Ageing, yielding, and rheology of nanocrystalline cellulose suspensions
Autocorrelation functions of the scattered intensity in the VV and VH geometries together with the ILT for a dilute NCC of g/ml at 20 °C. Top inset: VV and VH decay rates as a function of q 2. Bottom inset: Absolute light scattering intensity in the VV geometry in the Holtzer representation. The solid lines denote the form factor of nearly rigid rods of length L = 270 nm and diameter of d = 20 nm with size polydispersity of PD = 2.
Polarized optical micrographs of a 10 wt. % NCC suspension (a) at rest, and shear rates of (b) 0.1 s−1, (c) 1 s−1, and (d) 10 s−1. The arrows denote the direction of flow and cross polarizers.
(a) Frequency sweeps on a 10 wt. % NCC suspension. Black symbols are the results obtained prior to the experimental testing and white symbols are the results after performing all the experiments. Frequency dependence of G′ and G′′ falls on the same curve indicating the absence of artifacts, such as evaporation. (b)Dynamic strain sweep at ω = 1 Hz (6.28 rad/s) and 10 Hz (62.8 rad/s).
(a) Time sweeps for a 10 wt. % NCC at various strains and ω = 10 Hz (62.8 rad/s) measured for 1000 s. NCC samples exhibit aging as G′ begins to increase after a critical time (b) Viscosity with time at constant shear rates. Viscosity increases more rapidly with time at lower applied shear rates. At higher rates, the viscosity remains constant over the time period examined here.
Correlation functions for a 10 wt. % NCC sample under oscillatory shear measured from 0 to 300 s at (a) ω = 1 Hz (6.28 rad/s) and (b) ω = 10 Hz (62.8 rad/s). The evolution of the first echo is shown for various strains. The echo amplitude drops significantly from 1 at γ0 = 15% and γ0 = 27% at ω = 1 Hz (6.28 rad/s) and ω = 10 Hz (62.8 rad/s), respectively, exhibiting shear-induced irreversible rearrangement (yielding). This occurs at a strain corresponding to the cross-over of G′ and G′′ as shown in Fig. 3(b) . The inset in (b) shows the initial decay rate with strain at ω = 10 Hz (62.8 rad/s). The linear dependence indicates the absence of wall slip.
Time dependence of echo heights, P(mT), at several strains and ω = 10 Hz (62.8 rad/s). The solid line represents the correlation function under application of no shear (Brownian motions). The deviation from “no shear” correlation function is an indication of shear-induced irreversible rearrangements.
The strain dependence of the first and sixth echo heights at (a) ω = 1 Hz (6.28 rad/s) and (b) ω = 10 Hz (62.8 rad/s). Insets show the strain dependence of the relaxation time and power index obtained by the fits of a stretched-exponential function. The solid lines are drawn as an eye-guide.
Time dependence of echo heights, P, at several strains for a 10 wt. % NCC suspension at ω = 10 Hz (62.8 rad/s) measured from 600 to 900 s.
The strain dependence of the first and sixth echo heights for a 10 wt. % NCC sample under oscillatory shear [ω = 10 Hz (62.8 rad/s)] measured from 600 to 900 s. There is no strain dependence in the height of the first echo below γ 0 ∼ 33% at which point sample yields irreversibly. Inset shows the relaxation time and power index obtained by the fits of a stretched-exponential function.
Article metrics loading...
Full text loading...