Grawitter Josua, Stark Holger
Technische Universität Berlin, Institut für Theoretische Physik, Straße des 17. Juni 135, 10623 Berlin, Germany.
Soft Matter. 2021 Mar 11;17(9):2454-2467. doi: 10.1039/d0sm02082f.
Droplets move on substrates with a spatio-temporal wettability pattern as generated, for example, on light-switchable surfaces. To study such cases, we implement the boundary-element method to solve the governing Stokes equations for the fluid flow field inside and on the surface of a droplet and supplement it by the Cox-Voinov law for the dynamics of the contact line. Our approach reproduces the relaxation of an axisymmetric droplet in experiments, which we initiate by instantaneously switching the uniform wettability of a substrate quantified by the equilibrium contact angle. In a step profile of wettability the droplet moves towards higher wettability. Using a feedback loop to keep the distance or offset between step and droplet center constant, induces a constant velocity with which the droplet surfs on the wettability step. We analyze the velocity in terms of droplet offset and step width for typical wetting parameters. Moving instead the wettability step with constant speed, we determine the maximally possible droplet velocities under various conditions. The observed droplet speeds agree with the values from the feedback study for the same positive droplet offset.
液滴在具有时空润湿性模式的基底上移动，例如在光可切换表面上产生的润湿性模式。为了研究此类情况，我们采用边界元方法来求解液滴内部和表面流体流场的控制斯托克斯方程，并通过Cox-Voinov定律补充接触线动力学。我们的方法再现了实验中轴对称液滴的松弛过程，我们通过瞬间切换由平衡接触角量化的基底均匀润湿性来启动该过程。在润湿性的阶跃分布中，液滴向更高润湿性移动。使用反馈回路使阶跃与液滴中心之间的距离或偏移保持恒定，会诱导液滴以恒定速度在润湿性阶跃上滑动。我们针对典型的润湿参数，根据液滴偏移和阶跃宽度分析速度。改为以恒定速度移动润湿性阶跃，我们确定了各种条件下液滴的最大可能速度。观察到的液滴速度与相同正液滴偏移下反馈研究的值一致。
