Magneto-responsive surfaces for controlling dynamics of wetting and drop impact (SANDPIT)

磁响应表面（MRSs）为非磁性液体的远程、无损和实时（动态）可控操作提供了一种新的方法，近年来引起了研究人员的广泛关注。未来的技术应用范围包括基于液滴的微流体、微反应器、液体分布器、雾收集等。到目前为止，MRSs的动态润湿仍然不在研究范围之内。该项目旨在探索基于软磁活性弹性体（MAEs）的MRSs的潜力，以了解和调整其润湿特性和飞溅。MAEs是一种复合材料，由微米和纳米大小的铁磁颗粒嵌入到柔性聚合物（在我们的案例中是聚二甲基硅氧烷）基质中。我们计划i)研究基于磁场的非结构化MAE表面润湿和滴溅调节ii)研究基于磁场的微结构MAE表面润湿和滴溅调节。i)垂直于MAE薄膜表面施加几百mT的磁场，导致动态剪切模量增加（开关）几个数量级，表面粗糙度显著增加。此外，MAE的磁场诱导塑性效应允许人们在微米尺度上追溯平滑甚至调节MAE表面的粗糙度。这使得MAEs有希望成为单独阐明表面形貌和机械性能对润湿性能切换的影响的候选者。为此，我们将研究具有不同材料成分和初始表面粗糙度的非结构化MAE表面，从而导致材料刚度和磁场响应率的变化。ii)我们将研究磁场诱导表面形貌开关对具有层状和柱状微结构的MAE表面织构的影响。与表面结构的突出方向不平行的磁场会导致表面结构弯曲，从而极大地改变表面形貌。本文还将探讨毛细力对高纵横比柔性微结构挠度的影响。该方法的特点是使用了软MAEs的倾斜微观结构，这可能导致液滴扩散的不对称性。最后，我们将研究跌落对MAE表面的冲击，揭示微观和宏观尺度过程之间的关系，与快速接触线运动有关，以及跌落冲击的结果（沉积/弹跳/飞溅）。

Magneto-responsive surfaces (MRSs) have recently attracted attention of researchers because they provide a remote, nondestructive and real-time (dynamic) approach for controllable manipulation of nonmagnetic liquids. Prospective technological applications range from droplet-based microfluidics, microreactors, and liquid distributors, to fog harvesting etc. Hitherto, the dynamic wetting of MRSs remained out of the scope of research. The project aims to explore the potential of MRSs, based on soft magneto-active elastomers (MAEs) to understand and tune their wetting properties and drop splashing. MAEs are composite materials comprising micro- and nanometer-sized ferromagnetic particles embedded into a compliant polymer (in our case polydimethylsiloxane) matrix. We plan i) to investigate magnetic field-based regulation of wetting and drop splashing on non-structured MAE surfaces and ii) to investigate magnetic field-based regulation of wetting and drop splashing on microstructured MAE surfaces. i) Application of a magnetic field of a few hundred mT, perpendicular to MAE film surfaces, leads to the increase (switch) in the dynamic shear modulus over a few orders of magnitude and to the significant increase in the surface roughness. Furthermore, the magnetic-field-induced plasticity effect of MAEs allows one to retroactively smoothen or even modulate the roughness of the MAE surface on the micrometer scale. This makes MAEs promising candidates for separate elucidation of the effects of surface topography and the mechanical properties on switching of wetting properties. For this purpose, we will study unstructured MAE surfaces with different material composition and initial surface roughness, leading to the variable material stiffness and responsivity to a magnetic field. ii) We will study effects of magnetic-field-induced switching of surface topography on MAE surfaces textured with lamellar and pillared microstructures. A magnetic field which is not parallel to the protrusion direction of surface structures leads to their bending and therefore, drastically changes the surface topography. The effects of deflection of flexible microstructures with a high aspect ratio due to capillary forces will be also explored. The special feature of our approach is the usage of tilted microstructures from soft MAEs that may lead to asymmetry of drop spreading. Finally, we will study drop impact on MAE surfaces to unveil the relationships between micro- and macro-scale processes, related to fast contact line motion, and the outcome of drop impact (deposition / bouncing / splash).