- Influence of particle modulus on the rheological
properties of agar microgel suspensions
- S. Adams, W. J. Frith, and J. R.Stokes
- Fluids near a critical point obey a generalized Cox-Merz
rule
- Robert F. Berg
- Slip and flow in pastes of soft particles: Direct
observation and rheology
- Steven P. Meeker, Roger T. Bonnecaze, and Michel Cloitre
- Transient response of magnetorheological
fluids: Shear flow between concentric cylinders
- John C. Ulicny, Mark A. Golden, Chandra S. Namuduri, and Daniel J.
Klingenberg
- The linear viscoelastic behaviour of a series of
molecular weights of the TLCP HBA/HNA 73/2
- Elvira Somma and Maria Rossella Nobile
- Rheo-optical studies of the response of
entangled polymer solutions to step changes in shear rate
- J. P. Oberhauser, K. Pham, and L. G. Leal
- Thixotropy and rheopexy of aggregated dispersions
with wetting polymer
- Andrei Potanin
- A Successive Fine-Graining scheme for predicting
the rheological properties of dilute polymer solutions
- R. Prabhakar, J. Ravi Prakash, and T. Sridhar
- Nonlinear shear rheology of polystyrene melt with
narrow molecular weight distribution – experiment and theory
- Thomas Schweizer, Jan van Meerveld, and Hans Christian Öttinger
- Ordering kinetics of BCC morphology in diblock
copolymer solutions at low temperatures
- Zhizhong Liu, Montgomery T. Shaw, and Benjamin S. Hsiao
- Rheological properties of whey protein / gum
arabic coacervates
- Fanny Weinbreck, Roland H. W. Wientjes, Hans Nieuwenhuijse, Gerard W.
Robijn, and Cornelis G. de Kruif
- Inception of shear flow for ferromagnetic dispersions
- Meihua Piao, D. E. Nikles, A. M. Lane, and J. M. Wiest
- Effect of attractions on shear thickening in dense
suspensions
- V. Gopalakrishnan and C. F. Zukoski
S. Adams, W. J. Frith, and J. R.Stokes
Unilever R&D Colworth
Sharnbrook, Bedford MK44 1LQ. UK
Abstract
The linear viscoelastic and steady shear flow properties of high phase
volume suspensions of a range of agar microgel particles have been
measured and are found to depend upon the deformability (or modulus) of
the particles. Agar concentrations in the range 0.5 - 5 wt % are
utilised, giving a range of particle modulus spanning 2.4 to 185 kPa. On
increasing the particle modulus, in suspensions with phase volumes above
maximum packing, the storage modulus increases by two orders of
magnitude although the loss tangent (tan) also increases due to
increasing viscous dissipation. The flow properties of the suspensions
at high shear stresses also showed significant differences due to
changing particle rigidity. The suspensions containing the hardest
particles are found to display limited evidence of shear-thickening
behaviour at high stresses, while those containing the softest particles
continue to shear-thin. A high-shear plateau in the viscosity is
observed for suspensions with particles of medium rigidity. The
suspensions containing the stiffer particles also have a considerably
higher viscosity for the same degree of space filling. Empirical
expressions linking the viscoelastic and flow properties to the particle
modulus have been derived.
Return to top.
Robert F. Berg
Process Measurements Division
National Institute of Standards and Technology
Gaithersburg, MD 20899-8364
Abstract
Upon approaching the
liquid-vapor critical point, the spontaneous density fluctuations in a
small-molecule fluid increase in both size and lifetime. Similar
increases of the concentration fluctuations occur near the critical
mixing point of a binary liquid mixture. The presence of these large
fluctuations leads to an increase of the zero-shear viscosity, and their
persistence in time leads to viscoelasticity and shear thinning. These
rheological phenomena, which are already supported by theory and
experiment, are shown here to obey a generalized form of the Cox-Merz
rule. This relation formally equates the shear viscosity
h(g-dot)
measured at shear rate g-dot
with the magnitude of the linear complex viscosity
h*(w)
measured at frequency w.
