Unravelling 3D Dynamics and Hydrodynamics during Incorporation of Dielectric Particles to an Optical Trapping Site

Mapping of the spatial and temporal motion of particles inside an optical field is critical for understanding and further improvement of the 3D spatio-temporal control over their optical trapping dynamics. However, it is not trivial to capture the 3D motion, and most imaging systems only capture a 2D projection of the 3D motion, in which the information about the axial movement is not directly available. In this work, we resolve the 3D incorporation trajectories of 200 nm fluorescent polystyrene particles in an optical trapping site under different optical experimental conditions using a recently developed widefield multiplane microscope (imaging volume of 50 x 50 x 4 mu m(3)). The particles are gathered at the focus following some preferential 3D channels that show a shallow cone distribution. We demonstrate that the radial and the axial flow speed components depend on the axial distance from the focus, which is directly related to the scattering/gradient optical forces. While particle velocities and trajectories are mainly determined by the trapping laser profile, they cannot be completely explained without considering collective effects resulting from hydrodynamic forces.

Automated summary

In this work, the 3D motion trajectories of 200 nm fluorescent polystyrene particles in an optical trapping site were resolved using a widefield multiplane microscope. The particles showed a shallow cone distribution following preferential 3D channels near the focus. The radial and axial flow speed components depended on the axial distance from the focus, which was directly related to the scattering/gradient optical forces. The trapping laser profile determined the particle velocities and trajectories, but collective effects resulting from hydrodynamic forces also played a role.