| dc.description.abstracteng | In this thesis I present simulation-based studies of systems of self-
propelled particles in heterogeneous media. I consider the interaction
of particles with planar walls, single spherical obstacles or arrays of
randomly distributed obstacles. Active particles with different propulsion mechanisms and different interactions with each other and the
environment have been known to exhibit interesting universal phenomena; however, in conducting a generic theory explaining such
phenomena we still require further investigation of different types of
active systems. The aim of our studies in this thesis is to shed some
light on the emergent behavior of individual or large collections of
active particles with repulsive excluded volume interactions and linear propulsion in the presence of environmental heterogeneities. First, I describe the behavior of single active particles in the vicinity
of a simple environmental constraint: a planar wall. It is shown how
the activity increases the tendency of the particles to move along the
wall and spend long times in its vicinity. These results are consistent
with the behavior of a large class of biological and synthetic active
particles, reported previously. The distribution of the residence time
on the wall is found and its dependence on different parameters is
explored numerically.
Then I introduce spherical large obstacles to the system and extend
our observations to this case. It is illustrated how active particles tend
to reside longer on the obstacles as they flatten, and also how the residence times are affected by our model parameters, as compared to
the case of a planar wall.Next, I go on to study the collective behavior of active particles in
the presence of large obstacles. The accumulation and crystallization
of active particles around the obstacles are characterized: an interesting phenomenon that has been previously found in different active
systems with repulsive interactions only.
I further describe a particular phenomenon of collective rotation of
active particles around the obstacles. Given the purely repulsive interactions of particles with themselves and the obstacle, the absence
of any active torque on the particles, and the lack of any aligning or
synchronizing mechanism between the particles, such huge rotating
aggregates of particles is not a trivial state of the system. I explore
the origin of such rotations and using simple arguments explain why
they occur. Our suggested mechanism for driving the rotations also
describes some of their important properties such as the increase of their angular velocity as the rotations build up, and the scaling of the
total torque driving the rotating crystals by their mass.
Finally, I present some results on the behavior of active systems in
crowded environments. Increasing the crowdedness, the diffusion becomes non-Gaussian and slow. The decrease of the diffusion constant
with the obstacle density is a function of the activity. The effect of
activity on the particle’s exploration of the cages, made by the many
obstacles, is further investigated in this thesis. | de |