Measurement of viscous slip coefficient on a thin wire surface in rarefied gas flows

In rarefied gas flows, velocity slip phenomenon appears with increasing Knudsen number and has a strong influence on the flow field. It is important to evaluate the size of velocity slip. In this study, a novel approach was proposed to measure the viscous slip coefficient, which represents the magnitude of velocity slip in high Knudsen number flows, on the surface of a thin wire. A microchannel with coaxial-cylinders geometry consisting of a thin wire placed on the center axis of a circular microtube was prepared. Then, the mass flow rate through a gap between the thin wire and the microtube was measured using the constant-volume method. The measured results were subsequently compared with those obtained with the microtube only, i.e., without the thin wire, and the viscous slip coefficient on the thin wire was deduced by fitting the analytical mass flow rates to the measured values. From our measurements using a platinum thin wire inside a glass microtube, the viscous slip coefficients for helium and argon on the platinum surface were successfully obtained. From the viscous slip coefficients, the tangential momentum accommodation coefficients were also calculated. Our novel approach makes it possible to measure the velocity slip on metal surfaces, leading to further accumulation of the data on a wider variety of sample surfaces.