Microencapsulation and Stimuli-Responsive Controlled Release of Particles Using Water-in-Air Powders

In this work, we report a facile, one-step method to encapsulate hydrophilic particles (HP) (micro- or nanosize) using water-in-air powders. Hydrophobic silica nanoparticles were mixed with an aqueous phase containing HP in the presence of air under high shear, resulting in the self-assembly of silica nanoparticles on water droplets to make water-in-air powders with HP encapsulated in the aqueous phase within the silica shell. The encapsulated HP can be released on the basis of an external stimulus such as adding an external aqueous phase of a certain pH or a surfactant solution that alters the wettability of the encapsulating silica nanoparticles. A contact angle study was performed using surface-hydrophobized glass slides, which acted as a proxy for hydrophobic silica nanoparticles, to investigate the effect of these stimuli on surface hydrophobicity. Such encapsulation and a stimuli-responsive controlled release system has promising potential in subsurface petroleum engineering such as the delayed swelling of particles for conformance control and delayed acid stimulation.