Self-assembly of magnetic Janus colloids with radially shifted dipoles under an external magnetic field.

Abstract

Magnetic Janus particles (MJPs) with radially shifted dipoles exhibit a versatile platform for engineering responsive materials through field-directed self-assembly. Motivated by their potential in programmable soft matter, Brownian dynamics simulations are used to systematically investigate how the radial dipolar displacement <i>s</i> and the Langevin parameter <i>α</i> govern the aggregation pathways and emergent morphologies of MJPs in quasi-two-dimensional environments. We identified six distinct aggregation regimes: three arising under low magnetic fields (<i>α</i> ≲ 10) corresponding to the low-, intermediate-, and high-shift cases, and two emerging at intermediate (10 ≲ <i>α</i> ≲ 90) and high magnetic fields (<i>α</i> ≳ 90). These regimes exhibit a rich morphological evolution as <i>α</i> increases: from disordered loops (low <i>α</i>, low <i>s</i>), islands (low <i>α</i>, intermediate <i>s</i>), and worm-like clusters (low <i>α</i>, high <i>s</i>), transitioning through chiral and tangled chains (intermediate <i>α</i>, intermediate and high <i>s</i>), and culminating in fully aligned chains (intermediate <i>α</i> with low <i>s</i>, and high <i>α</i> for all <i>s</i>). A structure diagram predicted by considering a simple ratio of competing torques (<i>R</i>_Mag) effectively illustrates these transitions and specifies the conditions necessary for structural reorganization. This framework supports the rational design of adaptive colloidal architectures for applications in targeted delivery, soft microrobotics, and reconfigurable magnetic systems. Notably, the universal convergence to a growth exponent of <i>z</i> ≈ 0.473 under high magnetic fields (<i>α</i> ≳ 90) reveals a definitive kinetic signature of complete cluster alignment along the field direction, establishing a robust and tunable route to field-induced material organization.