Surface Tension Measurement at the Microscale by Passive Resonance of Capillary Waves

The properties of fluid interfaces increase in importance as the physical scale decreases and, hence, characterization of surface tension becomes all the more critical. However, there is to date no method to characterize this parameter on microscale surfaces. We propose here a simple method based on the resonance of capillary waves, which are naturally excited by thermal fluctuations, under one-dimensional spatial restrictions using single-beam dynamic light scattering. The principle was verified at methanol/air interfaces in polydimethylsiloxane (PDMS) microchannels having various widths. Characteristic comb-shape power spectra were experimentally obtained. Theoretical analysis showed that the spectral peaks correspond to the first or higher modes of the capillary wave resonance in the restricted space between the parallel channel walls. A useful relation between successive modes was derived to eliminate the effects of damping at the soft PDMS walls. Thus, for methanol, two values were calculated from three successive modes (24.8 and 21.2 mN/m); the literature value is 22.02 mN/m. For acetonitrile, the value obtained was 28.2 +/- 5 mN/m, close to the literature value of 28.6 mN/m. Although accuracy and precision require further elucidation, this novel method is expected to become a powerful tool at the micro/nanoscale.