Morphology and motion of single optically trapped aerosol particles from digital holography

Nonspherical particles play a key role in the atmosphere by affecting processes such as radiative forcing, photochemistry, new particle formation and phase transitions. In this context, measurements on single particles proved to be very useful for detailed investigations of the properties of the particles studied and of processes affecting them. However, measurements on single nonspherical particles are limited by the difficulties and lack of understanding associated with the optical trapping of such particles. Here, we aim at better understanding the optical trapping of nonspherical particles in air by comparing the motion of an observed nonspherical particle with simulated optical forces and torques. An holographic microscope is used to retrieve the 6D motion of a trapped peanut-shaped particle (3D for translation and 3D for rotation). Optical forces and torques exerted by the optical trap on the peanut-shaped particle are calculated by using FDTD simulations. Most of the main features of the particle motion are in agreement with the calculations while some specific aspects of the particle motion cannot yet be explained.