Comparisons of theoretical results and experimental data obtained
elsewhere demonstrate that fluids near a critical point obey the
somewhat generalized form h(kCMg-dot
= w)
= |h*(w)|,
with kCM = 0.4. The demonstration is simplified by
showing that the Carreau-Yasuda model in the form (1 + bAg|g-dot
t|)-p
represents the theories for h(g-dot)
and |h*(w)|
near critical points. (The product bAgt
is a time constant, and p = 0.022 is a universal critical
exponent.)
Return to top.
Steven P. Meekera), Roger T. Bonnecazeb),
and Michel Cloitrea,c)
a) Laboratoire Matičre Molle et Chimie (UMR
167, ESPCI-CNRS)
ESPCI, 10 rue Vauquelin, 75321 Paris Cedex, France
b) Department of Chemical Engineering and Texas
Materials Institute
The University of Texas at Austin
Austin, TX 78712, USA
Abstract
Microgel pastes and concentrated emulsions are shown to exhibit a
generic slip behavior at low stresses when sheared near smooth surfaces.
The magnitude of slip depends on the applied stress. Well above the
yield stress, slip is negligible compared to the bulk flow. Just above
the yield stress, slip becomes significant and the total deformation
results from a combination of bulk flow and slip. At and below the yield
stress, the bulk flow is negligible and the apparent motion is entirely
due to wall slip. By directly imaging the deformation of pastes and from
rheological measurements, we show that slip is characterized by
universal scaling properties, which depend on solvent viscosity, bulk
shear modulus and particle size. A model based on elastohydrodynamic
lubrication between the squeezed particles and the shearing surface
explains these properties quantitatively.
c)
Corresponding author :
michel.cloitre@espci.fr.
Return to top.
John C. Ulicny and Mark A. Golden
Materials & Processes Lab
General Motors R&D and Planning
GM R&D and Planning, MC 480-106-224
30500 Mound Rd.,
Warren, MI 48090-9055
Chandra S. Namuduri
Electrical & Controls Integration Lab
General Motors R&D and Planning
GM R&D and Planning, MC 480-106-390
30500 Mound Rd.,
Warren, MI 48090-9055
Daniel J. Klingenberg
Department of Chemical and Biological Engineering
and Rheology Research Center
University of Wisconsin
1415 Engineering Dr.
Madison, WI 53706
Abstract
An experimental investigation of the rheological response of
magnetorheological (MR) suspensions subjected to step changes in applied
magnetic field strength at fixed shear rate is reported. For small
applied field strengths, the shear stress increases rapidly to a steady
value. Above a critical field strength, the rapid initial increase in
shear stress is followed by a slow, transient increase in stress. The
critical Mason number corresponding to the critical magnetic field
strength at the onset of this transient depends on the particle volume
fraction as well as the shear rate. This is in contrast to a previous
analysis where the critical Mason number was predicted to depend on only
the particle volume fraction. The discrepancy is attributed to colloidal
forces that are significant in our experimental system, but were not
included in the analysis. Further comparison with the previous analysis
requires either including the effects of colloidal forces, or performing
experiments with systems in which colloidal forces are not important.
Return to top.
Elvira Somma and Maria Rossella Nobilea)
Department of Chemical and Food Engineering
University of Salerno
Fisciano (Sa) 84084 Italy
Abstract
In this work the rheological properties of the thermotropic liquid
crystalline polymer HBA/HNA 73/27 produced by Ticona have been
investigated over the molecular weight range 30000 - 51000. The TLCPs
show linear viscoelastic behavior at small strain amplitudes, as
detected in strain sweep experiments performed at a constant frequency
of 1, 5, 10, 30, 50 and 100 rad/s. The onset for nonlinearity (gc)
significantly decreases (from ~35% to ~5%) as the applied
frequency (w) increases,
independently of the molecular weight. When the results are plotted
in terms of G*/G*0 (the complex modulus
normalised to the corresponding linear viscoelastic value) as a function
of the stress amplitude, the different curves do not seem to collapse
upon each other in the large frequency range investigated in this paper.
