Experimental measurement of vibration of liquid droplet at low bond numbers using ESPI

The vibration of droplets finds multiple applications in inkjet printing, combustion sprays, drop atomization etc. In many of these processes, the primary interest is to estimate the resonant frequencies and mode shapes of the vibrating drops. Previous works show extensive characterization of vibrating droplets in a gravity dominated regime, where Bond numbers (ratio of gravitational and surface tension effects) are greater than 2. In the present work, vibrations of small size droplets, in the capillary regime, with Bond number in the range 0.24–1.37 are considered on hydrophilic and hydrophobic surfaces. The surface of the vibrating drop at resonance is characterized using a novel interferometric method: Electronic Speckle Pattern Interferometry (ESPI) is explored. The resonant frequencies obtained through the interferometric patterns of ESPI is found to be in good agreement with a theoretical model considering a 1D capillary-gravity wave.