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

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.

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.

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.