Simultaneous active and diffusive behaviour of asymmetric microclusters in a photophoretic trap

Active and diffusive motion in Brownian particles are regularly observed in fluidic environments, albeit at different time scales. Here, we experimentally study the dynamics of highly asymmetric microclusters trapped in air employing photophoretic forces generated from a loosely focused laser beam, where the trapped particles display active and diffusive dynamics simultaneously in orthogonal spatial directions. Thus, particle motion in the longitudinal direction ($z$) is enslaved to irregular kicks that naturally arise from an interplay of gravitational and photophoretic forces. This leads to a bimodal nature of the probability distribution function with a near-ballistic scaling of mean-squared displacement in the $z$ direction demonstrating active like dynamics, while the dynamics along the transverse ($x$) direction displays diffusive behaviour with a strong dependence on the motion along $z$. To explain these unique characteristics, we developed a 2D-Langevin model of a confined elliptic particle experiencing an additional stochastic force along $z$ to account for the arbitrary jumps. The numerical results show excellent qualitative agreement with the experimental observations. Our findings should pave the way for the design of high-efficiency Brownian engines in air, besides stimulating new research in the emerging field of photophoretic trapping.