Signatures of Structural Recovery in Colloidal Glasses

Colloids near the glass concentration are often taken as models for molecular glasses. Yet, an important aspect of the dynamics of molecular glasses, structural recovery, has not been elucidated in colloids. We take advantage of a thermosensitive colloidal suspension to study the structural recovery after concentration jumps by using diffusing wave spectroscopy. The three classical aging signatures observed in molecular glasses are studied and the results are compared with those typical of molecular glasses. For the intrinsic isotherms, unlike molecular glasses, the colloid shows huge changes in relaxation time at equilibrium while the times required to reach the equilibrium state are nearly constant. For asymmetry of approach, we find a similar nonlinearity to that observed in the molecular glasses. For the memory experiment, while a memory effect is seen, the response is qualitatively different from that in molecular glasses.

Figure 1

(a) Autocorrelation function $g_2$ vs $t$ at different aging time $t_w$ for a typical run in a temperature down-jump from $32$ to $31.3°\mathrm{C}$. (b) Master curve of $g_2$ vs $t/a_{tw}$ from time-aging time superposition.

Figure 2

(a) Relaxation time for a PNIPAAM colloidal suspension as a function of aging time at different temperature after temperature down-jump from $32°\mathrm{C}$. (b) Equilibrium relaxation time vs temperature, consistent with glasslike behavior.

Figure 3

Comparison of results from intrinsic isotherms experiments for (a) colloid (start temperature $32°\mathrm{C}$); (b) molecular glass (start temperature $40°\mathrm{C}$). (Data from Kovacs [5], figure from Zheng and McKenna [13].)

Figure 4

Comparison of the asymmetry of approach experiment results for (a) colloid, (b) molecular glass. (Data from Kovacs [5], figure from Zheng and McKenna [13].)

Figure 5

Comparison of the memory effect experiment results for (a) colloid, $T_0=32°\mathrm{C}$, waiting $\mathrm{\text{time}}=150\mathrm{s}$, $T_{\mathrm{Partial}\mathrm{recovery}}=31.2°\mathrm{C}$, $T_{\mathrm{final}}=31.5°\mathrm{C}$, the inset is the enlargement memory effect curve; (b) molecular glass. (Data from Kovacs [5], figure from Zheng and McKenna [13].)