Enhanced damping of capillary bridge oscillations using velocity feedback

Abstract

In reduced gravity, the stability of cylindrical liquid bridges and other systems having free surfaces is affected by ambient vibrations of the spacecraft. Such vibrations are expected to excite capillary modes. The lowest-order unstable mode of a liquid bridge is particularly susceptible to vibration as the length of the bridge approaches the stability limit. This mode is known as the (2,0) mode and is an axisymmetric varicose mode of one wavelength in the axial direction. In this work, an optical system is used to detect the (2,0)-mode amplitude. The derivative of the error signal produced by this detector is used to produce the appropriate voltages on a pair of annular disk electrodes which are concentric with the bridge. A mode-coupled Maxwell stress profile is thus generated in proportion to the modal velocity. Depending on the sign of the gain, the damping of the capillary oscillation can be either increased or decreased. This effect has been demonstrated in Plateau-tank experiments. Increasing the damping of the capillary modes on free liquid surfaces in space could be beneficial for containerless processing and other technologies.