Capillary detachment of a microsphere from a liquid-liquid interface

The attachment and detachment of microparticles at liquid-liquid interfaces are important for a number of material systems, from capillary suspensions and emulsions to coating applications. Hence, capillary forces become relevant to develop appropriate guidelines for designing these material systems. The required work to detach microparticles from a liquid-liquid interface is related to the shape of the meniscus; however, measuring capillary forces on a single microparticle at a liquid-liquid interface, while simultaneously imaging the meniscus, can be challenging. In this study, we correlate the detachment force with the shape of the meniscus by combining colloidal probe, atomic force microscopy (AFM) and laser scanning confocal microscopy. We measure the force and visualize the capillary bridge on a hydrophilic or hydrophobic microparticle, which is being pulled from a thin glycerol film surrounded by silicone oil. A fundamental model for detachment, based on capillary theory, is verified with different conditions. Moreover, we demonstrate that the interfacial tension and the continuous change of contact angle for a pinned contact line must be considered to accurately predict the detachment force.