  Ultrasoft colloidal particle fluctuates due to its flexibility. Such
fluctuation is essential for colloidal structure and dynamics, but is
challenging to quantify experimentally. We use dendrimers as a model system to
study the fluctuation of ultrasoft colloids. By considering the dynamic
polydispersity in the small-angle neutron scattering (SANS) model, and
introducing the fluctuation of invasive water into the contrast in SANS, we
reveal the fluctuating amplitudes of the size and shape of the dendrimer of
generation 6 at finite concentrations. The size fluctuation is suppressed while
the shape fluctuation increases as the weight fraction of dendrimers passes
11%. With neutron spin echo data, we suggest that such crossover originates
from the competition between the inter- and intra-particle dynamics. Further
investigation on lower-generation samples shows a contrary result, which
suggests a structural basis for these dynamic phenomena.
