Dipartimento di Fisica

In a recent numerical study published in Nature Physics, Nicoletta Gnan and Emanuela Zaccarelli, researchers from Institute of Complex Systems of CNR  (CNR-ISC) working in the Physics Department, have shown how the dynamic response of a dispersion of soft colloids is influenced by the ability to deform of the single constituents. Soft colloids are mesoscopic particles often made from polymeric material, i.e. flexible chains that provide elasticity to the particle, so that it can deswell and deform when densely packed.
Understanding their behaviour at high concentrations remains a challenge for modern soft matter physics and, in particular,  an important open question is whether softness controls the dynamics in such dense conditions. Despite several experimental attempts to establish a link between elasticity and dynamical behaviour, the lack of appropriate theoretical models able to capture the key ingredients of soft particles did not allow to provide a conclusive answer to this question. In this recent study, the team introduced a new numerical model of soft particles with an internal elasticity, represented by polymer rings, for simplicity studied in 2D, whose circular shape is maintained thanks to an additional Hertzian field in their centre.  The stronger the elastic interactions, the harder are the polymeric rings, while a smaller elasticity provides softer rings.
The connectivity, embedded in the model, allows them to deform, spontaneously storing the elastic energy (stress) which they then release when they manage to restore their circular shape. The ability to deform has a strong impact on the dependence of the relaxation time on the packing fraction which change from fragile-to-strong as observed in experiments of ionic microgels close to the glass transition. In addition, the dynamics is controlled by an intermittent particle motion which gives rise to a compressed exponential decay of the self-intermediate scattering function, also observed in highly stressed systems such as colloidal gels. This behavior can be rationalized in terms of deformed rings that act as an internal source of stress. This simple model thus provides microscopic insights into two mechanisms which are of a deep interest in soft matter: the fragility dependence on softness and the occurrence of a compressed exponential decay in dynamical correlation functions.

Reference:
Nicoletta Gnan and Emanuela Zaccarelli
Article - Nature Physics (2019)