Indeed, the stress onset for nonlinearity increases as the applied
frequency (w) increases (sc
~ 700 Pa at w = 1 rad/s to
sc ~ 2000 Pa at
w = 100 rad/s for Mw =
51000), opposite to the trend shown by the strain onset. The behavior of
these TLCPs seems, therefore, intermediate between having a
characteristic strain and stress level for the occurrence of
nonlinearity. The nonlinear onset dependence on the frequency has been,
then, analyzed by horizontally shifting the normalised G*(g)
profiles curves at different w values
and a master curve has been obtained.
The frequency behavior of these TLCPs has also been studied showing that
the lower molecular weight samples are characterized by a prevalent viscous
behaviour at high frequencies (G’’ > G’), while decreasing
w the storage modulus tends to level off and
dominates the loss modulus. On the other hand, the higher molecular weight
samples (Mw = 51000 and Mw > 51000) show
a linear viscoelastic behaviour that resembles that of conventional flexible
polymers with a terminal regime at the lower frequencies and a transition to
the rubber-like region at higher w. Our
results support, then, the hypothesis suggested in the literature [Romo-Uribe
(2001)] of the existence of entanglements in the HBA/HNA 73/27 thermotropic
copolyester, whose contribution increase with increasing the molecular
weight.
a) Author to whom all correspondence should be
addressed. E-mail: mrnobile@unisa.it
Return to top.
J. P. Oberhausera), K. Pham, and L. G. Leal
Department of Chemical Engineering
University of California at Santa Barbara
Santa Barbara, California 93106-5080
Abstract
Experimental birefringence data are reported for an entangled 4.86 wt%
polystyrene solution involving a narrow molecular weight distribution,
8.42 × 106 g/mol polymer in tricresyl phosphate (TCP). The
average number of entanglements per chain was estimated to be
approximately 22. The flows considered involve step changes in shear
rate from one steady value to another (including the case in which the
two shear rates have the same sign and that in which the sign changes
and the flow direction is reversed). Comparisons were also made for
these flows with data obtained for the same polymer/solvent combination,
but at 3.00 wt% where the estimated number of entanglements per chain
was approximately 10. The range of shear rates was chosen to span
Weissenberg numbers based on the longest Rouse time of O(1) and
thus provide data that can be used to explore the effects of modest
chain stretching on the dynamics. Although the long-time behavior was
found to be qualitatively similar to a startup flow, the short-time
dynamics was significantly different. Qualitative considerations based
on our current ideas about the dynamics of entangled polymers do not
offer a full explanation of these short-time dynamics.
a) Present address: Department of Chemical
Engineering, University of Virginia, Charlottesville, VA 22904-4741;
electronic mail:
oberhauser@virginia.edu
Return to top.
Andrei Potanina)
2D-37, Imation Corp.
Oakdale, MN 55128-3414
Abstract
Different types of thixotropy of aggregated dispersions of magnetic
metal, non-magnetic iron oxide and carbon black particles in organic
solvents with a polymer containing pendant wetting groups are studied.
Thixotropic behavior depends on the type of aggregation which determines
the type of structural recovery in and after shear. Metal particles form
denser aggregates combined into flocs or networks recoverable in or
after shear, which results in positive thixotropy with up-shear stress-vs.-shear-rate
curves running above down-shear curves. By contrast, iron oxide and
carbon black particles form looser aggregates which re-flocculate slower
in shear and do not re-flocculate at all at rest after shear, which
results in rheopexy with recovery strongly accelerated by shear and
negatively thixotropic loops. Viscoelastic moduli of the latter systems
are determined by their shear history, e.g., G’ decreases with
the preceding shear rate and doesn’t recover. The effect of the wetting
polymer on time-dependant phenomena is twofold: it suppresses thixotropy
by consolidating aggregates, but slows down recovery, so that rheopexy
is maximized at certain polymer-to-pigment ratios. Furthermore, two
types of rheopexy are discriminated. Recovery rate of carbon black
dispersions is controlled by the current shear rate only, resulting in
thixotropic loops strongly dependent on sweep time. On the other hand,
recovery rate of iron oxide dispersions has a long memory of shear
history, which leads to thixotropic loops almost insensitive to sweep
time. The difference is attributed to slow shear-induced
dilation/shrinkage of aggregates built from elongated iron oxide
particles, as opposed to stable aggregates of carbon black.
a)
E-mail: apotanin@imation.com
Return to top.
