Title: Formation of an advection lattice with biaxial magnetic fields: A powerful new method of heat and mass transfer Speaker: James E. Martin, Sandia National Laboratories Date/Time: Tuesday, January 26, 2010, 1:00 – 2:00pm Location: CSRI Building, Room 90 (Sandia NM) Brief Abstract: In recent years we have become interested in the dynamics of magnetic particle suspensions subjected to biaxial and triaxial ac magnetic fields. For suspensions of spherical particles we have convinced ourselves (through theory, simulation and experiment) that we understand what is going on, including some pretty weird stuff. We have now started to study suspensions of magnetic platelets and have observed, under some very particular field conditions, the formation of extraordinary flow patterns and surface instabilities. We have no idea of what is going on, but we have made a lot of interesting videos that are fun to puzzle over. Biaxial and triaxial ac magnetic fields are spatially uniform, time-dependent fields whose direction explores either two or three dimensions, with typical field component frequencies in the range of 102 - 103 Hz and magnitudes from 0.005 - 0.05 T. In spherical particles suspensions a biaxial field creates negative dipolar interactions that lead to particle sheets, which are static structures of little interest. All of the really interesting phenomena – fluid mixing [1], molecular-like particle clusterings [2], the formation of composites with exotic structures [3] – occur in triaxial fields, which create complex many-body interactions. We have discovered that magnetic platelets behave very differently, probably because of their tendency to rotate so as to confine the field vector to their plane. The resultant particle fluttering apparently creates complex flow because of hydrodynamic coupling. In the simplest case the flow can be described as a diagonal square lattice (relative to the directions of the field components) of “antiferromagnetic” flow cells, with the flow in each cell normal to the field plane. More complex flows can be stimulated, including helical flows, and these depend critically on both the frequency ratios of the field components and their phase relation. Understanding these flows is probably going to be a computational challenge of the first order, but their utility is clear: we can magnetically transport heat and mass at a terrific rate without a thermal gradient or gravity, and without pumps, hoses, connectors, seals or any moving parts. This has significant implications for cooling in space, or in any other circumstance where convection does not occur. CSRI POC: Louis Romero, (505) 845-7512 |