On the Meniscus Shape and Marangoni Flow in Capillary Tubes with Noncircular Cross-Section Shapes

The liquid–vapor interface remains a key point for improving the heat transfer efficiency of heat exchangers. In this paper, we report experimental results on the evaporation of ethanol in capillary tubes of different internal cross-sections as the bulk meniscus recedes inside the tube, since the chosen conditions allow for the development of a liquid film along tube's internal corners due to the presence of capillary forces. To achieve our aims, we used 3D video microscopy to monitor the behavior of the meniscus during evaporation and, also, calculated the capillary pressures for three channels with different cross-sectional shapes each, namely circular, square and equilateral triangle cross-sections, for three solid–liquid contact angles, i.e., 30°, 60°, and 90°. At the same time, we have combined infrared thermal imaging technology with particle imaging velocimetry (PIV) visualization technology to analyze the severe Marangoni convection at the included angle. To this end, the interfacial temperature distribution of Marangoni convection was measured by means of an infrared camera and the flow pattern by means of PIV, respectively, through the plane in various views. For capillary tubes with included angle, the temperature gradient at the meniscus reaches its maximum at the included angle. Moreover, both temperature and convective gradients exist in the liquid film with included angle. When the angle of the capillary tube is smaller, the capillary effect will increase, leading to the formation of a thicker film. Our investigation constitutes an "extreme" experiment of Marangoni flow at the corners of the capillary tube. Due to the capillary forces, liquid films can rise in the corners of polygonal tubes up to the entrance of the tubes. Furthermore, the wetting force of ethanol at the angle of the triangle tube is much larger than that in the case of the square tube. Finally, the Marangoni vortex flows from the relatively high-temperature region in the center of the liquid towards the included angle, where evaporation is faster and the recharge of material and energy is more concentrated. We anticipate that our study sheds light into the meniscus shape and Marangoni flow in capillary tubes of with cross-sections of different shapes, which is of fundamental importance for various applications and in particular microfluidic systems.