A fundamental study on transportation of a liquid droplet on a substrate having a spatial gradient in its surface free energy is underway at Tokai University. In this paper, the basic concept of calculation for droplet transportation will be reviewed. Capability of surface to make a motion of a droplet is discussed with several key parameters such as advancing and receding contact angles and surface free energy gradient. Measurements have shown that it is possible to predict the variation of the total surface energy of the system and the decreasing rate of the energy can be calculated for the gradient surface.
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