Investigation on the laser trapping mechanism of light-absorbing particles in air

We report on the micron-sized light-absorbing particle trapping in two configurations (horizontal and vertical), in order to elucidate the laser trapping mechanism based on the photophoretic force. Two types of carbon particles (irregular graphite particles and carbon microspheres) were tested in both Gaussian and hollow beam traps. By comparing the trapping efficiency and stability under various circumstances, we confirmed that there are two types of photophoretic forces: F-Delta alpha and F-Delta T forces on the laser irradiating particle. Furthermore, the forces and moments exerting the particles in various traps were analyzed, which explained the experimental phenomena very well. It was found that the F-Delta alpha force due to the thermal accommodation difference among the irregular particles helps the irregular particles to be balanced more easily and of higher trapping efficiency and stability. This work provides important references for people to choose a suitable trapping scheme according to the particles, which would be of significance in the applications of single-particle analysis. Copyright (C) EPLA, 2019