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.

In magnetorheological fluids, the viscosity varies with the applied magnetic field and the non-Newtonian character of these complex fluids may change significantly. In our laboratory we performed experiments to study these properties. We altered a commercial spin-coater to apply external magnetic fields of the order of 0.01 T.

Earlier, we used poly-disperse colloids of superparamagnetic particles, where we studied the morphological properties of the spin coated deposits [M. Pichumani et al.Magnetohydrodynamics 47 (2011) 191] (see also this highlight). Here, we provide a new method to measure the relative viscosity of a superparamagnetic colloid, (by applying a magnetic field during a spin-coating process) which involves evaporation of the solvent. We also defined a term that considers the discrete nature of the suspension.

Among other interesting results, we showed that it is possible to compare the thickness of the dried deposits of spin coated colloids under very different conditions. Thus, the application of an external magnetic field while spin coating allows to obtain the relative variation of the viscosity of the colloid (magnetorheology) by measuring the occupation factor (or height) of the colloidal deposit. We also discussed the morphologies of the final dried colloidal deposits and the possible mechanisms involved in their formation.

• • M. Pichumani et al. Soft Matter 9 (2013) 2506.

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

We thank A. Irigoyen Barrio for his help in preparing the colloidal suspension. We acknowledge C. Gómez-Polo for her generous loan of SQUID and fruitful discussions, and J.M. Pastor and M.A. Miranda for the magnetic characterization of the superparamagnetic colloidal particles. We thank Claire F. Woodworth for her help in correcting grammar mistakes while preparing the manuscript. This work is partly supported by the Spanish Government Contract no. FIS2011-24642. M. P. acknowledges the financial support from the "Asociación de Amigos de la Universidad de Navarra" and the Management, Sri Ramakrishna Engineering College, Coimbatore, India.

Comparison of the film thickness profiles for different colloids without and with applied magnetic field. Squares: SiO₂-MAG, H=0 T; Diamonds: SiO₂-MAG H=0.011 T; Circles: SiO₂-NM. For circles, the information is extracted from Figure 4a of [M. Giuliani et al. J. Phys. Chem. Lett. 1 (2010) 1481] reference experiment by doing a spatial average. Dashed line is a comparison to Cregan model [V. Cregan et al. J. Colloid Interface Sci. 314 (2007) 324]. For more information of the colloids see [M. Pichumani et al. Soft Matter 9 (2013) 2506]: