Colloidal rods in optical potential energy landscapes

Joshua L. Abbott, James A. Spiers, Yongxiang Gao, Dirk G A L Aarts and Roel P A Dullens

We study the static and dynamic behaviour of colloidal rods in an optical potential energy landscape. We explore the stable states of a colloidal rod in a single optical trap close to a flat wall. Here two metastable states are observed, horizontal and vertical, both of which experience a parabolic potential energy landscape. Next we place a colloidal rod into a one-dimensional sinusoidal optical potential energy landscape and introduce a constant driving velocity. When driven below the critical velocity, the particle is confined to a single potential energy minimum of the optical landscape and the equilibrium position of a particle is investigated. The equilibrium position of a rod is found to vary substantially from that of a sphere due to the drag coefficient of a rod, which is highly dependent on its proximity to an optical trap. The driving velocity is increased to enable the particle to traverse the periodic landscape and above the critical velocity, the average particle velocity increases as the square root of the driving velocity. When introducing oscillations to the driving velocity we observe dynamic mode locking and characterise the nature of synchronised motion of the rod-like particles.

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