Experimental description of bubble dynamics and heat transfer processes occurring on the pool boiling of water on biphilic surfaces

This study concerns the detailed description of the fluid dynamics and heat transfer mechanisms occurring during single bubble nucleation on biphilic surfaces (superhydrophilic/hydrophilic surfaces surrounding hydrophobic/superhydrophobic isles). A high-speed video camera is synchronized with a high-speed thermographic camera to relate the temporal evolution of bubble dynamics, from generation to detachment. The results allowed identifying several stages of bubble growth in all the biphilic surfaces tested, regardless of the size of the superhydrophilic regions. Bubble dynamics is affected by the size of the superhydrophobic regions. Since bubbles are constrained to the superhydrophobic region boundaries, which alter their base diameter, smaller superhydrophobic regions tend to promote a regular and stable bubble generation, due to the action of stronger surface tension forces acting on the boundary region. As the base diameter increases, the surface tension effects, which only act at the boundary with the hydrophilic region, are lessened and only affect the bubble at larger volumes. Consequently, smaller superhydrophobic regions are associated to higher evaporation mass transfer rates. Temperature gradients are larger at the hydrophilic/superhydrophobic boundaries and despite being small amplitude differences, they clearly promote induced convection of the cold liquid between superhydrophobic regions, as observed in the high-speed thermographic images.