Coalescence-Induced Jumping Bubbles during Pool Boiling

Surfaces with micro/nano-structures significantly enhance the critical heat flux of nucleate boiling by increasing the wickability of the liquid phase to delay film boiling. An alternative strategy, of removing surface bubbles at smaller sizes, is largely ignored. Here, they fabricate a rationally designed micro-structured surface that enables the coalescence-induced departure of microscopic vapor bubbles during the pool boiling of water at diameters an order of magnitude smaller than single-bubble buoyant departure. Arrays of micro-cavities or micro-grooves serve as tailored nucleation sites to nucleate close-packed vapor bubbles, which coalesce at unusually small sizes compared to a homogeneous surface. Two different modes of coalescence-induced bubble departure are observed and modeled: capillary-inertial jumping for smaller micro-bubbles and buoyant-inertial liftoff for larger micro-bubbles. Capillarity-enhanced pool boiling on micro-structured surfaces has the potential to dramatically enhance the heat transfer coefficient and critical heat flux of pool boiling. The surface energy released upon coalescence is exploited to remove surface bubbles during the pool boiling of water. Rational close-packed microstructures promote early coalescence events, resulting in bubble departure at sizes at least an order of magnitude smaller than with buoyant departure. Two modes of coalescence-enhanced departure are characterized: capillary-inertial jumping for smaller micro-bubbles and buoyant-inertial liftoff for larger ones.image