Air pocket stability and the imbibition pathway in droplet wetting

The stability of air pockets formed in grooves on a surface is relevant to contact angle hysteresis of droplet wetting and it is investigated by imbibition experiments and surface evolver (SE) simulations. Liquid drops of different wettabilities are placed atop a conical hole on a polymethyl methacrylate (PMMA) substrate. The stability of the air pocket depends on surface wettability. Four kinds of imbibition behaviors ranging from wetting to nonwetting are observed. The imbibition pathway for the kinetically unstable air pocket is observed by using the olive oil droplet. It involves an inward flow of a thin liquid film along the wall of the hole. The accumulation of liquid at the bottom leads to the rise of the air bubble. The energy-barrier profile associated with the imbibition pathway acquired by SE simulations is able to interpret the outcome of imbibition. The advancing and receding contact angles of various liquids on a PMMA substrate with drilled holes are also determined. Their wetting behaviors can be categorized into three types. Our experimental results for substrates with or without fluorination are in good agreement with the theory based on the stability of air pockets.