Abstract:
Liquid bridges between two solid surfaces have applications in low gravity such as the solidification of floating zones. Long bridges naturally become unstable to a symmetric mode by bulging near one end while the opposite end thins. For a cylindrical bridge in low gravity of radius R and length L, the slenderness S=L/2R has a natural (Rayleigh) limit of (pi) beyond which the bridge breaks. It has been demonstrated that acoustic radiation pressure may be used in simulated low gravity to produce stable bridges significantly beyond the Rayleigh limit with S as large as 3.6. The bridge (PDMS mixed with a dense liquid) has the same density as the surrounding water bath containing an ultrasonic standing wave. Modulation can be used to excite specific bridge modes [Morse et al., Phys. Fluids 8, 3--5 (1996)]. The shape of our bridge is optically sensed and the ultrasonic drive is electronically adjusted such that the radiation stress distribution dynamically quenches the most unstable mode. This active control simulates passive stabilization suggested for low gravity [J. Acoust. Soc. Am. 97, 3377(A) (1995)]. Feedback increases the mode frequency in the naturally stable region. [Work supported by NASA.]