Experimental Study of Bubble Growth and Departure at the Tip of Capillary Tubes with Various Wettabilities in a Stagnant Liquid

Abstract

In the present paper, the bubble growth and departure at the tip of capillary tubes with different wall wettabilities in a stagnant fluid is experimentally investigated by using a high-speed visual system. The visual experiments show that the bubble growth experienced three typical stages: the initial growth, the speed-up growth, and the speed-down growth, with distinct varying behaviors of tip contact angle and size of bubble. The formation mechanism of each growth stage is discussed individually. It can be deduced from the experimental results that the bubble breakthrough point for a hydrophilic capillary tube is resulting from the triple contact line rapidly advancing from the inner wall slightly beneath tube tip to the inside surface edge of the tube tip when the contact angle of the bubble on the inner wall approaches the receding contact angle. The wall wettability has a significant effect on bubble growth and departure. The departure size and growth cycle period of bubble for a Teflon tube with hydrophobic wall is obviously smaller than these for glass tube with hydrophilic wall. Furthermore, the triple contact line of the bubble locates at the inside surface edge of the tube tip for glass tube, while one locates at the outside surface edge for Teflon tube before bubble departure. The liquid incursion into the tube tip for glass tube has never arisen for the Teflon tube after bubble departing from the tube tip.