Spin-coating of colloids is an interesting method in obtaining colloidal crystals (see also here and here). In these kind of experiments, it is easy to control the parameters like the rotation rate, the initial concentration and the fluid phase. However, the effects of these parameters are not fully understood along with the mechanisms of clustering under stress. Using magnetorheological fluids we are able to change easily the effective viscosity by applying a magnetic field. With this, we get a better insight on the coating process.

In this work, we apply an axial magnetic field to a spinning colloidal dispersion of superparamagnetic particles (for more details please look at [M. Pichumani et al. Magnetohydrodynamics 47 (2011), 191]). For various magnetic fields and speeds of rotation, we measure morphological properties like occupation factors, size, shape and orientation of clusters in the dried coating.

Among other interesting results, we obtained that the applied field influences the transition mainly by affecting the dynamic viscosity of the magnetic colloidal dispersion spinning on the substrate. Also, the magnetic field acts on the magnetic particles, which controls the clustering behavior through inter-particle dipole interactions. The increasing applied magnetic field enhances the interaction between the particles, which drives to link the neighboring clusters together to form sub-monolayer deposits. But in the case of zero-field, the colloidal dispersion behaves as non-magnetic. Finally, as we increased the magnetic field, we obtained more eccentric clusters. However, they were not oriented in any specific direction.

• • M. Pichumani et al. Magnetohydrodynamics 47 (2011), 191.

  • M. Pichumani, Ph.D. thesis. Universidad de Navarra (2012)

We acknowledge C. Gómez-Polo for her generous loan of SQUID, and to J.M. Pastor and M.A. Miranda for the magnetic characterization of the colloidal particles. This work is partly supported by the Spanish Government (ref. FIS2008-01126) and by Departamento de Educación (Gobierno de Navarra). M. Pichumani acknowledges financial support from the "Asociación de Amigos de la Universidad de Navarra".

Dried substrate. Micrographies at 4 mm from the center of rotation for both the zero field case (left) and for the case where a field of 0.033 T (right) was applied during spinning (2000 rpm). Scale bars are 100 μm: