  Micrometer scale colloidal particles that propel in a deterministic fashion
in response to local environmental cues are useful analogs to self-propelling
entities found in nature. Both natural and synthetic active colloidal systems
are often near boundaries or are located in crowded environments. Herein, we
describe experiments in which we measured the influence of hydrogen peroxide
concentration and dispersed polyethylene glycol (PEG) on the clustering
behavior of 5 micrometer catalytic active Janus particles at low concentration.
We found the extent to which clustering occurred in ensembles of active Janus
particles grew with hydrogen peroxide concentration in the absence of PEG. Once
PEG was added, clustering was slightly enhanced at low PEG volume fractions,
but was reduced at higher PEG volumes fractions. The region in which clustering
was mitigated at higher PEG volume fractions corresponded to the region in
which propulsion was previously found to be quenched. Complementary agent based
simulations showed that clustering grew with nominal speed. These data support
the hypothesis that growth of living crystals is enhanced with increases in
propulsion speed, but the addition of PEG will tend to mitigate cluster
formation as a consequence of quenched propulsion at these conditions.
