Slow dynamics in gelation phenomena: From chemical gels to colloidal glasses

Abstract

We here discuss the results of three-dimensional Monte Carlo simulations of a minimal lattice model for gelling systems. We focus on the dynamics investigated by means of the time autocorrelation function of the density fluctuations and the particle mean-square displacement. We start from the case of chemical gelation, i.e., with permanent bonds, and characterize the critical dynamics as determined by the formation of the percolating cluster, as actually observed in polymer gels. By opportunely introducing a finite bond lifetime tau(b), the dynamics displays relevant changes and eventually the onset of a glassy regime. This has been interpreted in terms of a crossover to dynamics more typical of colloidal systems and a connection between classical gelation and recent results on colloidal systems is suggested. By systematically comparing the results in the case of permanent bonds to finite bond lifetime, the crossover and the glassy regime can be understood in terms of effective clusters.

Heterogeneous Diffusion in a Reversible Gel

“…In Fig. 3-b we show that viscous flow is mostly controlled by τ link , the rate for polymer extraction [2,15]. Flow in this system occurs when the percolating network slowly rearranges through polymer moves [12].…”

“…Gelation is thus qualitatively different from a glass transition where the plateau height remains constant with a dramatic increase of relaxation timescales [16]. Coincidence of gelation and percolation, put forward in [2] or dispelled in [7], happens whenever long-lived bonds make cluster restructuration very slow, but does not occur in systems where the bond lifetime is short at percolation [4].…”

“…Inspired by recent experimental work on gelation, a variety of "minimal" models have recently been studied to elucidate the connection between gelation and seemingly related phenomena: Geometric percolation [2,3], glass transition [4,5,6], phase separation [7,8]. Detailed experiments performed with colloidal particles with tunable interactions [9] revealed that a non-trivial interplay between phase separation and kinetic arrest may produce gel-like structures.…”

Relaxation dynamics in a transient network fluid with competing gel and glass phases

“…The cross linking of the permanent bonds between the constituent monomers happens during the synthesis process inducing a vulcanization transition once the density of the links exceeds the percolation threshold [8]. Different static and dynamical properties in the vicinity of this transition have been studied, both using simulations and theoretical models (e.g., see [9,10]). Physical gels are on the other hand low-density network structures with bonds that can be broken/realigned by thermal fluctuations within finite timescales [7].…”

“…In general, these paths lead to spatially heterogeneous structures. However, in recent times, considerable effort has been made to devise ways by which spatially homogeneous physical gels can be formed [10,[17][18][19][20][21][22][23][24][25][26]. For such gel-forming systems, a wide variety of relaxation functions have been reported: logarithmic [27][28][29][30], stretched [16] or compressed exponentials [21,31].…”