  We address experimentally the problem of how the structure of a surface
monolayer determines the visco-elasticity of the interface. Optical microscopy
and surface quasi--elastic light scattering have been used to characterize
aggregation of CaCO$_3$ particles at the air--water interface. The structures
formed by cluster-cluster aggregation are two dimensional fractals which grow
to eventually form a percolating network. This process is measured through
image analysis. On the same system we measure the dynamics of interfacial
thermal fluctuations (surface ripplons), and we discuss how the relaxation
process is affected by the growing clusters. We show that the structures start
damping the ripplons strongly when the two length scales are comparable. No
macroscopic surface pressure is measured and this is in contrast to lipid,
surfactant or polymer monolayers at concentrations corresponding to surface
coverage. This observation and the difficulty in fitting the ripplon spectrum
with traditional models suggest that a different physical mechanism might be
responsible for the observed damping of ripplons in this system.
