Evaporative heat transfer from an electrowetted liquid ribbon on a heated substrate

Evaporation of narrow water ribbons (of 5 and 7 μL volume) formed on a heated surface is investigated. Chemical and structural patterning of a silicon substrate is employed to fabricate a hydrophilic stripe that bisects hydrophobic pillar arrays of varied geometric roughness. Electrical heating of a 100 nm titanium layer on the back side of the device provides a constant heat flux. In the absence of electrical actuation, water introduced onto the substrate takes a bulging ribbon shape that is constrained to the immediate vicinity of the hydrophilic stripe. Electrowetting of the water ribbon spreads it into the hydrophobic pillar arrays on either side, leading to significant increases in maximum wetted width (up to 200%) and wettability (up to 80% reduction in contact angle). Infrared thermography is employed to characterize the cooling effect due to the spreading of the ribbon, while a goniometer monitors the ribbon shape. The heat transfer in each case is estimated through an energy balance analysis, and the results are compared with other electrowetting-based cooling techniques.