Windmill droplets: optically induced rotation of biphasic oil-in-water droplets del Pozo, Jesús J. Bonhome-Espinosa, Ana B. Sun, Wei Gutiérrez-Ariza, Carlos Rica-Alarcón, Raúl Alberto Rodríguez-Arco, Laura In the field of microdroplet manipulation, optical tweezers have been used to form and grow droplets, to transport them, or to measure forces between droplet pairs. However, the exploration of out-ofequilibrium phenomena in optically trapped droplets remains largely uncharted. Here, we report the rotation of biphasic droplets fabricated by co-emulsifying two immiscible liquids (i.e., hydrocarbon and fluorocarbon oils) with a refractive index mismatch in water. When trapped, droplets of a specific geometry rotate around the axis of the laser beam, in what appears to be a dissipative, out-ofequilibrium phenomenon. The rotational frequency, obtained from image analysis, is stable and proportional to the beam power. Remarkably, droplets that do not interact with the trapping beam can also be rotated indirectly. This is achieved by positioning the droplets at the center of a circular arrangement of multiple, sequentially activated traps, so that the droplet orients towards the location of the active trap. Altogether, our results demonstrate out-of-equilibrium phenomenology in optically trapped biphasic droplets, which would inspire the development of devices based on them (e.g., optically induced mixing, etc.). In addition, they may shed light on fundamental principles of optical manipulation of asymmetric particles. 2025-09-11T10:02:01Z 2025-09-11T10:02:01Z 2025-07-30 journal article Del Pozo, J. J., Bonhome-Espinosa, A. B., Sun, W., Gutiérrez-Ariza, C., Rica-Alarcón, R. A., & Rodríguez-Arco, L. (2025). Windmill droplets: optically induced rotation of biphasic oil-in-water droplets. Soft Matter, 21(33), 6575–6583. https://doi.org/10.1039/d5sm00273g https://hdl.handle.net/10481/106251 10.1039/d5sm00273g eng http://creativecommons.org/licenses/by-nc/4.0/ open access Atribución-NoComercial 4.0 Internacional Royal Society of Chemistry