Photophoretic Movement of a Micron-Sized Light-Absorbing Capsule: Numerical Simulation

Multilayer microparticles with a liquid core and a polycomposite light-absorbing shell (microcapsules) are important components of modern bio- and medical technologies. Opening of the microcapsule shell and payload release can be realized by optical radiation. The photophoretic force is due to the radiation-stimulated thermal gradient and arises from the temperature inhomogeneity of the microparticle. Photophoretic forces, as well as radiation pressure forces, are inherently mechanical forces and can cause microcapsules to move during the opening cycle. We numerically simulate the microcapsule photophoretic motion when illuminated by an intense laser pulse. Numerical calculations of the temperature field in a spherical microcapsule are carried out using the finite element method, taking into account the auxiliary nanoparticles, which are randomly distributed around the capsule and serve to enhance the heating of the capsule under short pulse exposure. The spatial distribution of the absorbed optical power as well as the temporal dynamics of microcapsule heating depending of its size are investigated in detail. We show, for the first time to our knowledge, that under the action of photophoretic gradient, the microcapsule can move along the laser incidence direction both forward and backward at the distance of several tens of nanometers depending on the particle size and conditions of optical absorption.