Abstract:

Marangoni instabilities in a sessile droplet of 1cSt silicone oil evaporating at constant contact angle mode on a heated copper substrate were investigated through a series of experiments. Results reveal that with the increase of the temperature of substrate, three kinds of convection modes occur in sequence, i.e., thermocapillary convection, steady Bénard-Marangoni (BM) convection and irregular oscillatory BM convection, respectively. For the steady BM convection, with the receding of the wetting radius, the cell number decreases one by one while the morphology of the BM cell does not vary. When the cell number is less than five, the individual cell becomes to be circular. With the contact angle increases, the critical Marangoni number (Ma_c) for the onset of the steady BM convection increases. The dimensionless cell number increases with the dimensionless wetting radius, which is independent on the contact angle.

Key words: sessile droplet, evaporation, constant contact angle mode, convection, instability

摘要：

实验观察了水平加热基板上1cSt硅油液滴在固定接触角蒸发模式下的Marangoni对流失稳模式的演化规律，分析了接触角和基板温度对Marangoni对流不稳定性的影响。结果表明，随着基板温度的升高液滴内依次呈现热毛细对流、稳定的Bénard-Marangoni（BM）对流和不规则振荡的BM对流。对于稳定的BM对流，涡胞数随润湿半径的减小逐渐减少；当涡胞数少于5时，涡胞变为圆形；随着接触角的增大，由热毛细对流转捩为稳定的BM对流时的临界Marangoni数（Ma_c）增大；蒸发过程中，液滴内无量纲涡胞数随无量纲润湿半径的增大而线性增大，与接触角无关。

关键词: 液滴, 蒸发, 固定接触角, 对流, 不稳定性