The ultrasound is responsible for generating capillary waves on the thin-layer of fluids, while OCT is utilized to measure the particle displacement on the fluid’s surface (left panel). Our demonstration highlights the system’s ability to capture capillary waves traveling on a water layer with an approximate thickness of only 300 µm (middle panel). The viscosity of the thin water layer was found to be 1.0086 ± 0.2282 mP·s, which is in excellent agreement with the standard value (right panel).

Rheological properties of biological fluids are closely linked with various physiological processes. Capillary waves are associated with rheological properties of fluids such as viscosity. However, existing rotational-based and tube-based rheometry devices are unsuitable for measuring the viscosity of biological fluids due to the need for sample contact and cleaning the testing chamber between each sample. Although capillary waves in deep regime are associated with the rheological properties of biological fluids such as viscosity, not all biological fluids can be sampled in significant volumes, as is the case with blood. Therefore, it is crucial to measure the viscosity of fluids with small volumes. Unfortunately, evaluating the viscosity of thin-layer fluids in a non-contact manner has not been widely reported and poses challenges using existing rotational-based and tube-based rheometry devices.

In this study, Dr. Hsiao-Chuan Liu and his collaborators present a non-contact rheometry method based on capillary waves in a shallow regime for evaluating the viscosity of thin-layer water (e.g., sub-millimeter to micrometer depth) using acoustic radiation force-based optical coherence elastography (ARF-OCE) with customized scan patterns. The experimental results were compared with theoretical simulations. Tap water in room temperature was used to create a thin layer with a thickness of only 300 µm. A focused transducer was employed to generate the ARF required for inducing capillary waves on the thin-layer fluids. A one-dimensional Fourier transform (1D-FT) was applied to evaluate frequency components to determine a suitable range of wave propagation distance for calculating attenuation. Dr. Liu and his collaborators successfully determined the viscosity of thin layer fluid using ARF-OCE in a non-contact fashion, which paves the way for viscosity measurements of various thin layer fluids such as glycerol, plasma, or biological fluids with minimal sample volumes in near future studies.

This technique can be further used to evaluate the infection of eyes such as dacryocystitis by evaluating the viscosity of tears.