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 crystals6–8
and hybrid composites9–14, including magnetically
active slippery surfaces12–14. However, these designs exhibit a
limited range of topographical changes and thus a restricted
scope of function. Here we introduce a hierarchical magnetoresponsive 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 magneticfield 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 pressures15–19 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.