Controllable magnetic systems
This invention relates to methods and devices for propulsion through a fluid, in particular at low Reynolds number. We describe a method of propelling one of a magnetic device and a fluid relative to the other, the magnetic device comprising a pair of magnetic moments linked by an elastic coupling element, one of said moments having a greater resistance to a change in orientation due to an external applied magnetic field than the other, the method comprising applying an elliptical or ellipsoidal rotating magnetic field to the device to cause a change in mutual attraction or repulsion between said magnetic moments to thereby change a physical configuration of said device, propelling said device relative to said fluid.
1. A magnetic swimming device for swimming in a fluid, the device comprising:
a first magnetic particle to provide a first magnetic dipole;
a second magnetic particle to provide a second magnetic dipole; and
an elastic coupling mechanically coupled between said first and second magnetic particles;
wherein along an axis joining said first and second magnetic particles said first magnetic particle has a greater magnetic anisotropy than said second magnetic particle; and
wherein said first magnetic particle is configured to provide one or both of a greater magnetic moment and a greater viscous resistance to said fluid than said second magnetic particle;
whereby in a rotating magnetic field said device swims at least in part by changing its orientation and by changing a distance between said first and second magnetic particles.
2. A magnetic swimming device as claimed in claim 1 wherein said first magnetic particle has a greater size in at least one dimension than a size of said second magnetic particle in a corresponding said dimension.
3. A magnetic swimming device as claimed in claim 1 wherein said first and second magnetic particles comprise different respective first and second magnetic materials, and wherein said first magnetic material has a greater intrinsic coercivity than said second magnetic material.
4. A magnetic swimming device as claimed in claim 1 wherein said elastic coupling has a greater bending modulus than Young's modulus of elasticity of said elastic coupling along said axis.
5. A magnetic swimming device as claimed in claim 1 wherein a separation between said first and second magnetic particles is greater than a maximum dimension of at least one of said magnetic particles.
6. A magnetic swimming device as claimed in claim 1 wherein said elastic coupling extends in a direction perpendicular to said axis.
7. A magnetic swimming device as claimed in claim 6 wherein said elastic coupling has the shape of a spheroid or loop.
8. A magnetic swimming device as claimed in claim 1 wherein said elastic coupling comprises an organic material or a biological entity.
9. A magnetic swimming device as claimed in claim 1 carrying a transported object, and wherein said elastic coupling at least in part comprises said transported object.
10. A magnetic swimming device as claimed in claim 1 configured as a pump for pumping a fluid, the pump comprising a fluid conduit and a magnetic swimming device as claimed in claim 1 attached in said conduit such that as said device swims said fluid is transported through said conduit.