Straub Benedikt B, Schmidt Henrik, Rostami Peyman, Henrich Franziska, Rossi Massimiliano, Kähler Christian J, Butt Hans-Jürgen, Auernhammer Günter K
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
Leibniz-Institut für Polymerforschung, Hohe Straße 6, D-01069 Dresden, Germany.
Soft Matter. 2021 Nov 17;17(44):10090-10100. doi: 10.1039/d1sm01145f.
The dynamics of wetting and dewetting is largely determined by the velocity field near the contact lines. For water drops it has been observed that adding surfactant decreases the dynamic receding contact angle even at a concentration much lower than the critical micelle concentration (CMC). To better understand why surfactants have such a drastic effect on drop dynamics, we constructed a dedicated setup on an inverted microscope, in which an aqueous drop is held stationary while the transparent substrate is moved horizontally. Using astigmatism particle tracking velocimetry, we track the 3D displacement of the tracer particles in the flow. We study how surfactants alter the flow dynamics near the receding contact line of a moving drop for capillary numbers in the order of 10. Even for surfactant concentrations far below the critical micelle concentration ( ≪ CMC) Marangoni stresses change the flow drastically. We discuss our results first in a 2D model that considers advective and diffusive surfactant transport and deduce estimates of the magnitude and scaling of the Marangoni stress from this. Modeling and experiment agree that a tiny gradient in surface tension of a few μN m is enough to alter the flow profile significantly. The variation of the Marangoni stress with the distance from the contact line suggests that the 2D advection-diffusion model has to be extended to a full 3D model. The effect is ubiquitous, since surfactant is present in many technical and natural dewetting processes either deliberately or as contamination.
润湿和去湿的动力学很大程度上由接触线附近的速度场决定。对于水滴，已经观察到添加表面活性剂即使在远低于临界胶束浓度（CMC）的浓度下也会降低动态后退接触角。为了更好地理解为什么表面活性剂对液滴动力学有如此显著的影响，我们在倒置显微镜上构建了一个专门的装置，在该装置中，当透明基板水平移动时，水滴保持静止。使用像散粒子跟踪测速技术，我们跟踪流中示踪粒子的三维位移。我们研究了表面活性剂如何改变移动液滴后退接触线附近的流动动力学，毛细管数约为10。即使表面活性剂浓度远低于临界胶束浓度（≪ CMC），马兰戈尼应力也会极大地改变流动。我们首先在一个考虑平流和扩散表面活性剂传输的二维模型中讨论我们的结果，并由此推导出马兰戈尼应力的大小和标度的估计值。建模和实验一致认为，几μN/m的微小表面张力梯度就足以显著改变流动剖面。马兰戈尼应力随与接触线距离的变化表明，二维平流 - 扩散模型必须扩展为完整的三维模型。这种效应很普遍，因为表面活性剂在许多技术和自然去湿过程中要么是有意存在的，要么是作为污染物存在的。
