Probing the intermolecular interaction mechanisms between lubricant and substrate of liquid-infused surfaces: A comparative study in air and in aqueous solution.

HYPOTHESIS: Liquid-infused surfaces (LISs) have drawn considerable attention for their exceptional self-cleaning and antifouling properties. The interaction forces between the infusing lubricant and the substrate surface play a pivotal role in determining LIS stability and performance. However, there is a lack of directly measured intermolecular forces as quantitative experimental evidence to determine the contribution of lubricant chemistry in different environments at the molecular scale.

EXPERIMENTS: In this study, the interaction forces of diverse functional groups representing typical infusing lubricants with polydimethylsiloxane (PDMS) and phenyltriethoxysilane (PTES) surfaces were directly measured using atomic force microscopy in air and in aqueous solution.

FINDINGS: The force measurement results in air indicated that the adhesion of the functional groups was larger on PTES compared to PDMS, and the adhesion was primarily determined by van der Waals interaction. In aqueous solution, the adhesion on PTES became lower than that on PDMS across almost all the functional groups, except for the CC. This reversal could be attributed to the enhanced electrostatic repulsion between functional groups and PTES, as PTES carried more negative charges than PDMS. The large adhesion for CC on PTES measured in aqueous solution could be due to the effective π interactions between CC and the benzene ring on PTES. The force measurement results were further supported by molecular dynamics simulations, suggesting that the adhesions between lubricants and substrate surfaces were predominantly determined by the intermolecular interactions. The measured adhesion forces had a positive correlation with the lubricant retention as a higher adhesion force resulted in a lower reduction ratio of drop sliding velocity after air and water shearing. Notably, the n-octanoic acid (COOH) exhibited the highest adhesion and the greatest resistance to shear-induced loss on PDMS both in air and under water. While 1-octene (CC) exhibited the highest stability on PTES in an aqueous environment. This work established a molecular-level design framework for engineering robust LIS systems, offering practical guidelines for selecting lubricants suited to specific substrate surfaces.