Dr. Alexey Snezhko

Argonne National Laboratory

Emergent Behavior, Self-Assembly and Novel Properties of Generated Phases in Magnetic Granular Ensembles

Thursday, March 27, 2008

4:00 p.m.

Cardwell 102

Large ensembles of interacting particles subject to an external forcing often exhibit unexpected nontrivial collective behaviors. A fundamental issue in the study of such systems is how collective ordering arises from the dynamics of discrete interacting components. Recently, self-assembly of materials into desired complex architectures has become one of the greatest prospects for nanoscience with the promise to enable a 'bottom up' approach that may supersede current complex nano-patterning technologies. Furthermore, novel self-organized phases often bring new sometimes unexpected physical properties.

Controlled pattern formation and novel collective dynamics in sub-mono-layers of magnetic microparticles subject to an external forcing by alternating electromagnetic fields will be discussed. Depending on the excitation parameters a rich variety of structures such as clusters, rings, chains and networks can be generated in the sub-monolayer of magnetic particles on the solid substrate.

Remarkable nontrivially ordered dynamic self-assembled structures ("magnetic snakes") are formed in the ensemble of magnetic microparticles suspended on the liquid/air interface and energized by an alternating magnetic field in a certain range of excitation parameters. These structures emerge as a result of the competition between magnetic and hydrodynamic forces. Strong induced vortex flows on the surface of the liquid finalize the rich hydrodynamic picture of the “magnetic snake”. Instability of the dynamic structures with respect to self-generated surface flows in the liquid will be demonstrated (self-assembled magnetic swimmers).

Self-assembled snakes have a complex magnetic ordering. The segments of the snake exhibit long-range antiferromagnetic ordering mediated by the surface waves, while each segment is composed of ferromagnetically aligned chains of microparticles.

The mechanism of the pattern formation and nontrivial magnetic properties of the generated dynamic self-organized patterns will be discussed.