[Todos] Coloquio extraordinario (viernes 14:30) en el Departamento de Física

[Pablo Dmitruk]

                       COLOQUIO EXTRAORDINARIO EN EL DEPARTAMENTO DE FÍSICA
                                         FCEyN - UBA

                              En el Aula Federman, 1er piso Pabellón I.
                                    Ciudad Universitaria

                         El próximo  viernes 28 de noviembre a las 14:30 hs


Physics of colloids: from collective assemblies to single swimmers

Francesc Sagues

Universidad de Barcelona

(Nota: la charla será en español)
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Abstract:

Two different aspects of the physics of driven colloidal particles will be
discussed. In the first part I will report on the collective organization of
paramagnetic particles placed above the periodic stripes of a uniaxial
magnetic film. An external field modulation induces vibration of the stripe
walls and produces random motion of the particles. Defects in the stripe
patterns favour particle nucleation into large clusters above a critical
density. Mismatch between particle size an pattern wavelength generates
assemblies with different morphological order. At even higher field
strengths, repulsive dipolar interactions between the particles induce
cluster melting. In the second part I will show how anisotropic  
paramagnetic colloidal particles dispersed in water and floating above  
a flat plate can be endowed with controlled propulsion when subjected  
to an horizontal precessing
magnetic field. 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. We combine a report of experimental observations with
a theoretical analysis that fully characterizes the swimming velocity in
terms of the relative strength and frequency of the actuating magnetic
field.