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DYNAMO Report Summary

Project ID: 335040
Funded under: FP7-IDEAS-ERC
Country: Spain

Mid-Term Report Summary - DYNAMO (Dynamics and assemblies of colloidal particlesunder Magnetic and Optical forces)

The ERC starting grant "DynaMO" powered a series of experimental research lines based on the use of magnetic and optical forces and targeting transport and organization of colloidal matter under near and out of equilibrium conditions. In these 2.5 years, the group focused on two separate lines, based on investigating the collective transport of colloids trough periodic potentials, and the static and dynamic assembly of anisotropic particles. In the first case, the group investigated in details the assembly and transport of paramagnetic colloids driven above ferrite garnet films (FGFs) when subjected to external magnetic field modulations. The FGFs are a thin transparent film growth by dipping liquid phase epithaxy, and characterized by ferromagnetic domains which can be assembled into lattices of parallel stripes or magnetic bubbles. For a striped pattern, the group discovered a new scenario of first-order phase transition that occurred via a complete inversion of the system energy landscape. This phenomenon was termed the “landscape inversion phase transition” (LIPT) and was observed by applying an external magnetic field to an assembly of paramagnetic colloids confined above the stripe pattern. The external field induced a solid to solid transition, by forcing the magnetic moments of the particles to transit from an anti ferromagnetic ordering to a ferromagnetic one. Further results showed a whole range of new phenomena related with the phase transition kinetics, the emergence of propagating fronts and the possibility to control the speed via an applied field.
For a FGF characterized by a magnetic bubble lattice, it was discovered a method to transport an ensemble of interacting particles under a rotating magnetic field. The measured colloidal current raised in integer and fractional steps with the field amplitude, and the stepwise increase was caused by excluded volume interactions between the particles, which formed composite clusters above the bubbles with mobile and immobile occupation sites. Furthermore, it was found that the colloidal current could be polarized to make selective use of type up or type down interstitials in the magnetic bubble lattice. Another development with the magnetic bubble lattice was the realization of a frustrated colloidal molecular crystal, when filling a honeycomb lattice of triangular shaped magnetic minima with exactly two particles per pinning site. It was proposed a new annealing method to obtain long range ordered stripes and random fully packed loops phases.
In relation to research line (ii), the group investigated the magnetic assembly of simple and complex anisotropic colloids. It was discovered a general method to assemble and propel highly maneuverable colloidal carpets which could be steered via remote control in any direction of the plane. These colloidal micropropellers were composed by an ensemble of spinning rotors and were used to entrap, transport, and release biological cargos on command via a hydrodynamic conveyor-belt effect. Also the propulsion behavior of microscopic colloidal rotors dynamically assembled into elongated chains was investigated in details.
Besides these experimental achievements, during the first two years of the project the group worked on the realization of two independent experimental platforms capable to combine magnetic and optical forces in order to arrange and manipulate colloidal matter. With one of these set-up it was realized a colloidal version of an artificial spin ice system. Artificial spin ices are lattices of interacting ferromagnetic nano-islands, and have been used to date as microscopic models of frustration induced by lattice topology. However, the realization of frustrated ice states in which individual spins can be manipulated in situ and their collective dynamics monitored in real time remain both challenging tasks. The colloidal spin ice was based on interacting paramagnetic colloids arranged into a lattice of bi-stable gravitational traps. Optical tweezers were used to locate the particle at a defined filling in the gravitational wells, while external magnetic fields were used to tune the pair interactions between the particles. With this experimental realization it was possible to generate frustrated states where the individual units could be manipulated in real time. Moreover, given the experimentally accessible time and length scale in colloidal systems, it was possible to record the particle motion, and thus get direct information on the system relaxation dynamics. By combining the magnetic and optical forces as independent arms, it was possible to introduce in the colloidal spin ice system monopole-like defects and Dirac strings and use loops with defined chirality as elementary units to store binary information.

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