Memory in aging colloidal gels with time-varying attraction

We report a study of gel formation and aging in suspensions of nanocolloidal spheres with a short-range attraction whose strength is tuned by changing temperature (T). Following a quench to a T below the gel point, the suspensions form gels that age through an increasing elastic shear modulus (G') and slowing, increasingly constrained microscopic dynamics. When the attraction strength is suddenly decreased during aging, the gel properties evolve non-monotonically like in the Kovacs effect in glasses, in which G' decreases and the microscopic dynamics become less constrained for a period before more conventional aging resumes. Eventually, the properties converge to those of a gel that has undergone aging at the lower attraction strength throughout. The time scale of the convergence increases as a power law with the age at which the attraction strength is decreased and decreases exponentially with the magnitude of the decrease. A model for gel aging in which particles attach and detach from the gel at rates that depend on their contact number qualitatively reproduces these trends and reveals that the non-monotonic behavior results from the dispersion in the rates at which the populations of particles with different contact number adjust to the new attraction strength.