Optical observation of high-frequency drop oscillations by a spectrum compression technique applied to the capillary pressure tensiometry.

Image acquisition and subsampling of periodic high-frequency drop oscillations is presented as an advantageous metrological procedure in capillary pressure tensiometry (CPT). The observation of a finite sequence of single tone or of multiharmonic cycles, subsampled in an expanded time-scale interval, allows the characteristics of the real oscillations to be well-reconstructed in a frequency-compressed spectrum, where each component is translated toward lower frequencies. The introduced technique is applied to nanoliter-sized water drops, oscillating in a hydrocarbon matrix up to 150 Hz frequency, by using a standard PAL CCD camera provided with an electronic shutter. Application examples show the important role of this technique in data analysis and interpretation of typical high-frequency oscillating drop/bubble experiments. In particular, this technique is effective to check the onset of critical hydrodynamic effects and allows for the determination of the intrinsic elasticity of the liquid/cell system as a function of frequency by comparison of the liquid volume, as displaced by a piezo-actuator, and the actually observed drop volume-amplitude oscillation. The knowledge of this quantity is fundamental for the calculation of the dilational viscoelasticity from the acquired pressure data in the CPT.