Title: Tuning the Hydrophilicity of Chemically and Physically Complex Surfaces – Hydration Dynamics, Adhesion, and Wettability
Advisors: Jacob Israelachvili and Songi Han
Surface hydrophobicity and hydrophilicity influence everyday phenomena such as oil-water separation, detergency, and the spreading (or lack thereof) of water on synthetic or natural surfaces. However, few metrics characterize surface-water interactions at the molecular scale. At the macroscale, effective surface hydrophilicity is often gauged by static contact angle measurements (i.e., wettability), but dynamic and time-dependent wetting phenomena on rough surfaces is often overlooked. This presentation addresses these long-standing issues using novel combinations of experimental, simulation, and theoretical efforts.
Molecular-scale properties of water at organic and inorganic surfaces were probed by measuring surface forces and adhesion with a surface forces apparatus, and by measuring surface water diffusivity/mobility with Overhauser dynamic nuclear polarization NMR. Certain solution additives can shrink the hydration shell around the polar groups of lipids and surfactants (i.e., decrease hydrophilicity), resulting in greater adhesion between lipid bilayers, and increased surface water diffusivity. Other additives can have the opposite effect, depending on the hydrogen bonding capabilities of the additive. Similar measurements were performed on silica surfaces with tunable hydrophilicity, where simulations also demonstrate the importance of the distribution of surface groups on surface hydrophilicity. The results provide an experimental correlation between hydration dynamics and equilibrium forces between surfaces, and suggest specific ways in which hydration should be modeled, depending on the chemical and physical makeup of surface groups. Surface structure/roughness on much larger length scales is also important to characterize surface hydrophilicity. When water contacts macroscopically rough surfaces, water either completely fills the crevasses, or rests on trapped vapor pockets, resulting in dramatically different observed contact angles. Using contact angle and microscopy measurements, I identified several novel factors that dictate the dynamics of this filling process.
The improved description of the chemical and physical mechanisms that govern hydrophilicity at molecular and macroscopic length scales enables better quantitative modeling of surface interactions and self-assembly in soft matter and colloidal systems, as well as adhesion of thin films and coatings.