Evaluation of gravity dependence of contact angle and surface tension for water
2020
Naganuma, N. (Meiji University, Kawasaki, Kanagawa (Japan). Graduate school of Agriculture) | Nogawa, K. | Maruo, Y. | Sato, N. | Noborio, K.
The driving force of water movement in porous media is the matric potential gradient due to capillary force. Although capillary force was observed under microgravity, a previous paper reported that water movement in porous media was slower under microgravity than under 1G. From the capillary rise theory, we hypothesized that water movement was delayed because the contact angle and surface tension of water are functions of gravity. A larger contact angle and smaller surface tension could slow water movement under microgravity. The objective of this research is to evaluate the gravitational dependence of the contact angle and surface tension of distilled water. The contact angles of water droplets and surface of water in the capillary tube were measured under microgravity induced by a 2 m drop facility. According to De Gennes' hypothesis, the effect of gravity on contact angle becomes dominant as the radius of a water droplet becomes larger than the capillary length (about 2 mm). We made water droplets with radii of 0.985, 2.73, 3.375, 3.925, and 4.535 mm on an acrylic plate and of 1.5 mm in an acrylic capillary tube. Surface tension was also measured using the maximum bubble pressure method under microgravity induced by a parabolic flight. Although no dependence of gravity was observed for the contact angle of droplets less than 2 mm in radius, the contact angle in a capillary tube that is 3 mm in radius slightly increased under microgravity. In addition, the gravity dependence of surface tension was not observed for distilled water.
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