Wet-etched asymmetric spherical nanoparticles with controllable pit structures and application in non-aqueous foams

The structure of colloidal particles is one of the factors that significantly affect their properties. Asymmetrical spherical particles with pit structures were prepared by using NH₄F to perform wet chemical etching on the designated positions of the partially masked particles. The depth and effectiveness of the pits were adjusted by varying the etching time. By changing the properties of the oil mixture, the oil repellency and foaming ability of the etched particles were characterized and compared. By controlling the wet etching time, the effective pit structures were etched on the particles. Within 10 d of being etched, the particles with pit geometry showed better foam properties than the original unetched particles. The pit structure on the particles improves the oil repellency of the particles in a series of oil mixtures with relatively lower surface tension. No significant difference was observed between the under-etched (18 h) particles and the non-etched particles. The ineffective geometry of the over-etched (15 d) particles results in insufficient robustness of the Cassie-Baxter state of the particles and reduces the volume of the generated foam.