  Tracking of particles, be it a passive tracer or an actively moving bacterium
in the growing bacterial colony, is a powerful technique to probe the physical
properties of the environment of the particles. One of the most common measures
of particle motion driven by fluctuations and random forces is its diffusivity,
which is routinely obtained by measuring the mean squared displacement of the
particles. However, often the tracer particles may be moving in a domain or an
aggregate which itself experiences some regular or random motion and thus masks
the diffusivity of tracers. Here we provide a method for assessing the
diffusivity of tracer particles within mobile aggregates by measuring the
so-called mean squared relative distance (MSRD) between two tracers. We provide
analytical expressions for both the ensemble and time averaged MSRD allowing
for direct identification of diffusivities from experimental data.
