ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

4pPA12. Capillary bridge modes driven with modulated acoustic radiation pressure.

Scot F. Morse

David B. Thiessen

Philip L. Marston

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

A liquid bridge between two solid surfaces is known as a capillary bridge. In the experiments to be described, the bridge liquid is a mixture of PDMS with a dense organic liquid. The bridge has the same density as the surrounding water bath such that its equilibrium shape between circular posts is a circular cylinder of length L and radius R. Low-frequency modes of such a bridge are known to exist where surface tension provides the restoring force. Even neglecting viscosity, the modal characteristic equation is complicated by the boundary conditions. It is demonstrated that low-frequency modulation of an ultrasonic standing wave in the surrounding bath can be used to excite specific axisymmetric or nonaxisymmetric capillary modes. Coupling to each mode depends on the location of the bridge in the standing wave. The coupling is a consequence of the space-time modulation of the radiation pressure as discussed previously for drops and bubbles [P. L. Martson, 15--26 (1980)]. The predicted decrease of mode frequency with increasing slenderness S=L/2R is confirmed as is the vanishing of the frequency of the most unstable mode as S approaches (pi). [Work supported by NASA.]