Liao Ying-Chih, Subramani Hariprasad J, Franses Elias I, Basaran Osman A
School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907-2100, USA.
Langmuir. 2004 Nov 9;20(23):9926-30. doi: 10.1021/la0487949.
Surfactants are routinely used to control the breakup of drops and jets in many applications such as inkjet printing, crop spraying, and DNA or protein microarraying. The breakup of surfactant-free drops and jets has been extensively studied. By contrast, little is known about the closely related problem of interface rupture when surfactants are present. Solutions of a nonionic surfactant, pentaethylene glycol monododecyl ether, or C12E5, in water and in 90 wt % glycerol/water are used to show the effects of surfactant and viscosity on the deformation and breakup dynamics of stretching liquid bridges. Equilibrium surface tensions for both solutions can be fitted with the Langmuir-Szyskowski equation. All experiments have been done at 24 degrees C. The critical micelle concentrations for C12E5 are 0.04 and 0.4 mM in water and the glycerol/water solution, respectively. With high-speed imaging, the dynamic shapes of bridges held captive between two rods of 3.15 mm diameter are captured and analyzed with a time resolution of 0.1-1 ms. The bridge lengths are 3.15 mm initially and about 5-7 mm at pinch-off. Breakup occurs after stretching for about 0.2-0.3 s, depending on the solution viscosity and the surfactant concentration. When the liquid bridges break up, the volume of the sessile drop left on the bottom rod is about 3 times larger than that of the pendant drop left on the top rod. This asymmetry is due to gravity and is influenced by the equilibrium surface tensions. Surfactant-containing low-viscosity water bridges are shown to break up faster than surfactant-free ones because of the effect of gravity. With or without surfactant, water bridges form satellite drops. Surfactant-containing high-viscosity glycerol/water bridges break up more slowly than surfactant-free ones because of strong viscous effects. Moreover, the shapes of the sessile drops close to breakup exhibit a "pear-like" tip; whether a satellite forms depends on the surface age of the bridge before stretching commences. These unexpected effects arising from the addition of surfactants are due to the capillary pressure reduction and Marangoni flows linked to dynamic surface tension.
表面活性剂常用于控制许多应用中的液滴和射流破碎，如喷墨打印、作物喷洒以及DNA或蛋白质微阵列。无表面活性剂的液滴和射流破碎已得到广泛研究。相比之下，对于存在表面活性剂时密切相关的界面破裂问题却知之甚少。使用非离子表面活性剂五乙二醇单十二烷基醚（或C12E5）在水以及90 wt%甘油/水溶液中的溶液，来展示表面活性剂和粘度对拉伸液桥变形和破碎动力学的影响。两种溶液的平衡表面张力都可以用朗缪尔-齐斯科夫斯基方程拟合。所有实验均在24摄氏度下进行。C12E5在水和甘油/水溶液中的临界胶束浓度分别为0.04 mM和0.4 mM。通过高速成像，捕捉并分析了夹在两根直径为3.15 mm的棒之间的桥的动态形状，时间分辨率为0.1 - 1 ms。桥的初始长度为3.15 mm，在 pinch-off时约为5 - 7 mm。破碎发生在拉伸约0.2 - 0.3 s后，这取决于溶液粘度和表面活性剂浓度。当液桥破碎时，留在底部棒上的固着液滴体积比留在顶部棒上的悬垂液滴体积大约3倍。这种不对称是由重力引起的，并受平衡表面张力影响。含表面活性剂的低粘度水桥由于重力作用比无表面活性剂的水桥破碎得更快。无论有无表面活性剂，水桥都会形成卫星液滴。含表面活性剂的高粘度甘油/水溶液桥由于强烈的粘性作用比无表面活性剂的桥破碎得更慢。此外，接近破碎时固着液滴的形状呈现出“梨形”尖端；是否形成卫星液滴取决于拉伸开始前桥的表面龄期。添加表面活性剂产生的这些意外影响是由于毛细管压力降低以及与动态表面张力相关的马兰戈尼流。
