailoring the non-wettability of membranes in membrane distillation and in optimizing heat and mass transfer in packed beds and falling film evaporators for humidification-dehumidification desalination systems. In collaboration with Kishor Nayar and Prof. John Lienhard at MIT, we measured the surface tension of seawater across a salinity range of 20 ⩽ S ⩽ 131 g/kg and a temperature range of 1 ⩽ t ⩽ 92 °C at atmospheric pressure using the Wilhelmy plate method. The uncertainty within measurements varied from 0.18 to 0.37 mN/m with the average uncertainty being 0.22 mN/m. See this publication for details.

Figure 3. Measuring the surface tension of seawater.