Shape-Evolving Structured Liquids via Ferromagnetic Active Particles

The motion of active matter is the basic form of locomotion in biology, a vital ingredient in many functions of cells, and an essential design challenge in nanorobotics. Here, we integrated active matter into structured liquids to harness its motions to perform work on liquid interfaces. The structured liquids, produced by interfacial jamming of nanoparticle-surfactants (NPSs), are reconfigurable and therefore provide an ideal platform for generating active energy-consuming systems. The liquid shape will evolve and respond to external stimuli when the interfacial tension is sufficiently low, i.e., >0.05 mN/m in this study. In this study, we  employed  ferromagnetic  active  swimmers  with  relatively large  momentum.  Nickel  particles  with  average  diameters ranging   from ~10 to ~60  microns  were  subjected  to  an  external  AC  magnetic  field  to  energize  rolling  motions  on  a  solid  substrate.  When  they  are  encapsulated  in  a  droplet  of  structured  liquids,  the  collisions  of  particles  resulted  in  directional  shape  changes  and  translational  motions  of  the  droplet.  This strategy would provide a route to a new class of biomimetic, reconfigurable, and responsive materials, delivering mechanical responses unlike those of conventional materials.