Measurement of contact angle under different gravity generated by a long-arm centrifuge

The contact angle, as a vital measured parameter of wettability of a material surface, is pertinent to adhesion, nucleation, capillary phenomena, and liquid management in space. With the development of space exploration, the relationship between the contact angle and gravity has attracted increasing attention. In this study, we designed and manufactured a long-arm centrifuge (d =6 m) and a matched low-rate dynamic contact angle measurement system, which has the capacity to measure the advancing and receding contact angles of sessile drops under 1G-8G. The advancing and receding contact angles of water on smooth and rough surfaces were measured, and the results showed that 1) this apparatus is reliable for measuring the advancing and receding contact angles of sessile drops under 1G-8G, 2) the contact angle is not changed by gravity, while the measured apparent contact angles are changed by gravity owing to they are slope angles of liquid-vapor interface away from the solid surface due to limitation of the resolution of measurement system, and 3) wetting state of the rough surface can be changed by gravity.