Attractive-repulsive dynamics on light-responsive chiral microparticles induced by polarized tweezers

Raul Josue Hernandez , Alfredo Mazzulla , Alfredo Pane , Karen Volke Sepulveda and Gabriella Cipparrone
Multifunctional colloidal micro and nano-particles with controlled architectures have very promising properties for applications in bio and nanotechnologies. Here we report on the unique dichotomous dynamical behaviour of chiral spherical microparticles, either fluid or solid, manipulated by polarized optical tweezers. The particles are created using a reactive mesogen mixed with a chiral dopant to form cholesteric droplets in water emulsion. The photopolymerization enables to freeze the chiral supramolecular configurations in solid particles. Different internal architectures in the supramolecular structures, guided by the interfacial chemistry, enable to obtain global-optically isotropic or anisotropic spherical objects. Their optical manipulation reveals particular features. We show that the light can exert a repulsive or attractive force on the same particle depending on the handedness of its circular polarization, for particles having radial configuration of the cholesteric helices. We demonstrate that the unique selective reflection property of the cholesteric phase is responsible of the observed effect. On the other hand very exotic dynamics is observed in case of anisotropic chiral particles. Depending on the light handedness, they behave like Janus spherical particles with dissimilar optical properties that mean dielectric particles having a surface that is in part transparent and in part reflecting. The combination of optical manipulation with other control parameters (electric or magnetic), as well as the integration of other functionalities suggest exciting developments for applications of these micro-devices in the micro and optofluidics, microphotonics and materials science.
DOI