Contact angle hysteresis generated by the residual gravitational field of the Space Shuttle

When the two fluid phases of a substance are present in a cylinder, one of the possible equilibrium configurations is for two liquid phases to be pres ent, one above the vapor phase and one below. If surface tension dominates the gravitational effects, the two-interface configuration is the thermodyn amically favored one. When the system is in the two-interface configuration , the difference in pressure between the two liquid phases is predicted to be the same as it would have been had no vapor phase been present! Although the pressure profile cannot be measured directly, it is predicted to cause the contact angle value at the upper three-phase line to be smaller than t hat at the lower three-phase line. This difference in contact angles can be measured, and from the measured values, the theory can be used to determin e the value of the gravitational intensity. In an experiment conducted on a Space Shuttle flight, the configuration adopted when glass cylinders of di fferent diameters were each partially filled with water was recorded. The f luid in each cylinder was found to adopt the two-interface configuration, a s predicted. A 56 mm diam glass cylinder that had a height of 86 mm was obs erved to have a contact angle at the upper three-phase line of 6.7 +/- 1.3 degrees and 26.5 +/- 4.0 degrees at the lower. The value of the gravitation al intensity inferred from the measured contact angles agrees with that rep orted from the (NASA) electronic Space Acceleration Measurement System (SAM S). This agreement supports the prediction that contact angle hysteresis is generated by a difference in pressure between the two liquid phases of the two-interface configuration. (C) 2000 American Institute of Physics. [S002 1-9606(00)51116-7].