Controlling collective rotational patterns of magnetic rotors

Abstract

Magnetic actuation is widely used in engineering specific forms of controlled motion in microfluidic applications. A challenge, however, is how to extract different desired responses from different components in the system using the same external magnetic drive. Using experiments, simulations, and theoretical arguments, we present emergent rotational patterns in an array of identical magnetic rotors under an uniform, oscillating magnetic field. By changing the relative strength of the external field strength versus the dipolar interactions between the rotors, different collective modes are selected by the rotors. When the dipole interaction is dominant the rotors swing upwards or downwards in alternating stripes, reflecting the spin-ice symmetry of the static configuration. For larger spacings, when the external field dominates over the dipolar interactions, the rotors undergo full rotations, with different quarters of the array turning in different directions. Our work sheds light on how collective behaviour can be engineered in magnetic systems.

Microfluidic devices powered by integrated elasto-magnetic pumps

“…In this way, the pump operates in the bulk, distinct from similar previous studies on elasto-magnetic pumps investigating surface flows produced in open channels. [19][20][21][22]62 The inlet and outlet channels are 1000 μm in width, and 900 μm in depth. This pumping module is connected via polytetrafluoroethylene (PTFE) tubing, of length 4 cm and internal diameter 860 μm, to a separate reservoir module.…”

Degenerate states, emergent dynamics and fluid mixing by magnetic rotors

“…2(d)) patterns. Note that we introduce a parameter R to characterize the rotational pattern, 9 defined as.…”

“…Magnetic forces provide a useful way of driving small units in viscous fluids. For example, in recent years magnetic driving has been extensively used in microfluidic devices, for applications such as magnetic swimmers, [1][2][3][4] magnetic spinners, [5][6][7] magnetic pumps 8,9 and cilia, [10][11][12][13][14] and to facilitate particle sorting and segregation. [15][16][17][18] Many of the approaches are designed to control a single magnetic unit or to actuate many units in exactly the same manner.…”

“…In a recent work, 9 we analyzed the collective motion of a square array of magnetic rotors and found a surprisingly rich dynamics under an external magnetic field which oscillated along one axis of the array. By changing the relative strength of the external field and the dipolar interactions between the rotors, different collective rotational patterns emerged.…”

Field synchronized bidirectional current in confined driven colloids

“…In this context, a particularly appealing class of systems are driven magnetic particles, which present emergent collective dynamics [25,26] apart from their potential applications in technological fields such as microfluidics and lab-on-a-chip [27,28]. However, in most cases, the colloidal current is usually produced either via magnetic torque applied to anisotropic particles [29][30][31][32][33][34][35], hydrodynamic flows [36][37][38][39][40], or from the presence of external field gradients acting over isotropic spheres [41][42][43]. Forces resulting from gradients of external fields are usually more difficult to control over spatially extended surfaces or to be switched on/off quickly on command.…”