Electric field assisted assembly of colloidal discoid crystals with reconfigurable orientation

We demonstrate that cyclic application of electric fields can reconfigure the orientation of colloidal discoid crystals. 4-μm sulfate-modified polystyrene discoids are self-assembled into a monolayer crystal in an isopropanol-water mixture held between two conductive coverslips with a 120-μm separation. After sedimentation, the discoid minor axes are perpendicular to the substrate (homeotropic alignment). Adding an AC electric field (1 kHz) of ≥ 0.50 V switches the minor axes parallel to the substrate (planar alignment) with high fidelity within ~100 s. Switching off the AC field orients the discoids back into the homeotropic alignment within ~300 s. Kinetic modelling based on the torques induced by electric fields and gravity (for discoids in contact with the substrate) provides good agreement with the measured time constants for both switching directions; these torques are estimated to be levels that overcome thermal fluctuations, so that the discoids achieve a steady orientation, consistent with experimental observations. We further support that the field application generates orientational switching by observing reconfigurable structural color in the discoid crystals; this switching can be maintained for at least 10 cycles. The cyclic application of electric fields provides a promising avenue to reconfigure structure and functional properties of colloidal crystals assembled from spheroids.