Shearing particle monolayers: Strain-rate frequency superposition

We report surface shear rheological measurements on monolayers of silica nanoparticles at the air-water interface. We have used the method of strain-rate frequency superposition (SRFS) to characterize the structural relaxation. We show that the rheological properties of the layers have the same universal linear and nonlinear behavior as three-dimensional soft materials. We also discuss the original healing properties of these monolayers.

Figure 1

(Color online) Strain amplitude sweep of a silica particle (possessing 34% SiOH) monolayer at the air-water surface. $\Gamma =50\text{mg}{\text{m}}^{-2}$ and $\omega =2\text{Hz}$.

Figure 2

Surface pressure and BAM images of the silica particle monolayer (34% SiOH) at the air-water surface (a) Surface pressure vs surface concentration. (b)-(d) BAM images at surface concentrations equal to (b) $20\text{mg}{\text{m}}^{-2}$, (c) $50\text{mg}{\text{m}}^{-2}$ and (d) same as (c) but fractured just after the beginning of an expansion. The scale bars correspond to 1 mm.

Figure 3

BAM images of a fracture in the monolayer of particles with $x=34\mathrm{\%}$ at a surface concentration equal to $50\text{mg}{\text{m}}^{-2}$, after (a) 1 s, (b) 300 s, (c) 3600 s. The fracture is no longer visible at this scale after 15 h. The scale bars correspond to $500\mu \text{m}$, and the arrows indicate the position of the fracture.

Figure 4

(Color online) Rapid recovery and self-healing of moduli of silica particle (34% SiOH) monolayer, $\Gamma =50\text{mg}{\text{m}}^{-2}$. (a) Strain amplitude as a function of time for different cycles. (b) Variation of $G^{\prime }$ and $G^{″}$ in multicycle amplitude sweep. The inset graphic shows the moduli relaxation dynamics in interval 4. The numbers in the top figure correspond to the different stress cycles as explained in the text.

Figure 5

(Color online) Strain rate frequency superposition measurements of monolayers with $\Gamma =50\text{mg}{\text{m}}^{-2}$. Data were taken for different ${\dot{\gamma }}_0$ varying from ${10}^{-4}$ to 1.5 or $2.0{\text{s}}^{-1}$. Left curves: constant shear rate sweep measurements shifted onto a single master curve: (a) $x=20\mathrm{\%}$, (c) $x=34\mathrm{\%}$, (e) $x=62\mathrm{\%}$. Right curves: corresponding amplitudes (a, right ordinate) and frequency (b, left ordinate) scaling factors vs ${\dot{\gamma }}_0$.