Wetting of Silicone Oil Drops and Air Bubbles in Ethylene Glycol/Water Mixtures

Contact angles of silicone oil droplets and air bubbles settled on a solid substrate and confined in binary ethylene glycol/water mixtures were studied as a function of ethylene glycol concentration. Despite similar reductions in interfacial tensions for both systems, distinct wetting behaviors were observed. While the air bubble contact angle increased with ethylene glycol concentration, the silicone oil droplet exhibited a more complex response, characterized by a stable contact angle at low ethylene glycol concentrations followed by an increase at higher concentrations. This suggests that droplet morphology can remain relatively stable even as interfacial tensions change. A phenomenological description based on the imbalanced forces highlights the crucial role of the contact angle in pure water environment, in predicting the wetting behavior that may follow the reduction of interfacial tension by further surfactant or cosolvent adsorption.