The phenomenology of the glass transition in complex fluids, especially molecular liquids, is reasonably well understood. At the same time, there is a significant amount of work on the “nominal” jamming transition in colloids that make a strong link between the glass transition in colloidal systems and the glass transition in molecular systems. The question that is relevant to the present investigation is “what are the limits of validity for this analogy between the two systems?” To address this question, we have undertaken a series of experiments on aqueous dispersions made of PNIPAAM particles that are thermos-responsive, i.e., upon changing the temperature one changes the particle diameter. Hence, at constant number fraction of particles, one can change the concentration of the dispersion by changing the temperature. Because the volume fraction is the control variable (in equilibrium) of the dynamics of the colloidal dispersion, this makes possible to carry out concentration jumps in the neighborhood of the colloidal glass transition to mimic the classical experiments performed by Kovacs in the characterization of the structural recovery of glassy materials in the neighborhood of the glass transition temperature. The “Kovacs signatures” are: 1) the intrinsic isotherms; 2) asymmetry of approach; and 3) memory effect that result from performing temperature jumps near to the glass temperature. The three important effects evidenced by these experiments are a) the volume (or structure) of the glass varies with a kinetics that slows rapidly with distance below the glass transition (intrinsic isotherms); b) the structural recovery is nonlinear as evidenced in the asymmetry of approach experiment; c) the response to complex histories is additive in the Boltzmann sense of linear superposition of the responses in reduced time (memory effect).
By carrying out temperature (concentration) jumps using the thermos-responsive particles in the vicinity of the glass concentration we have been able to establish points of similarity and difference between the behaviors of the colloidal glasses and molecular glasses in terms of the Kovacs signatures. Of importance is that the intrinsic isotherm type of experiment, while showing rapid divergence of the relaxation time of the material, does not show the same divergence of behaviors for the structural recovery times to achieve equilibrium. This is unlike molecular glasses where the times for both increase rapidly (at least exponentially if not super-exponentially) as the temperature below the glass transition decreases. Furthermore, while the asymmetry of approach occurs in the colloidal glass, the asymmetry is qualitatively different especially in that the time for the approach to equilibrium is the same for both the up-jump and down-jump conditions, whereas for molecular glasses the up-jump condition takes upward of one order of magnitude in time to achieve equilibrium. Also, we find that the memory effect in the colloidal glass is different from that of the molecular glass and, in some colloidal systems is even missing. As a result, we suggest that the differences demonstrate a need to revisit the view of the colloidal glass as a model for the glass transition event in molecular systems.
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