Impingement-assisted self-assembly of ferrofluid droplets under magnetic field

The unique liquid nature and strong magnetic response ability enable ferrofluid droplets to split and self-assemble into reconfigurable three-dimensional structures. Although the manipulation of a ferrofluid droplet in a static or quasi-static state has been well studied, the splitting and self-assembly of ferrofluids in a dynamic state remain unexplored. Here, we report the impingement-assisted self-assembly of ferrofluids on superhydrophobic surfaces, whereby the required magnetic field is greatly reduced. In particular, the coupling between the magnetic field strength and Weber number endows a precise regulation of the ferrofluid post-impact dynamics, which facilitates the optimization of the self-assembly behavior. Finally, we demonstrate that the impingement-assisted self-assembly of ferrofluids can be generalized on various surfaces with different wettability. The fundamental understanding and the ability to self-assembly during droplet impingement can provide important insight into the dynamics of ferrofluid droplets as well as the applicability in multifunctional analytical and fluidic devices.