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(Color online) Cutaway drawing of a water droplet levitating within a spherical brass cavity connected to a high voltage source, within the 50 mm-diameter magnet bore. A nozzle in the base of the cavity directs an air pulse at the underside of the droplet. A optical fiber inside the nozzle shines light from a laser at the droplet. Light refracted through the droplet is transmitted to a photodiode, connected to an oscilloscope, via a second fiber.
(Color online) Top: oscillations in laser light intensity falling on the photodiode reveal the oscillations of a droplet with radius 7.5 mm. Center: the peaks in the power spectrum of the oscillations show clearly the l = 2–5 resonant frequencies of the droplet. The measured frequencies are slightly higher than the frequencies predicted by Eq. (2), indicated by dashed lines, owing to the effect of the magnetogravitational potential trap. Bottom: the peaks shift to lower frequency with increasing charge Q. In order of increasing Q, the peaks correspond to Q = 0 (black curve), 1.4nC (red), 2.2nC (green), 2.9nC (blue), 3.6nC (pink), for the 7.5 mm radius drop.
(Color online) Frequency shift of the fundamental l = 2 mode, , as function of charge Q for drops with radii a between 4.5 and 7.5 mm.
(Color online) Top: fan plot showing the shift in frequency with charge Q for the l = 2–4 resonances. The frequency has been scaled by the time scale and the charge by . Bottom: when the frequency shift is further scaled by l(l − 1), the data for all the modes collapse onto one line. This panel includes the measured peak positions of modes . Only selected data points shown with error bars, for clarity.
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