Dual-responsive coloration of Janus droplets via total internal reflection and interference applied as single-use freezing indicators.

Responsive optical materials offer real-time visual feedback to external stimuli by converting them into visible light signals, holding great promise in display, sensing, and information encryption. Typically, heterogeneous Janus droplets can exhibit vivid coloration induced by total internal reflection and interference (TIRI) at concave liquid-liquid interfaces. Yet, achieving stimuli-responsive TIRI-based structural colors remains a challenge due to the difficulty in controlling interface curvature and reflective behavior under external stimuli. Here, we report the fabrication of Janus droplets with tunable TIRI structural colors that respond to both surfactant composition and temperature. By tailoring surfactant formulation, the droplet morphology and interface curvature can be adjusted, enabling reversible modulation of structural color appearance, including both iridescent and non-iridescent modes depending on incident light conditions. Furthermore, the structural color can be reversibly or irreversibly switched on/off in response to temperature changes, due to phase transitions that alter light reflection at the concave interface. Owing to their temperature-dependent optical response and real-time visual signaling, these droplets can serve as single-use security labels for monitoring freeze history of freeze-sensitive pharmaceuticals, providing reliable indication of product integrity.