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When saturated vapor passes over a colder substrate, liquid drops nucleate and grow by coalescence with surrounding drops. Typically speaking, nucleation and growth rates of water droplets are faster on a hydrophilic surface than on a hydrophobic surface. However, heat transfer efficiency degrades once surface becomes filmwise condensation. In this paper, vapor condensing on a gradient surface to prevent filmwise condensation is studied. New gradient surfaces are fabricated. It is demonstrated that 10% increase of condensation heat flux can be achieved on a silicon wafer with C = 1 mm gradient surface. The main mechanism for heat transfer enhancement is found to be that drops condensing on C = 1 mm gradient surface begin to move at a much smaller size compared with those on silicon wafer without modification.

This research studies the improvement of droplet movement on a vertical surface. Silicon substrates fabricated with different surface tension gradients are used to enhance heat transfer efficiency in a vapor condensing system. Experimental results find that the heat transfer efficiency of gradient surface with strip width C=1 mm can be 10% higher than a hydrophilic surface. The mechanism for the gradient surface is also studied by both theoretical and experimental methods. A characteristic length scale Lc, defined as Lc = radicY/pg where gamma, p and g represent the liquid surface Pg tension coefficient, liquid density and gravity, is found in the system. When the gradient surface length scale C is much higher than the length scale Lc, the system can be considered as an only gravity driven system. When length scale C of the gradient surface is smaller than or equals to the length scale Lc, the surface tension gradient driven force becomes dominant force in the system. It is found that C=1 mm gradient surface can cause smaller droplets to move and it is believed this is the major mechanism responsible for the better heat transfer efficiency.

Using phase change of working fluid to remove heat in a heat exchanger device is a high efficiency method. When superheated vapor passes over a sub-cooled substrate, water droplets nucleate and grow by coalescence with the surrounding drops. The merging droplets exhibit two-dimensional random motion somewhat like the Brownian movements of colloidal particles. If surface energy patterns are designed on the substrate surface, the random condensing droplets will nucleate and grow to a certain size and move toward the more hydrophilic side of the surface. Powered by this forces, condenser surface will not grow into film-wise condensation situation. Thus condensation speeds are faster than those of typical surfaces without any surface modification. This effect has implications for passively enhancing heat transfer in heat exchangers or heat pipes.