R. Prabhakar, J. Ravi Prakasha), and T. Sridhar
Department of Chemical Engineering,
Monash University
Clayton, Victoria - 3800, Australia
Abstract
A new method for refining the predictions of a finitely extensible
bead-spring chain model with hydrodynamic interactions is introduced.
The suggested Successive Fine-Graining procedure involves the
representation of a long but finite macromolecule of NK
Kuhn-steps with a coarse-grained bead-spring chain model, and
subsequently fine-graining the representation by progressively
increasing the number of beads, N. Extrapolating the results
obtained using exact Brownian dynamics simulations for several values of
N to the limit N - 1 ® NK,
leads to improved estimates for the behaviour of the actual polymer
chains. The extrapolated results for the extensional viscosity are found
to be in good agreement with the recent experimental observations of
Gupta et al. [2000]. The scaling of the extensional viscosity with
molecular weight is also examined.
a) Corresponding author.
Return to top.
Thomas Schweizer, Jan van Meerveld, and Hans Christian
Öttinger
ETH Zurich, Department of Materials, Institute of
Polymers, CH-8093 Zurich
Abstract
Measurements of the shear viscosity and the first and second normal
stress coefficients are shown at 175°C for a nearly monodisperse
polystyrene melt with Mw = 200 kg/mol. Tests are
performed on a cone-partitioned plate shear rheometer and cover a range
of Weissenberg numbers (tdg-dot)
from 0.13 to 40. Experimental problems encountered are the axial
compliance of the rheometer and the normal force capacity of the
transducer. The later limits the maximum shear rate to
tdg-dot
= 40. Experimental data are compared with the models of Öttinger (termed
TCR, J. Rheol. 43 (1999) 1461-1493) for the convective constraint
release parameter d2 = 0,
1, and 2 and Mead, Larson & Doi (termed MLD, Macromolecules 31 (1998)
7895-7914) for d2 =1. The
steady state and transient values of p21, N2,
and N1 agree qualitatively well between both models
and the experiment. The predicted normal stress ratio –N2/N1
is sensitive to the magnitude of d2
in the TCR model, similar to the extinction angle. The MLD model yields
|N2| and Y values lower
than both experiments and the TCR model with d2
= 1. From a comparison with the chain stretch time
ts (0.065 s) it can be shown
that the overshoot O of |N2| and p21
are linked to chain orientation, whereas O(N1) is
associated with chain stretching. The magnitude of the overshoot for all
shear rates increases as O(N1) < O(N2)
< O(h) for PS 200k. In comparison, a
polydisperse polystyrene melt shows stronger shear thinning of –N2/N1
and an increase of the magnitude of the overshoot as O(N1)
< O(h) < O(N2).
Return to top.
Zhizhong Liu, Montgomery T. Shaw
Department of Chemical Engineering and Institute of Material Science,
University of Connecticut
Storrs, Connecticut 06269
Benjamin S. Hsiao
Department of Chemistry
State University of New York at Stony Brook
Stony Brook, New York 11794-3400
Abstract
The ordering kinetics of a
body-centered cubic (BCC) mesophase in squalane solutions of
poly(styrene-b-ethylene-alt-propylene) (SEP) at low temperatures
was studied using rheological and small-angle X-ray scattering (SAXS)
methods. At temperatures more than 110°C
below the order-to-disorder transition temperature (TODT),
the evolution of the storage modulus, G', and the absolute value
of the complex modulus, |G*|, was found to be describable by the
Avrami equation, assuming a “series” mechanical model for the
combination of a soft initial phase and a growing hard phase. The
ordering half time, t0.5, of the SEP/squalane
solutions decreased with temperature from 50 to 120°C
and increased with concentration from 5 to 10 wt%, indicating the
diffusion-controlled nature of the ordering process in these micellar
systems at low temperatures. The temperature dependence of t0.5
followed the Arrhenius relationship with the activation energy showing
negligible concentration dependence. Furthermore, the elastic modulus of
the BCC phase was found to increase almost linearly with temperature,
which is qualitatively consistent with the assumption of entropic
elasticity of the corona chains in the micellar BCC lattice.
