Experimental investigation of the liquid interface reorientation upon step reduction in gravity

Experiments have been carried out to investigate the settling behavior of a free liquid/gas interface in a partly filled right circular cylinder upon step reduction in gravity. Microgravity conditions were obtained within milliseconds after the release of a drop capsule in the drop tower facility in Bremen. In the experiments the cylinder radius was varied from 10 mm to 20mm with static contact angles of 2 degrees to 60 degrees. In addition to a series of silicone fluids, two different test liquids with a refractive index matched with the value of the cylinder material were used for experiments. The use of a digital high-speed recording system with a recording frequency of up to 500 fps allowed both an observation of the entire free liquid interface and a detail view on the moving contact line. Digital image processing techniques were applied to detect the contour of the free surface. For the initial condition the system is dominated by hydrostatic forces. In this case the equilibrium of the free liquid surface is characterized by a high Bond number yielding a flat surface and a small liquid ascent at the cylinder wall depending on the static contact angle. After transition to reduced gravity with a very low Bond number, capillary forces govern the flow and a capillary driven reorientation of the liquid to the new equilibrium position is established in a damped oscillation. The particular interest of this study is the investigation of the initial behavior of the free surface reorientation. It was found that the initial rise velocity of the contact point is determined by the Morton number and the static contact angle. Experimental results are presented that show an increasing rise velocity for decreasing Morton numbers. Further results characterize the initial behavior of the free surface at the center point as a function of characteristic time scales.