Dynamic structure of thermoreversible colloidal gels of adhesive spheres
Abstract
The dynamic structure factor f(q,t) of suspensions of adhesive colloidal spheres has been characterized as a function of temperature over the volume fraction range 0.010<φ<0.075. Below a critical temperature that is volume fraction dependent, the suspensions underwent an abrupt, reversible transition in dynamic structure. Below their gel points suspensions became nonergodic, and the time decay of f(q,t) was arrested by as many as five decades. Static light scattering demonstrated that the adhesive spheres formed a fractal cluster gel structure. A recent model of the dynamics of fractal clusters [A. H. Krall and D. A. Weitz, Phys. Rev. Lett. 80, 778 (1998)] was applied to extract the temperature and volume fraction dependence of the characteristic decay times of f(q,t). Immediately above the gel temperature a single stretched exponential decay of f(q,t) was observed. The temperature dependence of the decay time was τ_{α}~ɛ^{1.15+/0.06}, where ɛ=(TT_{gel})/T_{gel}, and T_{gel} is the gelation temperature. The argument of the stretched exponential decay p decreased monotonically as the temperature was lowered toward the gel point, until, at gelation, p~0.5. Below the gel temperature, an initial stretched exponential decay of f(q,t) was followed by a plateau. Finally, at long times, an additional exponential decay of the gel f(q,t) was observed. By applying the fractal cluster dynamics model, it was found that the initial decay time, τ_{β}~ɛ^{1.00+/0.07}. The plateau in f(q,t) was due to an upper bound of the meansquared displacement of gel segments, denoted δ^{2}. The typical magnitude of δ^{2} was not much greater than the square of the particle radius. The data showed δ^{2}~ɛ^{1.05+/0.07}. The additional exponential decay at long times, τ_{γ}, depended only weakly on ɛ. Its dependence on the scattering vector was τ_{γ}~q^{0.53+/0.06}. The argument of the stretched exponential decay of the gel f(q,t) and volume fraction dependence of τ_{β} and δ^{2} indicate that the spatial scaling of the gel compliance is consistent with the gel network bonds possessing angular rigidity. The ɛ dependence of the characteristic times τ_{α} and τ_{β} could not be fully explained by the fractal cluster dynamics model. The long time decay of f(q,t) exhibited behavior that differed from that recently reported for dilute gels of aqueous colloidal polystyrene [Cipelletti et al., Phys. Rev. Lett. 84, 2275 (2000)]. We hypothesize that the longtime decay in f(q,t) of the gels studied here is due to rare bond disaggregation processes that occur because of the relatively weak interaction between the adhesive spheres (ΔE_{min}/kT~10) of the thermoreversible gel.
 Publication:

Physical Review E
 Pub Date:
 May 2001
 DOI:
 10.1103/PhysRevE.63.051402
 Bibcode:
 2001PhRvE..63e1402S
 Keywords:

 82.70.Gg;
 61.20.Lc;
 82.70.Dd;
 61.43.Hv;
 Gels and sols;
 Timedependent properties;
 relaxation;
 Colloids;
 Fractals;
 macroscopic aggregates