Hybrid colloids composed of micron-sized ferromagnetic (carbonyl iron) and diamagnetic (silica) particles suspended in cyclohexanone, behave as, non-Newtonian, magnetorheological fluids. We measure and compare the magnetic field-dependent viscosity of hybrid diluted colloids using spin-coating and conventional magnetorheometry. We extend a previously developed model for simple colloids to this kind of hybrid colloids. As in the previous model, the viscosity of the colloidal suspension under applied fields can be derived from the surface coverage of the dry spin-coated deposits for each type of particles, and from the viscosity of the colloid at zero field. Also, our results allow us to obtain the evaporation rate of the solvent as a function of the rotation speed. Finally, we also measure the viscosity of the same suspension with a torsional parallel plate magnetorheometer under uniaxial DC magnetic fields aligned in the velocity gradient direction of a steady shearing flow. The experimental results under different conditions and the effect of each component on the magnetorheological properties of the resulting colloid are discussed. Standard spin-coating technique can be used both to characterize smart materials-complex fluids as well as to fabricate films with arbitrary solvents by tuning their viscosity by means of external fields.

Polycrystals of thin colloidal deposits, with thickness controlled by spin-coating speed, exhibit axial symmetry with local 4-fold and 6-fold symmetric structures, termed orientationally correlated polycrystals (OCPs). While spin-coating is a very facile technique for producing large-area colloidal deposits, the axial symmetry prevents us from achieving true long-range order. To obtain true long-range order, we break this axial symmetry by introducing a patterned surface topography and thus eliminate the OCP character. We then examine symmetryindependent methods to quantify order in these disordered colloidal deposits. We find that all the information in the bond-orientational order parameters is well captured by persistent homology analysis methods that only use the centers of the particles as input data. It is expected that these methods will prove useful in characterizing other disordered structures.

Spin-coating technique is very fast, cheap, reproducible, simple and needs less material to fabricate films of particulate systems/colloids. Their thickness and uniformity may be controlled by means of external fields. We apply magnetic fields during the spin-coating of a moderately concentrated superparamagnetic colloid (made of silica coated magnetite particles). We study the influence of different magnetic field configurations (homogeneous and inhomogeneous) on the resulting spincoated deposits and compare experimental results under various conditions. Superparamagnetic colloids behave as, non-Newtonian, magnetorheological fluids. Their viscosity vary significantly under applied magnetic fields. We measure and compare the effect of uniform and non-uniform magnetic fields on their relative effective viscosity, using the spin-coated deposits and a previously existing model for simple colloids. The mechanisms involved in the deposits formation under different experimental conditions are also discussed. In particular, we show that the magnetophoretic effect plays an important role in the spin-coating of magnetic colloids subjected to non-uniform magnetic fields. We characterize an effective magnetoviscosity in non-uniform magnetic fields that is largely influenced by the magnetophoretic effect that enhances the flow of the magnetic fluid.

Abstract. Pattern formation in colloids by weak ac fields in vertical deposition-like configuration at different temperatures has been studied experimentally. At low evaporation (room temperature), the effect of the field leads to the evolution of a one-dimensional array of clusters along the contact line and columnar colloidal dried deposits are obtained at higher evaporation. We investigate the flow dynamics involved in this pattern formation. Homogeneous variation of the contact angle by electrowetting effect becomes unstable and breaks the translational symmetry at the meniscus. Electrokinetic forces together with capillary forces result in the accumulation of particles for pattern formation. The movement of electrically charged colloidal particles is controlled by weak ac electric field even at higher temperatures. We observe the effect of increasing initial particle concentration on the behavior of the clusters for various field frequencies. The average distance between clusters increase monotonically with an increase in the initial particle concentration. We also observe that the average width of columns increases according to the applied field strength.

We report experimental results on the patterns that are formed during spin-coating of magnetic colloids at moderate concentrations and compare them with results obtained in diluted colloids. We show that, for moderate concentrations, the magnetic interaction between the (ferro)magnetic particles and with the external field is strong enough to overcome the centrifugal force. We study two different configurations for the magnetic field. The first one consists on an axial uniform field, where we obtain spikes perpendicular to the substrate with a well defined order which decreases as rotation rate increases. The second one consists on a radial non-uniform field, where we obtain elongated deposits radially disposed on the substrate. The effect of magnetic fields at moderate concentrations on the effective viscosity is confirmed to be much more important in the case of a uniform magnetic field, by increasing the hydrodynamic timescale which gives the ferromagnetic particles enough time to strongly interact to form the spikes.