Flow and Fracture in Drying Nanoparticle Suspensions

Drying aqueous suspensions of monodisperse silica nanoparticles can fracture in remarkable patterns. As the material solidifies, evenly spaced cracks invade from the drying surface, with individual cracks undergoing intermittent motion. We show that the growth of cracks is limited by the advancement of the compaction front, which is governed by a balance of evaporation and flow of fluid at the drying surface. Surprisingly, the macroscopic dynamics of drying show signatures of molecular-scale fluid effects.

Figure 1

Bright-field image of a drying suspension of silica nanoparticles. Inset: a schematic of the experimental geometry. Water evaporates from the drying edge, $y=0$. The crack tips extend to $y_C$, just behind the compaction front, at $y_F$. The inner meniscus lies at $y_M$.

Figure 2

(a) Trajectories of the tips of two cracks in the same sample. (b) The average position of all the crack tips in a single sample plotted against the square root of time.

Figure 3

(a) Compaction front trajectories in suspensions of spheres with radii from 6 to 26 nm, offset for clarity. (b) When scaled by fitted values of $y_0$ and $t_0$, the trajectories collapse onto the predictions of Eq. (2), solid line. (c) The dependence of the crossover length, $y_0$, with sphere radius, $a$.

Figure 4

(a) Compaction front trajectories in suspensions varying only in initial volume fraction. (b) When scaled by fitted values of $y_0$ and $t_0$, the trajectories collapse onto the predictions of Eq. (2), solid line. (c) The dependence of the crossover length, $y_0$, with the initial volume fraction, ${\varphi }_D$.