Evaluating phase velocity of capillary waves on thin layer fluids using acoustic radiation force-based optical coherence elastography

Rheological properties of biological fluids are closely linked with various physiological processes. Capillary waves are associated with rheological properties of fluids such as viscosity and surface tension. The phase velocity of capillary waves is a primary parameter for measuring rheological properties of fluids. For a fluid layer in a shallow fluid regime (the fluid depth is smaller than 0.05 times the wavelength), capillary waves play an important role especially in cell and molecular biology. Therefore, evaluating phase velocity of thin layers of a fluid is a key mechanism for understanding the rheological properties of the fluids in small scale. However, evaluating phase velocity of thin layer fluids with a non-contact has not been widely reported, and is challenging by using existing rotational-based and tube-based rheometry devices. Here we first report that phase velocities of capillary waves on thin layer fluids in shallow fluid regime can be determined. The acoustic radiation force (ARF) was used to create capillary waves on the thin layer fluids and a spectral domain optical coherence tomography (SD-OCT) was used to detect particle motions of the waves. The experimental results were compared with the theoretical analysis. A 7.5 MHz single element transducer was used to produce the ARF to create capillary waves. The phase velocity of capillary waves on thin layer fluids were successfully determined by using the proposed elastography technique with the non-contact fashion, which paves the way for measuring viscosity of thin layer fluids in our near future study.