Final Activity Report Summary - SLODYGEL (Slow dynamics and structural arrest in gelation phenomena)
Gels are a special case of slow and disordered systems: Due to the low volume fraction, their slow dynamics is intimately related to the formation of an open structure. The understanding of the connection between the structure of these systems and their complex dynamics is fundamental in the research on complex fluids, such as biological systems, polymeric materials, and colloids.
We have studied, by means of different models and approaches, the connections between structure and dynamics in a number of gelling systems. The main results are listed in the following points:
1) In colloidal suspensions with competing attraction and repulsion, at low volume fraction and temperature, dynamical arrest occurs in the experiments and in numerical simulations via the growth of elongated structures, that aggregate to form a connected network at gelation. We have been able to show for the first time that, in the region of parameter space where gelation occurs, the stable thermodynamical phase is a crystalline columnar one. Near and above the gelation threshold, the disordered spanning network slowly evolves and finally orders to form the crystalline structure. These findings strongly suggests that the transition to the gel phase observed in experiments happens in a super-cooled region, namely in a disordered phase which is metastable with respect to crystallisation. Of course the precursors of the modulated structures will have defects such as local inhomogeneities and branching points. Moreover, the viscosity of the solvent and to the imperfect shape of the particles may also hinder the ordered phase, producing long-living metastable disordered states.
2) The increase of the viscosity observed in attractive colloidal systems by varying the temperature or the volume fraction, can be related to the formation of structures due to particle aggregation. In particular we have studied the non-trivial dependence of the viscosity from the temperature and the volume fraction in the copolymer-micellar system L64 in collaboration with S.H. Chen at MIT and F. Mallamace (Italy). The comparison of the experimental data with the results of numerical simulations in a simple model for gelation phenomena suggests that this intriguing behaviour can be explained in terms of cluster formation and that this picture can be quite generally extended to other attractive colloidal systems.
3) We have analysed the heterogeneities of the dynamics in terms of the fluctuations of the intermediate scattering functions in a model for irreversible gels: We have shown that this dynamical susceptibility increases with the time until it reaches a plateau. At the lowest wave vector, approaching the gelation threshold it diverges with the same exponent as the mean cluster size. Our findings suggest a new alternative way of measuring critical exponents in a system undergoing chemical gelation and contribute to the understanding of connections between gels and other disordered systems.
4) In direct collaboration with D. Hellio-Serughetti and M. Djabourov at ESPCI we have studied the kinetic of bond formation in crosslinking of gelatin gels. In chemical crosslinking of gelatin solutions, two different time scales affect the kinetics of the gel formation in the experiments. We have complemented the experimental study with a numerical study. This approach has shown that the two characteristic time scales are related to the formation of single bonds crosslinker-chain and of bridges between chains. In particular we have shown that their ratio turns out to control the kinetics of the gel formation.
5) Within the collaboration with W. Kob at LCVN (France), we have been able to show that, due to the presence of the open spanning network the relaxation dynamics in colloidal gels at low volume fraction shows a non-trivial dependence on the wave-vector which is very different from the one observed in dense glass-forming liquids. At high wave vectors the relaxation is due to the fast cooperative motion of the branches of the gel network, whereas at low wave vectors the overall rearrangements of the heterogeneous structure produce the relaxation process.
We have studied, by means of different models and approaches, the connections between structure and dynamics in a number of gelling systems. The main results are listed in the following points:
1) In colloidal suspensions with competing attraction and repulsion, at low volume fraction and temperature, dynamical arrest occurs in the experiments and in numerical simulations via the growth of elongated structures, that aggregate to form a connected network at gelation. We have been able to show for the first time that, in the region of parameter space where gelation occurs, the stable thermodynamical phase is a crystalline columnar one. Near and above the gelation threshold, the disordered spanning network slowly evolves and finally orders to form the crystalline structure. These findings strongly suggests that the transition to the gel phase observed in experiments happens in a super-cooled region, namely in a disordered phase which is metastable with respect to crystallisation. Of course the precursors of the modulated structures will have defects such as local inhomogeneities and branching points. Moreover, the viscosity of the solvent and to the imperfect shape of the particles may also hinder the ordered phase, producing long-living metastable disordered states.
2) The increase of the viscosity observed in attractive colloidal systems by varying the temperature or the volume fraction, can be related to the formation of structures due to particle aggregation. In particular we have studied the non-trivial dependence of the viscosity from the temperature and the volume fraction in the copolymer-micellar system L64 in collaboration with S.H. Chen at MIT and F. Mallamace (Italy). The comparison of the experimental data with the results of numerical simulations in a simple model for gelation phenomena suggests that this intriguing behaviour can be explained in terms of cluster formation and that this picture can be quite generally extended to other attractive colloidal systems.
3) We have analysed the heterogeneities of the dynamics in terms of the fluctuations of the intermediate scattering functions in a model for irreversible gels: We have shown that this dynamical susceptibility increases with the time until it reaches a plateau. At the lowest wave vector, approaching the gelation threshold it diverges with the same exponent as the mean cluster size. Our findings suggest a new alternative way of measuring critical exponents in a system undergoing chemical gelation and contribute to the understanding of connections between gels and other disordered systems.
4) In direct collaboration with D. Hellio-Serughetti and M. Djabourov at ESPCI we have studied the kinetic of bond formation in crosslinking of gelatin gels. In chemical crosslinking of gelatin solutions, two different time scales affect the kinetics of the gel formation in the experiments. We have complemented the experimental study with a numerical study. This approach has shown that the two characteristic time scales are related to the formation of single bonds crosslinker-chain and of bridges between chains. In particular we have shown that their ratio turns out to control the kinetics of the gel formation.
5) Within the collaboration with W. Kob at LCVN (France), we have been able to show that, due to the presence of the open spanning network the relaxation dynamics in colloidal gels at low volume fraction shows a non-trivial dependence on the wave-vector which is very different from the one observed in dense glass-forming liquids. At high wave vectors the relaxation is due to the fast cooperative motion of the branches of the gel network, whereas at low wave vectors the overall rearrangements of the heterogeneous structure produce the relaxation process.