Glass Transition and Aging in Dense Suspensions of Thermosensitive Microgel Particles

We report a thermosensitive microgel suspension that can be tuned reversibly between the glass state at low temperature and the liquid state at high temperature. Unlike hard spheres, we find that the glass transition for these suspensions is governed by both the volume fraction and the softness of the particles, where softer suspensions form a glass at higher effective volume fractions. In the glass state, these suspensions show aging where the relaxation times increase linearly with age, irrespective of the degree of particle softness. This relaxation scaling is in contrast with hard sphere behavior but consistent with the soft glassy rheology model.

Figure 1

$G^{\prime }$ (open symbols) and $G^{\prime \prime }$ (solid symbols) of a $7\%\mathrm{w}/\mathrm{w}$ suspension at $25°\mathrm{C}$ plotted versus $\omega$ (a) or $\omega t$ (b) for $t_w=3$ (○), 30 (□), 300 (▽), and 3000 s (△). Lines represent the SGR model ($x=0.55$, $G_p=410\mathrm{Pa}$).

Figure 2

Evolution of $G^{\prime }$ (solid symbols) and $G^{\prime \prime }$ (open symbols) of the $7\%\mathrm{w}/\mathrm{w}$ suspension from glassy, at $T=35$ (a) and $37°\mathrm{C}$ (b), to liquid behavior at $T=38$ (c) and $40°\mathrm{C}$ (d). Lines are the best fitting SGR curves. Data in (a) and (b) were plotted versus $\omega t$ to collapse the curves for $t_w=3$ (○), 30 (□), and 300 s (▽). Data in (c) and (d) were plotted versus $\omega$ to reach collapse.

Figure 3

(a) Relative noise temperature $x$ versus $T$ at $c=3.9$ (△), 5 (◇), 7 (○), and $8\%\mathrm{w}/\mathrm{w}$ (□). (b) The same data as a function of $\varphi$. (c) Phase behavior (△: liquid, ▲: glass) in the $c\mathrm{\text{−}}T$ plane and the transition temperature $T_g(c)$ (line). (d) Transition volume fraction ${\varphi }_g$ versus the particle elasticity $G_p$. Lines are a guide to the eye.

Figure 4

$G^{\prime }$ (open symbols) and $G^{\prime \prime }$ (solid symbols) of two different suspensions with $\varphi =1.85$ plotted versus $\omega t$ for $t_w=3$ (△), 30 (▽), 300 (◇), 3000 (□), and 30 000 s (○). (a) $c=3.9\%\mathrm{w}/\mathrm{w}$ at $20°\mathrm{C}$; lines are the best fitting SGR curves for $\omega t<100$ $x=0.53\pm 0.02$, $G_p=12\pm 1\mathrm{Pa}$). (b) $c=8\%\mathrm{w}/\mathrm{w}$ at $35°\mathrm{C}$ ($x=0.72\pm 0.03$, $G_p=245\pm 30\mathrm{Pa}$).