Abstract
Rheology of microfibrillated cellulose (MFC) water
suspensions was characterized with a rotational
rheometer, augmented with optical coherence tomography
(OCT). To the best of the authors' knowledge, this is the
first time the behavior of MFC in the rheometer gap was
characterized by this real-time imaging method. Two
concentrations, 0.5 and 1 wt% were used, the latter also
with 10-3 and 10-2 M NaCl. The aim was to follow the
structure of the suspensions in a rotational rheometer
during the measurements and observe wall depletion and
other factors that can interfere with the rheological
results. The stepped flow measurements were performed
using a transparent cylindrical measuring system and
combining the optical information to rheological
parameters. OCT allows imaging in radial direction from
the outer geometry boundary to the inner geometry
boundary making both the shear rate profile and the
structure of the suspension visible through the rheometer
gap. Yield stress and maximum wall stress were determined
by start-up of steady shear and logarithmic stress ramp
methods and they both reflected in the stepped flow
measurements. Above yield stress, floc size was inversely
proportional to shear rate. Below the yield stress, flocs
adhered to each other and the observed apparent constant
shear stress was controlled by flow in the depleted
boundary layer. With higher ionic strength (10-2 M NaCl),
the combination of yield stress and wall depletion
favored the formation of vertical, cylindrical, rotating
floc structures (rollers) coupled with a thicker water
layer originating at the suspension-inner cylinder
boundary at low shear rates
Original language | English |
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Pages (from-to) | 1261-1275 |
Number of pages | 14 |
Journal | Cellulose |
Volume | 21 |
Issue number | 3 |
DOIs | |
Publication status | Published - 2014 |
MoE publication type | A1 Journal article-refereed |
Keywords
- Flocculation
- microfibrillated cellulose
- optical coherence tomography
- rheology
- wall depletion
- yield stress