Inspired by responsive liquid surfaces found everywhere from insect feet to lungs, a new concept in adaptive materials design emerged. A team combined an active liquid with a solid textured surface to create dynamic surface coatings. They can seamlessly resculpt the surface features of these hybrid materials into an endless variety of complex topographies. The team can sculpt both small (nanoscale) and large (macroscale) features. Triggered by a simple magnetic field, topographies form and reconfigure as the liquid flows above and within the textured surface. The complex surface geometries emerge from a rich interplay involving the fluid, solid, magnetic forces, and energy-conversion pathways.
The novel material provides outside-the-box strategies for controlling surface processes. Such materials have an array of useful properties. For example, the materials could direct the movement and assembly of tiny particles, regulate the flow of droplets, or turn adhesive properties on and off. Scientists can make one material with several properties. The impact? The materials can lead to breakthroughs in energy-efficient buildings, microelectronics, self-cleaning surfaces, and more.
Although polymer networks, liquid crystals, and composite materials can be programmed for a restricted number of structure-specific reconfigurations, a liquid surface is far more dynamic and versatile—but requires a robust way to control it over multiple time and spatial scales. Scientists have developed a strategy to manipulate the energy flow and intricate competition between forces emerging from the fluid’s interaction with the underlying microtexture and the response of suspended magnetic particles to an imposed magnetic field. The magnetic field prompts the microscopic flow of fluid, first above and then within the microstructure, and this redistribution changes the initially smooth fluid surface into a wide range of unique ordered topographies, with precise control over geometry, structural regularity, location, and timing. The hybrid system provides versatility and modularity in designing the dynamic surfaces and responses, and the specific topographical patterns can be finely tuned by controlling the strength, pattern, and orientation of the magnetic field, the responsive properties of the fluid, and the geometry of the substrate.
The resulting surface responses not only display intriguing spatial and time-based fluid dynamics and energy conversion properties in themselves, but also introduce a wide range of interesting phenomena and novel functions when interfaced with other solids and liquids. The demonstrated applications—new forms of reversible particle self-assembly, manipulation, and transport of non-magnetic matter in a magnetic field; precisely timed chemical reactions through precise mixing of clusters; and rewritable, spatial addressing of directional adhesion, friction, and biofilm removal—are only a small representative sample of the possibilities this new concept opens up to the imagination.
The Department of Energy, Office of Science, Basic Energy Sciences (experiments) and the National Science Foundation (theory) supported the work. Additional support was provided a Humboldt foundation fellowship (W.W.); European Commission through the Seventh Framework Programme (J.V.I.T.); and the Max Planck Society (D.M.D., M.S. and W.W.).
W. Wang, J.V.I. Timonen, A. Carlson, D.M. Drotlef, C.T. Zhang, S. Kolle, A. Grinthal, T.S. Wong, B. Hatton, S.H. Kang, S. Kennedy, J. Chi, R.T. Blough, M. Sitti, L. Mahadevan, and J. Aizenberg, “Multifunctional ferrofluid-infused surfaces with reconfigurable multiscale topography.” Nature 559, 77 (2018). [DOI: 10.1038/s41586-018-0250-8]
Harvard University press release: A multifunctional, multiscale, reconfigurable surface
Materials Today article: Magnetic fluid helps create highly versatile dynamic surface
C&E News article: Magnets tune the friction of a designed surface
Chemistry World article: Dynamic surfaces can be switched by magnetic fields