Self-assembly is ubiquitous at all scales in nature. Most studies have focused on the self-assembly of
micron-scale and nano-scale components. In this study, we explore the self-assembly of millimeterscale
magnetic particles in a bubble-column reactor to form 9 different structures. Two component
systems (N–N and S–S particles) assemble faster than one-component systems (all particles have N–S
poles) because they have more numerous bonding pathways. In addition, two-components add control
to process initiation and evolution, and enable the formation of complex structures such as squares,
tetrahedra and cubes. Self-assembly is collision-limited, thus, the formation time increases with the total
number of bonds required to form the structure and the injected power. The dimensionless Mason
number captures the interplay between hydrodynamic forces and magnetic interactions: self-assembly
is most efficient at intermediate Mason numbers (the system is quasi-static at low Mason numbers with
limited chances for particle interaction; on the other hand, hydrodynamic forces prevail over dipole–
dipole interactions and hinder bonding at high Mason numbers). Two strategies to improve yield involve
(1) the inclusion of pre-assembled nucleation templates to prevent the formation of incorrect initial
structures that lead to kinetic traps, and (2) the presence of boundaries to geometrically filter unwanted
configurations and to overcome kinetic traps through particle–wall collisions. Yield maximization
involves system operation at an optimal Mason number, the inclusion of nucleation templates and the
use of engineered boundaries (size and shape).
