Analysis of Capillary Rise and Surface Tension in Micro-Tubes Using Computational Modelling

Abstract

Microfluidics technology is widely used in modern engineering, such as medical diagnostic chips and micro-electronic cooling devices. In small-scale devices, the fluid flow is mainly controlled by capillary force and surface tension, while the effect of gravity is less dominant. Capillary rise is not a new phenomenon, but physical experiment can be difficult and costly because the water movement happens very fast and needs accurate equipment. Therefore, this study simulates the capillary rise in micro-tubes using Computational Fluid Dynamics (CFD). The objectives of this study are to observe the transient flow behaviour, compare the effect of different tube radius, and check whether a simplified 2D model can give reasonable result. ANSYS Fluent was used to simulate the two-phase flow between water and air using the Volume of Fluid (VOF) method. The simulation was done using a half-domain 2D model with symmetry boundary, and the mirror feature was only used for clearer contour visualization. A Grid Independence Test (GIT) was carried out to select a suitable mesh. The Continuum Surface Force (CSF) model and laminar viscous model were used to include surface tension effect without adding artificial turbulence. Three tube radius were studied, which were 0.5 mm, 1.0 mm, and 1.5 mm. The results showed underdamped capillary rise oscillation for all cases. The smaller tube had stronger viscous damping and became stable faster. The final capillary heights were 14.75 mm, 7.85 mm, and 6.20 mm for 0.5 mm, 1.0 mm, and 1.5 mm radius, respectively. The result shows that capillary height decreases when tube radius increases, which agrees with the modified Jurin Law for 2D parallel plates. Overall, the 2D VOF model can predict the main capillary rise behaviour with good accuracy.