Spin-coating of moderately concentrated superparamagnetic colloids in different magnetic field configurations

Abstract

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.

Pattern formation in spin-coating of hybrid colloids in different magnetic field configurations

“…On the other hand, the evaporation of the solvent freezes the spikes in the three phase contact line front when complete evaporation of the solvent occurs. If the applied magnetic field is radial and not uniform, magnetophoresis induces a reduction of the effective viscosity as has been observed in previous works with superparamagnetic colloids [32]. The length of the structures (elongated deposits of particles) increases with the distance to the rotation center.…”

“…If the applied magnetic field is radial and not uniform, magnetophoresis induces a reduction of the effective viscosity as has been observed in previous works with superparamagnetic colloids [32]. The length of the structures (elongated deposits of particles) increases with the distance to the rotation center.…”

“…On the one hand, the typical magnetic field strength in this configuration is much lower (1.8 kA m −1 ) than in the previous one (39.7 kA m −1 ), leading to a reduced magnetic effect. On the other hand, it has been previously shown that, for superparamagnetic particles colloids, the effective viscosity under the current configuration diminishes [32]. As a consequence, the typical length of the deposits are much smaller than the length of the spikes.…”