Shape-induced alignment and voltage-controlled switching in cholesteric droplets for smart film applications
摘要
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 N0 ≈ 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.