  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.
