Topographical Design and Thermal-Induced Organization of Interfacial Water Structure to Regulate the Wetting State of Surfaces.

Abstract

Smart surfaces with superhydrophobic/superhydrophilic characteristics can be controlled by external stimuli, such as temperature. These transitions are attributed to the molecular-level conformation of the grafted polymer chains due to the varied interactions at the interface. Here, tunable surfaces were prepared by grafting two well-known thermo-responsive polymers, poly(N-isopropylacrylamide) (PNIPAM) and poly(oligoethylene glycol)methyl ether acrylate (POEGMA188) onto micro-pollen particles of uniform morphology and roughness. Direct Raman spectra and thermodynamic analyses revealed that above the lower critical solution temperature, the bonded and free water at the interface partially transformed to intermediate water that disrupted the “water cage” surrounding the hydrophobic groups. The increased amounts of intermediate water produced hydrogen bonding networks that were less ordered around the polymer grafted microparticles, inducing a weaker binding interaction at the interface and a lower tendency to wet the surface. Combining the roughness factor, the bulk surface assembled by distinct polymer-grafted-pollen microparticles (PNIPAM or POEGMA188) could undergo a different wettability transition for liquid under air, water, and oil. This work identifies new perspectives on the interfacial water structure variation at a multiple length scale, which contributed to the temperature-dependent surface wettability transition. It offers inspiration for the application of thermo-responsive surface to liquid-gated multiphase separation, water purification and harvesting, biomedical devices, and printing.