Shape-induced alignment and voltage-controlled switching in cholesteric droplets for smart film applications

Oblate cholesteric droplets confined in the polymer film can host a toroidal director field with a large uniform region, but the geometric conditions for maximizing this state and its electric-field response were not established. Using simulations and experiments on two polymer-dispersed cholesteric systems with opposite dielectric anisotropy, we show that droplet flattening preserves the toroidal topology and expands the uniformly aligned central region. The optimal structure occurs near relative chirality N 0 ≈ 3 and compression ratio δ ≈ 0.2, where the aligned region reaches a/d ≈ 0.9. These conditions were realized experimentally, yielding dark homeotropic states with a/d ≥ 0.8 at zero field. Under an electric field, droplets with positive dielectric anisotropy switch through a defect-mediated pathway and show color tuning, whereas droplets with negative dielectric anisotropy reorient smoothly and reversibly without twist-wall formation. The results demonstrate the principal opportunity of forming the droplet structure with the uniformly aligned central region by geometric compression without electric field effect, that provides a simple route to reversible electro-optical cholesteric films to reversible electro-optical cholesteric films.