Capillary-driven flow in tubes connected to the containers under microgravity condition

Abstract

The capillary-driven liquid flow in tubes connected to containers under a microgravity condition is systematically studied in a drop tower experimentally. The microgravity time lasts up to 3.6 s and the working liquids are mixtures of ethanol and deionized water with different ratios. Theoretically, based on the previous theory for tubes directly immersed in fluid, a modified formula is developed to describe the change tendency of the height of meniscus with microgravity time for such a container/tube system exposed to a microgravity environment. From the theoretical formula, the numerical results of meniscus height at different microgravity time can be obtained, utilizing the geometrical parameters of container/tube systems and the relevant physical quantities of Eth/H2O mixtures with different ratios. By comparing the numerical results with experimental results for different contact angles between working liquid and container in different container/tube systems, we show that the theoretical model is able to quantitatively predict the capillary-driven flow in tubes connected to containers, and the numerical results have good consistence with the experimental results. In addition, the experimental results also show that though the ratio of ethanol to deionized water can change the contact angle remarkably, it has little influence on the capillary flow if the geometrical parameters of the container/tube systems are the same. This is because not only the contact angle, but also the surface tension and viscosity coefficient of the working liquid change with the ratio of ethanol to deionized water. It is found that when the contact angle increases from 42° to 66°, the surface tension increases from 0.0328 N/m to 0.0443 N/m correspondingly, but the viscosity coefficient decreases from 2.11 cSt to1.49 cSt. As a result, the changes of surface tension and viscosity coefficient offset the influence of the change of contact angle, which can be explained by our theoretical model. Compared with the extensively studied system in which tubes are directly immersed into liquid, the container/tube system studied in this paper is more similar to many actual systems such as fluid transfer systems in the microgravity condition and in micro-fluidic devices. Therefore, this study is useful for predicting and analyzing the capillary flows of these actual systems.