CBET-EPSRC: Analysis and Optical Control of Surfactant Effects for Increased Lubrication of Liquid Flows in the Cassie State.

Engineered superhydrophobic surfaces possess properties similar to those of naturally occurring superhydrophobic surfaces such as the lotus leaf. For example, they provide lubrication such that droplets roll off of them rather than adhere to them and are thus "self-cleaning." Flows of water over superhydrophobic surfaces promise to benefit various technologies, such as lab-on-chip and thermal management of electronics via microchannel cooling, because of the lubrication they may provide.

However, such lubrication has been severely hindered, or even eliminated, due to the presence of trace amounts of surfactant molecules in the water. The proposed NSF - UKRI Engineering and Physical Sciences Research Council collaborative project seeks to exploit surfactants to enhance rather than hinder lubrication. To accomplish this goal, photosurfactants will be added to water and light will be used to manipulate their distributions in the water.

This will produce a chromocapillary stress to pump water in preferred directions that can overwhelm the deleterious effects of background surfactants. The liquid-vapor interfaces (menisci) that form on superhydrophobic surfaces are ideal vehicles to exploit such chromocapillary stresses. Results of the project will benefit a variety of microfluidic technologies and could provide guidance for methods of propulsion of underwater objects.

The project will be carried out in collaboration with applied mathematicians at Imperial College London who possess complimentary and essential skills that are synergistic with those of the mechanical engineers on the US side of the project.

Robust models will be developed to predict the behavior of flows over superhydrophobic surfaces in the presence of chromocapillary stresses imposed by the strategic irradiation of menisci with light of various wavelengths in photosurfactant-containing water. The modeling will couple the species equations of the surfactant, both on the meniscus and in the bulk water, and the associated chemical kinetics to the hydrodynamic problem using analytical methods and in-house numerical codes.

A major component of the work will be experimental validation of the models using micro particle image velocimetry to measure velocity profiles in flows of water through superhydrophobic microchannels. Shapes of menisci and flow resistance of the microchannels will be computed and measured. The result will be experimentally-validated models for the apparent slip length, the parameter that quantifies lubrication in the presence of chromocapillary stress.

Results from the project will advance the general state of modeling of flows over superhydrophobic surfaces by capturing the effects of micelle formation at sufficiently-high surfactant concentrations, another means to enhance lubrication. Finally, the use of chromocapillary stresses to propel and steer superhydrophobic objects submerged in water will be explored.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.