Speeding the directed self-assembly under toggled magnetic fields

Toggled field self-assembly is a useful way to circumvent kinetic arrest of the induced structures in field-directed self-assembly and, in particular, in magnetic suspensions such as magnetorheological (MR) fluids. During the field-off period, Brownian motion permits rearrangement of the particles and classical system-spanning chains (typically formed under uniaxial DC fields) collapse into dense ellipsoidal aggregates that are more energetically favorable. Here we show, through the use of experiments and particle-level simulations, that the process is very sensitive to the field configuration (e.g., frequency and strength), and, in particular, it can be dramatically accelerated when the magnetic field strength is increased. Moreover, when the field strength reaches sufficiently high values, the classical two-step aggregation process becomes a single one and morphological differences in the final structures emerge.