Francesc Sagués

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.

Reaction-subdiffusion equations

To analyze possible generalizations of reaction-diffusion schemes for the case of subdiffusion we discuss a simple monomolecular conversion A --> B. We derive the corresponding kinetic equations for the local and concentrations. Their form is rather unusual: The parameters of the reaction influence the diffusion term in the equation for a component A, a consequence of the non-Markovian nature of subdiffusion. The equation for the product contains a term which depends on the concentration of A at all previous times. Our discussion shows that reaction-subdiffusion equations may not resemble the corresponding reaction-diffusion ones and are not obtained by a trivial change of the diffusion operator for a subdiffusion one.

Nonlinear chemical dynamics

The interdisciplinary field of nonlinear chemical dynamics has grown significantly in breadth and depth over the past three decades. Its subject matter and applications encompass all branches of chemistry as well as areas of mathematics, physics, biology and engineering. In this Perspective, we present an overview of some of the key results of nonlinear chemical dynamics, with emphasis on those areas most likely to be of interest to inorganic chemists. We discuss the range of phenomenology from chemical oscillation to chaos to waves and pattern formation, as well as experimental methods, mechanistic considerations, theoretical techniques, and the results of coupling and external forcing.

Pattern morphologies in zinc electrodeposition

Abstract A systematic experimental study of quasi two-dimensional zinc electrodeposition in an electrochemical cell with parallel electrodes is reported. The well-known different growth regimes, from anisotropically dendritic to irregularly disordered fractal, described previously in radial cells, are shown together with a novel mixed texture composed of dendritic backbones with ramified open branches. Experiments conducted by varying the cell thickness and electrode separation show significant effects by the current on the pattern morphology. Finally, the self-similarity of the electrodeposits is examined for two different growth regimes.

Growth and forms in quasi-two-dimensional electrocrystallization

Abstract A review on the morphologies and corresponding growth dynamics observed in quasi-two-dimensional electrocrystallization is presented. Emphasis is devoted to the understanding of the specific and growth mode-dependent role played by the different transport mechanisms, i.e. diffusion, migration and convection, as well as the chemical processes. The role of chemical reactions is decissive for the understanding of the well-known morphological transitions and is at the origin of the finger-like texture.

Dynamics of Turing patterns under spatiotemporal forcing

We study, both theoretically and experimentally, the dynamical response of Turing patterns to a spatiotemporal forcing in the form of a traveling-wave modulation of a control parameter. We show that from strictly spatial resonance, it is possible to induce new, generic dynamical behaviors, including temporally modulated traveling waves and localized traveling solitonlike solutions. The latter make contact with the soliton solutions of Coullet [Phys. Rev. Lett. 56, 724 (1986)]] and generalize them. The stability diagram for the different propagating modes in the Lengyel-Epstein model is determined numerically. Direct observations of the predicted solutions in experiments carried out with light modulations in the photosensitive chlorine dioxide-iodine-malonic acid reaction are also reported.

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 ]

Two representations in multifractal analysis

Two representations in multifractal analysis, the so-called q and tau representations, are discussed theoretically and computed practically. Complementary to the standard q-representation, the so-called tau -representation is especially suited to resolving the most rarified subsets of the distributed measure. Moreover, these two representations are especially adapted, respectively, to the well known fixed-size and fixed-mass box-counting algorithms. Both strategies are first applied to iteratively constructed mathematical measures. Once tested in this way, we use them to analyse the mass distribution and the growth probability distribution of an experimental electrodeposited pattern.

Chiral shape and enantioselective growth of colloidal particles of self-assembled meso-tetra(phenyl and 4-sulfonatophenyl)porphyrins

The self-assembly of diprotonated phenyl and 4-sulfonatophenyl meso-tetrasubstituted porphyrins gives a spontaneous chiral symmetry breaking, but only for H2TPPS3−, which forms helicoidal colloidal particles; the selection of the resulting chirality sign by the hydrodynamic forces of a stirring vortex can be demonstrated.