Dipartimento di Fisica e Scienze della Terra  
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Photon correlation spectroscopy, performed with coherent X-rays, directly accesses dynamical heterogeneity in arrested systems.
What do snow tires share with chocolate and plasters for transdermal drug delivery? The notable properties of all these systems are determined by molecular mobility, which is decreased when a dynamical arrest transition occurs. The dynamic of an arrested system has been characterized in a novel way by a group of researchers from our Department in a recent experiment performed at the beamline ID10 of the European Synchrotron ESRF in Grenoble.
Strange "Phase Transitions" are frequent in Soft Matter, which are due to an arrest of the dynamic of the system with no structural transition associated. Examples are the processes that lead to the formation of gels, as well as the glass transition, which is responsible of many properties of polymers and of various foods including butter and chocolate. To date there is no unifying theory describing all these transitions, at difference with the "classic" phase transitions such as e.g. crystallization or evaporation. However, there is a broad consensus that these transitions are characterized by heterogeneous and non-Gaussian dynamics . This is evident in numerical simulations, but direct experimental evidence is scarce.
A recent experiment of X-ray photon correlation spectroscopy (XPCS), described in a paper due to appear in Physical Review Letters, has allowed the direct measurement of space-time correlation functions in a two-dimensional gel formed by gold nanoparticles at the air-water interface. In particular, four times correlation functions have been measured for the first time in this way, allowing to identify and characterize directly the presence of dynamic heterogeneities in the sample. The result is that, with increasing particle concentration, relaxation times increase in parallel with the increase of interfacial mechanical moduli. More importantly, the tendency of nanoparticles to move in a collective fashion has been quantified, as the degree of cooperativity grows on approaching the arrested state.
It is expected that this experimental result shall help to rationalize the complex phenomenology of phase transitions due to dynamical arrest.
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