Rotation, oscillation and hydrodynamic synchronization of optically trapped oblate spheroidal microparticles

Abstract

While the behavior of optically trapped dielectric spherical particles has been extensively studied, the behavior of non-spherical particles remains mainly unexplored. In this work we focus on the dynamics of oblate spheroidal particles trapped in a tightly focused elliptically-polarized vortex beam. In our experiments we used polystyrene spheroids of aspect ratio of major to minor axes equal to 2.55 and of a volume equal to a sphere of diameter 1.7μm. We demonstrate that such particles can be trapped in three dimensions, with the minor axis oriented perpendicular to both the beam polarization (linear) and the beam propagation, can spin in a circularly polarized beam and an optical vortex beam around the axis parallel with the beam propagation. We also observed that these particles can exhibit a periodic motion in the plane transversal to the beam propagation. We measured that the transfer of the orbital angular momentum from the vortex beam to the spheroid gives rise to torques one order of magnitude stronger comparing to the circularly polarized Gaussian beam. We employed a phase-only spatial light modulator to generate several vortex beam traps with one spheroid in each of them. Due to independent setting of beams parameters we controlled spheroids frequency and sense of rotation and observed hydrodynamic phase and frequency locking of rotating spheroids. These optically driven spheroids offer a simple alternative approach to the former techniques based on birefringent, absorbing or chiral microrotors.

Transitional behavior in hydrodynamically coupled oscillators

“…Finally, we note that the nature of synchronization mechanisms studied here derives from the symmetry of the rotors. However, the way in which these processes are modified by reductions in rotor symmetry is yet to be fully understood [37].…”

Behavior of oblate spheroidal microparticles in a tightly focused optical vortex beam

“…48 With this configuration we generated simple vortex beams with controlled beam width, intensity and topological charge, respectively, at the input aperture of the focusing objective. It determined the beam properties (diameter, intensity and phase profile) in the objective focal plane and consequently provided control of the 3D trapping and behavior of OSPs.…”

“…Manufacturing of OSPs was based on the published methods. 48 OSPs are of an ellipsoidal shape defined by two longer axes with equal lengths 2a and by a shorter axis of length 2b. We define the OSP aspect ratio s as s = a/b and the equivalent spherical radius R s = (a 2 b) 1/3 giving the same sphere volume as the OSP.…”

Optical sorting of nonspherical and living microobjects in moving interference structures

“…Figure 8 illustrates the possibility to sort even objects of different shapes but almost equal volume. Spheroids were manufactured from polystyrene spheres [59][60][61] and after the procedure they were mixed with the original spheres and diluted by water. Travelling fringes were used to separate spheres and spheroids.…”