Developing adaptive materials with geometries that change in response to external stimuli provides fundamental insights into the links between the physical forces involved and the resultant morphologies and creates a foundation for technologically relevant dynamic systems1,2. In particular, reconfigurable surface topography as a means to control interfacial properties3 has recently been explored using responsive gels4, shape-memory polymers5, liquid crystals6,7,8 and hybrid composites9,10,11,12,13,14, including magnetically active slippery surfaces12,13,14. However, these designs exhibit a limited range of topographical changes and thus a restricted scope of function. Here we introduce a hierarchical magneto-responsive composite surface, made by infiltrating a ferrofluid into a microstructured matrix (termed ferrofluid-containing liquid-infused porous surfaces, or FLIPS). We demonstrate various topographical reconfigurations at multiple length scales and a broad range of associated emergent behaviours. An applied magnetic-field gradient induces the movement of magnetic nanoparticles suspended in the ferrofluid, which leads to microscale flow of the ferrofluid first above and then within the microstructured surface. This redistribution changes the initially smooth surface of the ferrofluid (which is immobilized by the porous matrix through capillary forces) into various multiscale hierarchical topographies shaped by the size, arrangement and orientation of the confining microstructures in the magnetic field. We analyse the spatial and temporal dynamics of these reconfigurations theoretically and experimentally as a function of the balance between capillary and magnetic pressures15,16,17,18,19 and of the geometric anisotropy of the FLIPS system. Several interesting functions at three different length scales are demonstrated: self-assembly of colloidal particles at the micrometre scale; regulated flow of liquid droplets at the millimetre scale; and switchable adhesion and friction, liquid pumping and removal of biofilms at the centimetre scale. We envision that FLIPS could be used as part of integrated control systems for the manipulation and transport of matter, thermal management, microfluidics and fouling-release materials.
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This work is supported by DOE under award number DE-SC0005247 (experiment) and by the NSF under award number DMREF-1533985 (theory). This work was performed in part at the Center for Nanoscale Systems at Harvard University (CNS), which is supported by the NSF under award number ECS-0335765. We thank J. C. Weaver for assistance with 3D printing, M. Khan for assistance with preparing micro-textured silicon master, P. Kim and J. Alvarenga for assistance with force measurements, K. Davey at American Electromechanics for assistance with designing the electromagnet, and N. Vogel, I. Morrison, Y. Hu, P. Kim, D. Daniel, M. Kreder and O. Ahanotu for discussions. W.W. thanks the Humboldt foundation for a fellowship. J.V.I.T. was supported by the European Commission through the Seventh Framework Programme (FP7) project DynaSLIPS (project number 626954). D.-M.D., M.S. and W.W. are supported by the Max Planck Society.