This is a collaborative work with the group of Prof. A. Yethiraj at Memorial University of Newfoundland, St. John's (Canada). Actually, the experiments presented here have been done in his laboratory, although we also do experiments with our spin-coater. In some previous experiments, we proved that colloid spin-coating is related to the appearance of Orientationally Correlated Polycrystals [C. Arcos et al. Phys. Rev. E 77 (2008) 050402(R)], which present a long range orientational order without translational order. These symmetries arise from the very first stages of the experiments [M. Giuliani et al. J. Phys. Chem. Lett. 1 (2010) 1481] and avoid formation of good quality monocrystals. In this experiment, we apply a strong ac electric field in the reference frame of the rotating substrate, which breaks the symmetry. Our experimental way of proceeding is similar to previous experiments [C. Arcos et al. Phys. Rev. E 77 (2008) 050402(R)][M. Giuliani et al. J. Phys. Chem. Lett. 1 (2010) 1481] except for the construction of a home-made device that allows to apply the electric field.

We studied the orientation angle of the crystallites (or domains) and how it correlates with the geometrical angle which forms the crystallite with respect to the center of rotation (see below). We also prove that it is the dielectrophoretic localization what breaks the symmetry.

Our main contribution was in the experiments (M. Pichumani), in the experimental design (M. Giuliani) and in the analysis / discussion of data (W. González-Viñas).

We obtained (among other interesting things [A.P. Bartlett, M. Pichumani et al.Langmuir 28 (2012) 3067]) that dielectrophoresis is able to break the radial symmetry in the spin-coating of colloids. This phenomenon prevents the formation of Orientationally Correlated Polycrystals and allows to improve the quality of the colloidal crystals obtained by the (fast) technique of spin-coating.

• • A.P. Bartlett, M. Pichumani et al. Langmuir 28 (2012) 3067.

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

We acknowledge Edward Hayden and Andrew R. Morrow for their assistance during the experiments, and Claire F. Woodworth for carefully reading the manuscript in order to correct grammatical errors. This work is partly supported by NSERC and the Spanish Government Contract No. FIS2008-01126. M.G. and M.P. acknowledge financial support from the "Asociación de Amigos de la Universidad de Navarra".

(A): Spin-coated sample at zero field which shows four-arms pattern characteristic of Orientationally Correlated Polycrystals; B: Spin-coated sample with field (0.95 kV/mm, 3 kHz). Here there is no axial symmetry.

Dominant domain orientation angle vs angle describing crystallite position. Field condition is 0.95 kV/mm and 3 kHz:

Order parameter for the Orientationally Correlated Polycrystals structure: