Microscale Magneto-Elastic Composite Swimmers at the Air-Water and Water-Solid Interfaces Under a Uniaxial Field

Abstract

Self-propulsion of magneto-elastic composite microswimmers is demonstrated under a uniaxial field at both the air-water and the water-substrate interfaces. The microswimmers are made of elastically linked magnetically hard Co-Ni-P and soft Co ferromagnets, fabricated using standard photolithography and electrodeposition. Swimming speed and direction are dependent on the field frequency and amplitude, reaching a maximum of 95.1 µm/s on the substrate surface. Fastest motion occurs at low frequencies via a spinning (air-water interface) or tumbling (water-substrate interface) mode that induces transient inertial motion. Higher frequencies result in low Reynolds number propagation at both interfaces via a rocking mode. Therefore, the same microswimmer can be operated as either a high or a low Reynolds number swimmer. Swimmer pairs agglomerate to form a faster superstructure that propels via spinning and rocking modes analogous to those seen in isolated swimmers. Microswimmer propulsion is driven by a combination of dipolar interactions between the Co and Co-Ni-P magnets and rotational torque due to the applied field, combined with elastic deformation and hydrodynamic interactions between different parts of the swimmer, in agreement with previous models.

Microfluidic devices powered by integrated elasto-magnetic pumps

“…It is shown to operate on a fully enclosed bulk fluid rather than just surface flows as is shown previously for a number of elasto-magnetic devices. [19][20][21][22]46 The pumping mechanism does not alter the fluid in any way through chemical interactions such as found in studies exploring Janus particles or diodes. 24,[31][32][33] This system does not rely on the maintenance of phase separated fluids, as found in some acoustic actuation methods 29 which may be adversely effected by long term storage or transport.…”

“…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.…”

“…12 Generating net motion in the Stokes regime has been a lively area of study over the last few decades, underpinned by the work of Purcell 13 and Taylor, 14 and has grown into a diverse field with a multitude of swimming and pumping mechanisms being developed both theoretically and experimentally. Research in this field has explored a vast array of driving mechanisms, including magnetic torque, [15][16][17][18][19][20][21][22][23] electric fields, 24,25 light, 26,27 acoustic waves [28][29][30] and chemical energy. [31][32][33] Many of these driving mechanisms have been studied in the context of microscopic pumping solutions.…”

“…[31][32][33] Many of these driving mechanisms have been studied in the context of microscopic pumping solutions. 19,20,24,[34][35][36][37][38][39][40][41][42][43][44][45][46] While there is no formal way of comparing these systems directly, it is clear that many of these integrated pumping designs do not offer optimal solutions for use in a broad range of low-cost, portable POCT devices. For example, a device utilising capillary flow 45 can be fabricated and operated easily, although cannot produce tuneable or reversible fluid flow and is not appropriate for prolonged or repeated use.…”

Locomotion of magnetoelastic membranes in viscous fluids

“…Magnetically controlled microrobots have applications in drug delivery [1][2][3][4], sensing [5,6], micromixing [7], detoxification [8,9] and microsurgery [10,11]. Such versatile use of magnetic microrobots is possible because magnetic fields can penetrate organic matter, do not interfere with biological or chemical functions, do not require fuel, and, most importantly, can be externally con-…”