Droplet movement on a vertical gradient surface

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=1mm 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 L-c defined as L-C = root gamma/rho g where gamma, rho and g represent the liquid surface 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 L-c, 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 L-c, the surface tension gradient driven force becomes dominant force in the system. It is found that C=1mm gradient surface can cause smaller droplets to move and it is believed this is the major mechanism responsible for the better heat transfer efficiency.