Effect of Pore Structure of Inverse Opals on Wetting Transitions and Liquid Imbibition

Infiltration of liquids into three-dimensionally ordered macroporous structures, such as inverse replicas of colloidal crystals called inverse opals, is highly sensitive to the geometry of the interconnecting necks between adjacent pores. Here, we demonstrate that the neck angle connecting the individual pores of an inverse opal can be rationally controlled by adjusting the softness of the templating colloidal particles via their glass transition temperature and the oven temperature during the self-assembly process. This allows systematic tuning of the critical contact angle of a liquid that is the threshold to enable spontaneous infiltration into the porous network of the inverse opal. We experimentally determine this critical contact angle as a function of their neck angle. We rationalize these finding via a geometric model that predicts the critical contact angle as a function of the neck angles. Importantly, our results show that the wetting transition is surprisingly robust to neck irregularities and surface roughness of the inverse opals. Finally, we study the dynamics of the wetting process by measuring imbibition and evaporation rate of a water droplet sitting on inverse opal-based thin films with varying neck angles. Our results provide a comprehensive understanding of how the pore geometry of inverse opals controls both static and dynamic liquid transport, offering a design strategy for tailored wetting and transport properties such well-defined porous materials.