The internal dynamics of fibrin gels grown at room temperature from fibrinogen solutions at different NaCl concentrations were investigated by means of dynamic light scattering (DLS). By varying the salt concentration, it is possible to obtain either "fine" gels characterized by thin fibers, or "coarse" gels with much thicker fibers. The observed dynamics of these two gels are quite different. Whereas for "fine" gels the dynamic structure factor shows an initial monoexponential decay followed by a stretched exponential decay, typical of semiflexibles polymers, for "coarse" gels the dynamic structure factor decay to a plateau whose value depends on the scattering wavevector q. Moreover a stretched exponential decay is observed, also at the fastest decay time. This behavior can be interpreted as given by the contributions of the internal elastic modes of many different length scales.
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