@article{StraubePagesTiernoetal., author = {Straube, Arthur and Pag{\`e}s, Josep M. and Tierno, Pietro and Ign{\´e}s-Mullol, Jordi and Sagu{\´e}s, Francesc}, title = {Collective dynamics and conformal ordering in electrophoretically driven nematic colloids}, series = {Phys. Rev. Research}, volume = {1}, journal = {Phys. Rev. Research}, doi = {10.1103/PhysRevResearch.1.022008}, pages = {022008}, abstract = {We present a theoretical framework to understand the collective dynamics of an ensemble of electrophoretically driven colloidal particles that are forced to assemble around a single topological defect in a nematic liquid crystal by an alternating current electric field. Our generic model combines phoretic propulsion with electrostatic interactions and liquid-crystal-mediated hydrodynamics, which are effectively cast into a long-range interparticle repulsion, while nematic elasticity plays a subdominant role. Simulations based on this model fully capture the collective organization process observed in the experiments and other striking effects as the emergence of conformal ordering and a nearly frequency-independent repulsive interaction above 10Hz. Our results demonstrate the importance of hydrodynamic interactions on the assembly of driven microscale matter in anisotropic media.}, language = {en} } @article{StoopStraubeJohansenetal., author = {Stoop, Ralph L. and Straube, Arthur and Johansen, Tom H. and Tierno, Pietro}, title = {Collective directional locking of colloidal monolayers on a periodic substrate}, series = {Phys. Rev. Lett.}, volume = {124}, journal = {Phys. Rev. Lett.}, doi = {10.1103/PhysRevLett.124.058002}, pages = {058002}, abstract = {We investigate the directional locking effects that arise when a monolayer of paramagnetic colloidal particles is driven across a triangular lattice of magnetic bubbles. We use an external rotating magnetic field to generate a two-dimensional traveling wave ratchet forcing the transport of particles along a direction that intersects two crystallographic axes of the lattice. We find that, while single particles show no preferred direction, collective effects induce transversal current and directional locking at high density via a spontaneous symmetry breaking. The colloidal current may be polarized via an additional bias field that makes one transport direction energetically preferred.}, language = {en} } @article{TiernoJohansenStraube, author = {Tierno, Pietro and Johansen, Tom H. and Straube, Arthur}, title = {Thermally active nanoparticle clusters enslaved by engineered domain wall traps}, series = {Nature Commun.}, volume = {12}, journal = {Nature Commun.}, doi = {10.1038/s41467-021-25931-7}, pages = {5813}, abstract = {The stable assembly of fluctuating nanoparticle clusters on a surface represents a technological challenge of widespread interest for both fundamental and applied research. Here we demonstrate a technique to stably confine in two dimensions clusters of interacting nanoparticles via size-tunable, virtual magnetic traps. We use cylindrical Bloch walls arranged to form a triangular lattice of ferromagnetic domains within an epitaxially grown ferrite garnet film. At each domain, the magnetic stray field generates an effective harmonic potential with a field tunable stifness. The experiments are combined with theory to show that the magnetic confinement is effectively harmonic and pairwise interactions are of dipolar nature, leading to central, strictly repulsive forces. For clusters of magnetic nanoparticles, the stationary collective states arise from the competition between repulsion, confinement and the tendency to fill the central potential well. Using a numerical simulation model as a quantitative map between the experiment and theory we explore the field-induced crystallization process for larger clusters and unveil the existence of three different dynamical regimes. The present method provides a model platform for investigations of the collective phenomena emerging when strongly confined nanoparticle clusters are forced to move in an idealized, harmonic-like potential.}, language = {en} } @article{CerecedaLopezOstinatoOrtizAmbrizetal., author = {Cereceda-L{\´o}pez, Eric and Ostinato, Mattia and Ortiz-Ambriz, Antonio and Straube, Arthur and Palassini, Matteo and Tierno, Pietro}, title = {Excluded volume induces buckling in optically driven colloidal rings}, series = {Phys. Rev. Research}, volume = {6}, journal = {Phys. Rev. Research}, doi = {10.1103/PhysRevResearch.6.L012044}, pages = {L012044}, abstract = {In our combined experimental, theoretical and numerical work, we study the out of equilibrium deformations in a shrinking ring of optically trapped, interacting colloidal particles. Steerable optical tweezers are used to confine dielectric microparticles along a circle of discrete harmonic potential wells, and to reduce the ring radius at a controlled quench speed. We show that excluded-volume interactions are enough to induce particle sliding from their equilibrium positions and nonequilibrium zigzag roughening of the colloidal structure. Our work unveils the underlying mechanism of interfacial deformation in radially driven microscopic discrete rings.}, language = {en} }