Stabilization of electrically conducting capillary bridges using feedback control of radial electrostatic stresses and the shapes of extended bridges

Electrically conducting, cylindrical liquid bridges in a density-matched, e lectrically insulating bath were stabilized beyond the Rayleigh-Plateau (RP ) limit using electrostatic stresses applied by concentric ring electrodes. A circular liquid cylinder of length L and radius R in real or simulated z ero gravity becomes unstable when the slenderness S=L/2R exceeds pi. The in itial instability involves the growth of the so-called (2, 0) mode of the b ridge in which one side becomes thin and the other side rotund. A mode-sens ing optical system detects the growth of the (2, 0) mode and an analog feed back system applies the appropriate voltages to a pair of concentric ring e lectrodes positioned near the ends of the bridge in order to counter the gr owth of the (2, 0) mode and prevent breakup of the bridge. The conducting b ridge is formed between metal disks which are grounded. Three feedback algo rithms were tested and each found capable of stabilizing a bridge well beyo nd the RP limit. All three algorithms stabilized bridges having S as great as 4.3 and the extended bridges broke immediately when feedback was termina ted. One algorithm was suitable for stabilization approaching S=4.493... wh ere the (3, 0) mode is predicted to become unstable for cylindrical bridges . For that algorithm the equilibrium shapes of bridges that were slightly u nder or over inflated corresponded to solutions of the Young-Laplace equati on with negligible electrostatic stresses. The electrical conductivity of t he bridge liquid need not be large. The conductivity was associated with sa lt added to the aqueous bridge liquid. (C) 2000 American Institute of Physi cs. [S1070-6631(00)00505-5].