Collective dynamics of driven colloids on ordered and disordered magnetic landscapes*

In this talk I will discuss recent results obtained in my group by using paramagnetic colloidal particles driven across two dimensional periodic and random magnetic landscapes. These

landscapes are generated by thin ferromagnetic films that provide an array of cylindrical ferromagnetic domains, also named “magnetic bubbles”. An external rotating magnetic field can modulate the periodic energy landscape and induce a directed particle transport via a travellingwave like mechanism [1,2].

In the first part of the talk, I will show that when the particles are forced to cross a direction that intersect two crystallographic axes of the lattice, collective effects induce transversal current and directional locking at high density via a spontaneous symmetry breaking [3,4]. In the second part of the talk, I will explore the case of disordered systems, and the corresponding current density relationship. I will explain two novel effects. The first one originates from particle sizes nearly commensurate with the substrate in combination with attractive pair interactions that we explain by an exactly solvable model of constrained cluster dynamics. It governs the colloidal flow at small densities and leads to a superlinear current increase. The second effect is a defect-induced breakup of coherent cluster motion, leading to an effective jamming of particle flow at higher densities. Finally, I will show that a lattice gas model with parallel update is able to capture the experimental findings of this complex manybody system.

[1] P. Tierno, T. H. Johansen, T. M. Fischer Phys. Rev. Lett. 99, 038303 (2007).

[2] P. Tierno, T. M. Fischer, Phys. Rev. Lett. 112, 048302 (2014).

[3] C. Reichhardt, C. J. Olson Reichhardt, Phys. Rev. Lett. 100, 167002 (2008).

[4] R. L. Stoop, A. V. Straube, T. H. Johansen, P. Tierno, Phys. Rev. Lett. 124, 058002 (2020).