Between Wenzel and Cassie: The life of water at tailored silica bubble interfaces

HYPOTHESIS: The behavior of water confined at the surface of particles adsorbed at interfaces plays a critical role in the stability and functionality of Pickering systems such as silica-stabilized microbubbles. Changes in the hydrophobicity of the adsorbed silica particles are expected to influence the macroscopic stability of these systems. We hypothesized that the high incoherent neutron cross-section of hydrogen can be exploited to distinguish differences in wetting behavior between these systems. EXPERIMENTS: Microbubbles stabilized by silica nanoparticles with varying degrees of surface hydrophobicity were formulated to test this hypothesis. Micropipette aspiration of the microbubbles was used to confirm the air-in-water structure. Using a combination of three neutron spectrometers the dynamics of water at the interface of these Pickering-stabilized structures was studied. The resulting water mobility was interpreted based on their surface chemistry. FINDINGS: Hydrophilic silica particles promoted a Wenzel-like wetting state, characterized by increased solid- water contact and reduced water mobility. In contrast, hydrophobic particles resulted in a configuration like the Cassie-Baxter state with entrapped air pockets, leading to enhanced water mobility and bulk-like behavior. These changes were associated with nanoscale surface interactions. These findings indicate that neutron spectroscopy provides a new method to determine the wetting of interfacially adsorbed Pickering particles in situ and therewith can help to better formulate Pickering systems.