Influence Of Molecular-Surface Interactions On Osmotic Flow In Nanochannels

We study how solute-solid surface interaction at nanoscale influences the direction and magnitude of the induced water flow (i.e., diffusioosmosis) in the nanofluidic system, when there is a concentration gradient of solutes across the nanofluidic system. Using the velocity measurement technique based on a signal from fluorescent molecules in nanochannels, we show that the attractive nature of the electrostatic interaction is responsible for the flow from high to low solute concentration for electrolytes. However, the role of the molecular interaction for neutral species, e.g., polyethylene glycol polymers and ethanol molecules, appears to be more subtle. Despite a natural tendency of those molecules adsorbing onto the surface, we observe a water flow from low to high solute concentration, which evidences that a dynamic interaction between the solute molecules and surface, rather than a static interaction, is responsible for the generation of diffusioosmosis.