The magnetic needle interfacial shear rheometer is a valuable tool for the study of the mechanical properties of thin fluid films or monolayers. However, it is difficult to differentiate the interfacial and subphase contributions to the... more
The magnetic needle interfacial shear rheometer is a valuable tool for the study of the mechanical properties of thin fluid films or monolayers. However, it is difficult to differentiate the interfacial and subphase contributions to the drag on the needle. In principle, the problem can be addressed by decreasing the needle diameter, which decreases the bulk contribution while the interfacial contribution remains essentially
We demonstrate a size sensitive experimental scheme which enables bidirectional transport and fractionation of paramagnetic colloids in a fluid medium. It is shown that two types of magnetic colloidal particles with different sizes can be... more
We demonstrate a size sensitive experimental scheme which enables bidirectional transport and fractionation of paramagnetic colloids in a fluid medium. It is shown that two types of magnetic colloidal particles with different sizes can be simultaneously transported in opposite directions, when deposited above a stripe-patterned ferrite garnet film subjected to a square-wave magnetic modulation. Due to their different sizes, the particles are located at distinct elevations above the surface, and they experience two different energy landscapes, generated by the modulated magnetic substrate. By combining theoretical arguments and numerical simulations, we reveal such energy landscapes, which fully explain the bidirectional transport mechanism. The proposed technique does not require pre-imposed channel geometries such as in conventional microfluidics or lab-on-a-chip systems, and permits remote control over the particle motion, speed and trajectory, by using relatively low intense magn...
Hydrodynamic interactions (HIs), namely, solvent-mediated long-range interactions between dispersed particles, play a crucial role in the assembly and dynamics of many active systems, from swimming bacteria to swarms of propelling... more
Hydrodynamic interactions (HIs), namely, solvent-mediated long-range interactions between dispersed particles, play a crucial role in the assembly and dynamics of many active systems, from swimming bacteria to swarms of propelling microrobots. We experimentally demonstrate the emergence of long-living hydrodynamic bound states between model microswimmers at low Reynolds numbers. A rotating magnetic field forces colloidal hematite microparticles to translate at a constant and frequency-tunable speed close to a bounding plane in a viscous fluid. At high driving frequency, HIs dominate over magnetic dipolar ones, and close propelling particles couple into bound states by adjusting their translational speed to optimize the transport of the pair. The physical system is described by considering the HIs with the boundary surface and the effect of gravity, providing an excellent agreement with the experimental data for all the range of parameters explored. Moreover, we show that in dense su...
Los fluidos magneto-reoleogicos son dispersiones de nano-parteculas magneticas suspendidas en un fluido no magnetico. Los momentos magneticos de las particulas tienden a alinearse en la direccion del campo aplicado. Aparecen entonces... more
Los fluidos magneto-reoleogicos son dispersiones de nano-parteculas magneticas suspendidas en un fluido no magnetico. Los momentos magneticos de las particulas tienden a alinearse en la direccion del campo aplicado. Aparecen entonces fuerzas de naturaleza dipolar, atractivas en la direccion paralela al campo y repulsivas en direccion normal. Debido al caracter anisotropo de la interaccion magnetica, las particulas tienden a formar agregados lineales, paralelos a la direccion del campo magnetico externo. Esto ocurre siempre que la interaccion mmagnetica sea lo suficientemente intensa como para superar a las interacciones que regulan la estabilidad de las particulas. Por otro lado, si existen ligaduras entre las particulas que constituyen estos agregados lineales, entonces algunas de estas estructuras lineales son ?permanentes?, es decir, que mantienen su geometria aun en ausencia del campo magneticoexterno. La respuesta al campo externo de las particulas magneticas, la geometria line...
