Dynamic advancing and receding contact angles of adaptive surfaces

表面科学的一大挑战是定量地了解自适应表面的动态接触角。这里，我们关注的是表面，它在液体或其蒸气存在时自发地发生变化。在我们最近发展的理论中，我们将适应过程的动力学与动态接触角联系起来。该理论的一个结果是，适应有两个重要的后果：(a)它导致接触角滞后和(b)前进和后退的接触角依赖于速度。因此，表面适应可以被认为是接触角滞后（除了粗糙度和非均质性）和动态接触角变化（除了流体力学）的一个原因。适应可以特别解释在接触线非常低的速度下观察到的接触角的变化。我们项目的目的是检验这一理论。为了达到这一目的，我们计划测量聚合物薄膜的适应过程动力学。利用动力学和自适应理论，可以计算出动态进退接触角。最后，将预测的动态接触角与在相同聚合物薄膜上测量的运动液滴的速度相关接触角进行比较。为了测量前进和后退的接触角与接触线的速度，我们将使用我们自制的下降附着力仪器（DAFI）。在DAFI中，无柄液滴通过一根电线以规定的速度在表面上移动，前进和后退的接触角与力平行光学测量。由于DAFI被限制在速度< 5厘米/秒，我们计划建立一个倾斜的平面设置与摄像机，以监测移动的速度高达1米/秒滴的动态接触角。作为模型系统，我们计划使用涂有不同聚合物刷膜的氧化硅表面。表面引发原子转移自由基聚合将用于聚合物薄膜的合成。对于极性液体，我们将使用聚n -异丙基丙烯酸酰胺（PNIPAM）和聚2-羟乙基甲基丙烯酸酯（PHEMA）。对于非极性液体，我们计划合成聚甲基丙烯酸甲酯（PMMA）和聚苯乙烯（PS）刷。由PS和聚（2-乙烯基吡啶）（PVP）制成的二元刷膜也将作为更复杂的适应的一个例子进行研究。通过x射线反射、表面等离子体共振（SPR）和激光扫描共聚焦显微镜研究了分子暴露于溶剂时的溶胀动力学和（再）组织。如果我们可以验证我们的理论，测量动态接触角的速度依赖性将允许计算表面的适应动力学。如果成功，我们将有一个相对通用的框架，用于动态接触角和适应过程引起的接触角滞后的定量建模。

One of the big challenges in surface science is to quantitatively understand dynamic contact angles of adaptive surfaces. Here, we are concerned with surfaces, which spontaneously change in the presence of the liquid or its vapor. In our recently developed theory, we related the kinetics of adaptation processes to dynamic contact angles. One result of the theory is that adaptation has two important consequences: (a) it leads to contact angle hysteresis and (b) advancing and receding contact angles become velocity dependent. Therefore, surface adaptation can be established as one cause for contact angle hysteresis (in addition to e.g. roughness and heterogeneity) and for changing dynamic contact angles (in addition to e.g. hydrodynamics). Adaptation may in particular explain the observed changes in contact angle at very low speed of the contact line.The aim of our project is to test the theory. To reach this aim, we plan to measure the kinetics of the adaptation process for thin polymer films. With the kinetics and our adaption theory, we can calculate the dynamic advancing and receding contact angles. Finally, the predicted dynamic contact angles will be compared with the velocity-dependent contact angles of moving drops measured on the same polymer films. To measure advancing and receding contact angle versus speed of the contact line we will use our home built Drop Adhesion Force Instrument (DAFI). In DAFI, sessile drops are moved across surfaces at defined velocity by a wire and the advancing and receding contact angles are measured optically in parallel with the force. Since DAFI is limited to velocities < 5 cm/s we plan to build up a tilted plane setup with cameras to monitor the dynamic contact angles of moving drops at velocities up to 1 m/s. As model systems, we plan to use silicon oxide surfaces coated with different polymer brush films. Surface-initiated atom-transfer radical poly¬meri¬za¬tion will be used to synthesize the polymer films. For polar liquids, we will use poly(N-iso¬propyl¬acryl¬amide (PNIPAM) and poly(2-hydroxyethyl methacrylate) (PHEMA). For non-polar liquids, we plan to synthesize polymethyl-meth¬acrylat (PMMA) and polystyrene (PS) brushes. Binary brush films made from PS and poly(2-vinylpyridine)) (PVP) will be investigated, too, as one example of a more complex adaptation. The swelling kinetics and (re)organizations of molecules upon exposure to solvents will be investigated by X-ray reflectometry, surface plasmon resonance (SPR) and laser scanning confocal microscopy.In case we can validate our theory, the measurement of the velocity dependence of the dynamic contact angles will allow calculating the adaption kinetics of surfaces. If successful, we would have a relatively universal framework for quantitative modeling of dynamic contact angles and contact angle hysteresis caused by adaptation processes.