Nonlinear Viscoelastic Dynamics of Nanoconfined Wetting Liquids

The viscoelastic dynamics of nanoconfined wetting liquids is studied by means of atomic force microscopy. We observe a nonlinear viscoelastic behavior remarkably similar to that widely observed in metastable complex fluids. We show that the origin of the measured nonlinear viscoelasticity in nanoconfined water and silicon oil is a strain rate dependent relaxation time and slow dynamics. By measuring the viscoelastic modulus at different frequencies and strains, we find that the intrinsic relaxation time of nanoconfined water is in the range 0.1–0.0001 s, orders of magnitude longer than that of bulk water, and comparable to the dielectric relaxation time measured in supercooled water at 170–210 K.

Figure 1

$F_L$ and $\omega$ in water as a function of $d$ at $\omega =955.3\mathrm{Hz}$, and for three different $X_0$ values, (a) $X_0=0.4\mathrm{nm}$. (b) $X_0=0.66\mathrm{nm}$. (c) $X_0=1.32\mathrm{nm}$. The phase for $d>1\mathrm{nm}$ is not shown because it fluctuates randomly.

Figure 2

$G^{\prime }$ and $G^{\prime \prime }$ in water as a function of tip-sample distance. The shadowed area, $d<0.2\mathrm{nm}$, is not discussed in this Letter because the gap size is smaller than a water molecular dimension. $\omega$ is 955.3 Hz, and the $X_0$ is 0.4 nm for (a’) and (a”), 0.66 nm for (b’) and (b”), and 1.32 nm for (c’) and (c”).

Figure 3

At $d=0.4\mathrm{nm}$, $G^{\prime }$ and $G^{\prime \prime }$ in water as a function of ${\gamma }_0=X_0/d$, and $\omega$ is 52.02 Hz for (a’) and (a”), 955.3 Hz for (b’) and (b”), and 1.9689 kHz for (c’) and (c”). The insets show the results for OMCTS at $d=1.4\mathrm{nm}$.

Figure 4

$\tau$ vs $\dot{{\gamma }_0}$ for water at $d=0.4\mathrm{nm}$. The dashed line is the fitting with Eq. (4) for $K=0.95\pm 1.49$ and $\nu =0.84\pm 0.29$ [20]. In the inset, the results are shown for OMCTS at $d=1.4\mathrm{nm}$.