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

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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).