Effect of gravity on the macroscopic advancing contact angle: an experimental investigation

The contact angle of an advancing sessile drop on a solid surface is important in many natural and industrial processes. This work is to study the effect of gravity on the advancing contact angle for a sessile drop. Purified water (millipore direct-Q) has been used as the liquid in this experiment. Teflon-coated silicon wafers were used as solid substrates. Images of the advancing drop were taken during the reduced gravity period. The advancing rates of the three-phase contact line were 6.5-9.0 mm/min. The high linear regression coefficient indicated that the advancing rate of the three-phase contact line can be considered almost constant for a particular pumping speed. Similar experiments were also performed on ground for comparison. The contact angles for the reduced gravity images were calculated by two different methods. The first method is axisymmetric drop shape analysis-profile (ADSA-P). The second method, geometrical relations for a spherical cap were used to calculate the contact angle from the images of the drop. The results were good agreement indicating that gravitational effects were negligible compared to surface effects. The average of the advancing contact angle in reduced gravity was 121°. The average of ground-based contact angles was 126°. The results show that, for water sessile drops on Teflon-coated silicon wafers, the advancing contact angle in the reduced gravity environment is less than the advancing contact angle in 1-g by about 5°. The irreproducibility of the contact angle might be due to airplane vibration.