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

Abstract

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.

Aerosol Droplet Surface Measurement Methods

“…14,15 The technique relies on two phenomena to achieve aerosol droplet capture and surface tension analysis, the first is the use of a focused laser that acts as an optical tweezer, and the second is the use of quasi-elastic laser scattering (QELS). [32][33][34][35][36][37][38][39][40] Spontaneous spherical resonance occurs as a result of thermal fluctuations at the surface of the liquid droplet, which can be observed in the Hz range of frequencies in a QELS spectrum. Figure 6 shows the experimental setup used to immobilize aerosol droplets using an optical tweezer as well as the use of QELS to analyze the spherical resonance modes at the droplet surface.…”

Interfacial Phenomena and Fluid Control in Micro/Nanofluidics

“…The in situ interfacial tension measurement remains a significant challenge, although some attractive image-based methods, [132][133][134] a light scattering method, 135,136 and an electrochemical method 137 have been reported. These new challenges are expected to advance the science and technology in this field in the next years.…”

Faraday forcing of high-temperature levitated liquid metal drops for the measurement of surface tension

“…Similar methods have been studied by Wada et al 4 and Pigot and Hibara. 5 Resonant frequencies are determined by Iino et al 6 by purposefully exciting standing wave instabilities on an interface within a cylindrical container using electrostatic oscillations. A similar method was later employed by Tsukahara et al, 7 who also related the experimentally determined resonant frequency to the surface tension of multiple fluids using expressions for the fluids’ natural frequencies.…”