Physics of colloids: from collective assemblies to single swimmers

Francesc Sagues

Dep. Química Física, Universidad de Barcelona

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.