Pietro Tierno

Magnetically Actuated Colloidal Microswimmers

To achieve permanent propulsion of micro-objects in confined fluids is an elusive but challenging goal that will foster future development of microfluidics and biotechnology. Recent attempts based on a wide variety of strategies are still far from being able to design simple, versatile, and fully controllable swimming engines on the microscale. Here we show that DNA-linked anisotropic colloidal rotors, composed of paramagnetic colloidal particles with different or similar size, achieve controlled propulsion when subjected to a magnetic field precessing around an axis parallel to the plane of motion. During cycling motion, stronger viscous friction at the bounding plate, as compared to fluid resistance in the bulk, creates an asymmetry in dissipation that rectifies rotation into a net translation of the suspended objects. The potentiality of the method, applicable to any externally rotated micro/nano-object, is finally demonstrated in a microfluidic platform by guiding the colloidal rotors through microscopic-size channels connected in a simple geometry.

Propulsion of flexible polymer structures in a rotating magnetic field

We demonstrate a new concept for the propulsions of abiological structures at low Reynolds numbers. The approach is based on the design of flexible, planar polymer structures with a permanent magnetic moment. In the presence of an external, uniform, rotating magnetic field these structures deform into three-dimensional shapes that have helical symmetry and translate linearly through fluids at Re between 10(-1) and 10. The mechanism for the motility of these structures involves reversible deformation that breaks their planar symmetry and generates propulsion. These elastic propellers resemble microorganisms that use rotational mechanisms based on flagella and cilia for their motility in fluids at low Re.

Magnetically driven Janus micro-ellipsoids realized via asymmetric gathering of the magnetic charge.

Spherical microspheres are widely employed for the most disparate purposes, such us to assemble three-dimensional structures, [ 1 ] to realize photonic band-gap materials, [ 2 ] porous membranes, [ 3 ] or to transport and release chemicals. [ 4 ] During standard growth processes, colloidal particles acquire a spherical shape since the latter minimizes the interfacial energy. However, this shape limits the number of new structures which can be assembled by using these particles as building blocks. Thus colloids with tailored anisotropic shape or susceptibility, either electric [ 5 ] or magnetic, [ 6 ] will signifi cantly extend the possibilities to build up new microstructures, [ 7 ] to manipulate the particles via exertion of forces or torques, or to use them as active component in microfl uidics and lab-on-a-chip applications. [ 8 ]

Giant transversal particle diffusion in a longitudinal magnetic ratchet.

We study the transversal motion of paramagnetic particles on a uniaxial garnet film, exhibiting a longitudinal ratchet effect in the presence of an oscillating magnetic field. Without the field, the thermal diffusion coefficient obtained by video microscopy is D(0) ≈ 3 × 10(-4)  μm2/s. With the field, the transversal diffusion exhibits a giant enhancement by almost four decades and a pronounced maximum as a function of the driving frequency. We explain the experimental findings with a theoretical interpretation in terms of random disorder effects within the magnetic film.

Transport and separation of biomolecular cargo on paramagnetic colloidal particles in a magnetic ratchet.

Paramagnetic particles in a magnetic ratchet potential were transported in discrete steps in an aqueous solution on the surface of a magnetic garnet film. The proposed technique allows the simultaneously controlled, dispersion-free movement of an ensemble of paramagnetic particles across the surface. External magnetic modulations were used to transport the particles in a defined direction, and a current reversal upon changing the size of the particles was used to separate particles having different diameters. Doublets consisting of a larger and a smaller particle functionalized with complimentary oligonucleotides and bound via Watson-Crick base pairing were separated after melting the double stranded DNA.

Autonomously Moving Catalytic Microellipsoids Dynamically Guided by External Magnetic Fields

Advances toward the realization of smarter transport systemsrequire the design of artificial microengines with versatilemagneticproperties,capableofdistinguishingthedirectionsofmotion, and to make complete 1808 turns without losingcontrol.Here we report on a new technique for realizingparamagnetic catalytic microellipsoids, displaying all pre-viously mentioned properties. Our approach is based on theuseofjanusparamagneticellipsoidalparticleshalf-coatedwitha thin Pt layer and immersed in a H

Reconfigurable Swarms of Nematic Colloids Controlled by Photoactivated Surface Patterns

Different phoretic driving mechanisms have been proposed for the transport of solid or liquid microscopic inclusions in integrated chemical processes. It is now shown that a substrate that was chemically modified with photosensitive self-assembled monolayers enables the direct control of the assembly and transport of large ensembles of micrometer-sized particles and drops that were dispersed in a thin layer of anisotropic fluid. This strategy separates particle driving, which was realized by AC electrophoresis, and steering, which was achieved by elastic modulation of the nematic host fluid. Inclusions respond individually or in collective modes following arbitrary reconfigurable paths that were imprinted by irradiation with UV or blue light. Relying solely on generic material properties, the proposed procedure is versatile enough for the development of applications that involve either inanimate or living materials.

AC electrophoresis of microdroplets in anisotropic liquids: transport, assembling and reaction

We describe the realization and controlled transport of water-based microreactors dispersed in a nematic liquid crystal and transported by application of an alternating electric field through the mechanism of induced charge electrophoresis. We characterize the propulsion speed of these microreactors in terms of droplet size, strength and frequency of the applied field and show how to use them to transport microscale colloidal cargoes and coalesce water miscible chemicals. Controlled motion of microdoplets in anisotropic fluids is a rich field of research which unveils new perspectives in the transport of water miscible chemicals or drugs.

Friction-controlled bending solitons as folding pathway toward colloidal clusters

We study the conformational transition of an ensemble of magnetic particles from a linear chain to a compact cluster when subjected to an external magnetic field modulation. We show that the transient dynamics induced by switching the field from static to rotating is governed by the relative friction of adjacent particles in the chain. Solid particles show bending solitons counter-propagating along the chain while buckling of the chain is the mechanism preferred by ferrofluid droplets. By combining real-space experiments with numerical simulations we unveil the underlying mechanism of folding pathways in driven colloidal systems.

Synchronous vs. asynchronous transport of a paramagnetic particle in a modulated ratchet potential

We present a combined experimental and theoretical study describing the dynamical regimes displayed by a paramagnetic colloidal particle externally driven above a stripe-patterned magnetic garnet film. A circularly polarized rotating magnetic field modulates the stray field of the garnet film and generates a translating periodic potential which induces particle motion. Increasing the driving frequency, we observe a transition from a phase-locked motion with constant speed to a sliding dynamics characterized by a lower speed due to the loss of synchronization with the traveling potential. We explain the experimental findings with an analytically tractable theoretical model and interpret the particle dynamics in the presence of thermal noise. The model is in good quantitative agreement with the experiments.