Final 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 5 years we established a young and dynamic research group centered on these issues. The group focused on two separate research lines: (i) collective transport of colloids trough periodic or random potentials, (ii) dynamics and assembly of complex magnetic particles. In the first case, the group investigated in detail the assembly and transport of paramagnetic colloids 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 occurs via a complete inversion of the system energy landscape, that was termed the “landscape inversion phase transition”. With this system, the group also found a mixed order phase transition and non-standard kinetics. For a magnetic bubble lattice, it was discovered a method to transport at constant speed an ensemble of interacting particles under a rotating magnetic field. The 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, a frustrated colloidal molecular was realized when filling these potential minima with exactly two particles per pinning site. Thus, it was proposed a new annealing method to obtain long range ordered stripes and random fully packed loops. Finally, the microrheological properties of optically confined colloidal crystals perturbed by magnetic probes were also investigated in detail. The group realized a microscopic version of a two-dimensional (2D) Taylor-Couette geometry by combining magnetic manipulation and optical trapping. By varying the two independent fields, the system flow diagram was fully determined with the corresponding microrheological properties and velocity profiles along the colloidal structure. The demonstrated approach gives a unique microscopic view on how the structure of strongly confined colloidal matter weakens or strengthens upon shear, envisioning the engineering of similar rheological system at the smallest scale.Regarding research line (ii), the group investigated the assembly and dynamics of complex magnetic colloids including engineered hematite anisotropic particles that could be activated by light and steered via magnetic fields. Using blue light, we demonstrate the rapid formation of two-dimensional colloidal clusters and gels via nonequilibrium diffusiophoresis. Microscopic hematite particles were used to form long-living interstitial bonds that strongly glue passive silica microspheres. By varying the relative fraction of doping, we uncover a rich phase diagram including ordered and disordered clusters, space-filling gels, and bicontinuous structures formed by filamentary dockers percolating through a solid network of silica sphere. Isotropic paramagnetic colloids were also used 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. The mechanism of the motility of these carpets was found cooperative and based on the rectification of the hydrodynamic flow generated by each rotor close to a bounding wall. Moreover, tuning the amplitude of the external field allows to produce a novel self-healing mechanism based on the controlled colloidal flow above these magnetic membranes that enables complete crystallization after a few seconds of propulsion. This out-of-equilibrium colloidal model system allows to investigate crystallization in transported systems and could thus provide deep insight for similar processes occurring in systems at different length and time scales.Apart from these achievements, the group developed two independent experimental platforms capable to combine magnetic and optical forces in order to arrange and manipulate colloidal matter. With these setups the group developed a colloidal version of an artificial spin ice system. 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, 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. We further show how to realize a completely resettable “NOR” gate, which provides guidelines for fabrication of nanoscale logic devices based on the motion of topological magnetic monopoles.In terms of output, the ERC group give rise to a total of 41 publications in international journals, including 2 PNAS, 4 Nature Comm, 5 Phys. Rev. Lett., 2 Nano Letters and 1 Science Advances. The group (PI + team members) participated to more than 30 conferences worldwide and start/strengthen collaborations with leading scientists in the field of Soft Condensed Matter Physics.
