  The morphology and the microscopic internal dynamics of a bidimensional gel
formed by spontaneous aggregation of gold nanoparticles confined at the water
surface are investigated by a suite of techniques, including grazing-incidence
x-ray photon correlation spectroscopy (GI-XPCS). The range of concentrations
studied spans across the percolation transition for the formation of the gel.
The dynamical features observed by GI-XPCS are interpreted in view of the
results of microscopical imaging; an intrinsic link between the mechanical
modulus and internal dynamics is demonstrated for all the concentrations. Our
work presents, to the best of our knowledge, the first example of a transition
from stretched to compressed correlation function actively controlled by
quasistatically varying the relevant thermodynamic variable. Moreover, by
applying a model proposed time ago by Duri and Cipelletti [A. Duri and L.
Cipelletti, Europhys. Lett. 76, 972 (2006)] we are able to build a novel master
curve for the shape parameter, whose scaling factor allows us to quantify a
'long time displacement length'. This characteristic length is shown to
converge, as the concentration is increased, to the 'short time localization
length' determined by pseudo Debye-Waller analysis of the initial contrast.
Finally, the intrinsic dynamics of the system are then compared with that
induced by means of a delicate mechanical perturbation applied to the
interface.
