At the microscale, the increased surface area-to-volume ratio greatly enhances the effect of surface forces, making them essential for fluid control. In mixed systems, the composition of the interfacial layer often differs substantially from that of the bulk phase. While this difference has minimal influence on surface tension measurement at the macroscopic scale, where the interfacial layer’s thickness is comparable to the amplitude of surface waves, it becomes critical at the microscopic scale. Surface tension, a macroscopic thermophysical property, depends not only on the free energy at the interface but also on the adsorption of solute molecules in the adjacent interfacial layer. Thus, microscale variations play a crucial role in surface adsorption, necessitating the study of methods to measure liquid surface tension in microchannels under in situ conditions.

In this paper, we propose a novel experimental system designed for light scattering on reflective surfaces, with adjustable micrometer-scale channel widths. In addition, a microscale liquid level control platform is developed, incorporating three-dimensional motion and a rotary stage that rotates along the z-axis to control dimensions in the x, y, z, and φ directions. Precise movement in the x-direction is achieved using a one-dimensional digital displacement stage, which offers a stroke of 25.4 mm and a step accuracy of 1 μm. This system enables the creation of microchannels ranging from 10 to 100 μm in width, with the capability to continuously vary channel dimensions by several micrometers. The power spectrum equation for surface waves in microscale channels is derived based on strict boundary conditions. Surface tension is determined by applying this equation to the channel data, following time-domain data processing techniques such as zero-channel-point acquisition, data folding, and discrete fast Fourier transform.

The power spectra of surface waves confined within microchannels of different widths at 298.15 K and atmospheric pressure are obtained using isooctane. As the channel width increases, the power spectrum shifts to lower frequencies, while the peak value gradually rises. According to Eq. (6), as the channel width increases, the extracted wavelength also increases, leading to a decrease in scattering angle and intensity, thus raising the power spectrum’s peak (Fig. 5). Minor deviations between measurements of the three standard substances and the reference data, as well as slight variations in repeated single-point measurements, demonstrate that the new system offers enhanced precision, consistency, and reliability across a wide range of channel widths or wave numbers. Furthermore, the first-order approximate solutions for iso-octane, n-decane, and hexadecane increasingly deviate due to the neglect of body-phase dissipation near the critical oscillatory region. The primary approximation applies only to extremely large Y values (e.g., Y>100), indicating extremely low viscosities (Fig. 6). In most practical cases, fluids do not meet this condition, resulting in significant systematic deviations when determining surface tension using this approach.

We propose an experimental system for measuring light scattering on microscale reflective surfaces and assess its accuracy and reliability using iso-octane, n-decane, and n-hexadecane under ambient temperature and pressure. The key findings are as follows. First, the surface wave power spectrum equation for microscale channels is derived by considering the surface wave dispersion equation and boundary conditions. Second, by integrating a precisely adjustable microchannel (30 to 90 μm) with a reflective light scattering system, we successfully measure fluid surface tension with minimal sample volumes (about 2 μL). Third, we validate the new system and measurement method using reference materials, and the discrepancies between the experimental and theoretical surface tension values for the three alkanes are within 3%, with single-point measurements taking only a few seconds. This meets the requirements for high-precision surface tension measurement and sensing. Lastly, a comparison of the surface wave power spectrum equation and the first-order approximation equation shows that the former accurately calculates surface tension in microscale channels, while the latter introduces significant systematic deviations.