Single bubble dynamics on hydrophobic-hydrophilic mixed surfaces

Single bubble dynamics of distilled water were experimentally investigated on hydrophobic-hydrophilic mixed surfaces with hydrophobic dot diameters ranging from 50 μm to 6 mm. The heterogeneity of surface wettability could affect interfacial dynamics and cause the pinning phenomenon of the bubble interface, a contact angle transition, a 'stick-slip' behavior, and the interface necking during bubble growth. The triple line of a bubble initiated from a hydrophobic dot was pinned at its edge where a singularity of the surface wettability occurs. During this contact line pinning, the contact angle of bubble interface decreased. When the contact angle decreased to equal the receding contact angle of a bare hydrophilic surface, the triple line moved outward rapidly; this is called 'slip' behavior. After such process, the nucleated bubble vertically elongated and subsequently departed from the surface. The contact angle transition during the pinning and departing behavior was described by considering the capillary length of the bubble determined by the vertical deformation of bubbles. Using the bubble dynamics on the heterogeneously patterned surface with compactly arranged 50 μm dots, boiling heat transfer coefficient enhanced to 2.1 times that in the bare hydrophilic surface.