Return to top.
Fanny Weinbrecka) and Roland H. W. Wientjes
NIZO food research
P.O. Box 20, 6710 BA Ede, The Netherlands
Hans Nieuwenhuijse and Gerard W. Robijn
FCDF Corporate Research
P.O. Box 87, 7400 AB Deventer, The Netherlands
Cornelis G. de Kruif
NIZO food research, P.O. Box 20, 6710 BA Ede, The Netherlands
Van’t Hoff Laboratory, Debye Research Institute
University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands
Abstract
Complex coacervation in whey protein (WP) / gum arabic (GA) mixtures
occurred in a specific pH window between 2.5 and 4.8. After phase
separation, a concentrated polymer (also called coacervate) phase was
obtained, whose visco-elastic properties were studied at various pH
values. The viscosity of the WP/GA coacervate exhibited a surprisingly
low shear-rate dependence, especially at pH 4.0, from 0.3 s-1
to 30 s-1 and a shear thinning above 30 s-1,
indicating a structural change. Hysteresis in the flow curve was
measured at the pH values at which the electrostatic interactions were
the strongest. Hysteresis was due to a slow structural rearrangement of
the coacervate phase and, with time, the initial viscosity was
completely recovered, showing that structural changes were reversible.
In frequency sweep experiments, the values of G’’ were up to 10
times higher than the values of G’, indicating the highly viscous
character of the coacervates. pH 4.0 appeared to be the pH at which the
coacervate phase was the most concentrated in biopolymer (Cp
= 32%), and at which the highest viscosity was measured. By decoupling
the effect of biopolymer concentration and electrostatic interactions,
it appeared that the high viscosity of the WP/GA coacervates was mainly
due to the strong electrostatic interactions between WP and GA at low
pH. The weaker the electrostatic interaction was, the lower the
viscosity, especially at pH 4.5 and 3.0. The viscous behavior of the
coacervates showed parallels with that of concentrated latex
dispersions. WP/GA particles would consist of a GA polymer chain (as in
latex) but now cross-linked by the electrostatic interactions with WP.
a)
Author to whom correspondence should be addressed; electronic mail:
fanny.weinbreck@nizo.nl
Return to top.
Meihua Piao, D. E. Nikles, A. M. Lane, and J. M. Wiest
Department of Chemical Engineering and
Center for Materials for Information Technology,
The University of Alabama, Tuscaloosa, AL 35487
Abstract
Rheological data are presented for shear flows of concentrated
dispersions of acicular ferromagnetic particles; such dispersions are
widely used in the manufacture of data storage materials. In steady
shear flow these dispersions are strongly shear thinning. Upon the
inception of shear flow, they show stress overshoots for both the shear
stress and the primary normal stress difference. The data can be
rationalized in terms of a temporary junction network, resulting from
the strong magnetic interactions between the particles, if the network
consists of relatively weak magnetic links connecting strongly bound
flocs of particles. Application of a shear deformation results in
disruption of the initially sample spanning network. The overshoot in
the shear stress occurs when the flocs become arranged in the shear
planes. The overshoot in the primary normal stress difference occurs as
a result of deformation of the strongly bound flocs.
Return to top.
W. E. Smith and C. F. Zukoskia)
Department of Chemical and Biomolecular Engineering
University of Illinois
Urbana, IL 61801
Abstract
A class of structured particles are developed such that the effects of
particle interactions are adjusted enabling suspension mechanics to be
studied without the influence of interparticle forces other than volume
exclusion, Brownian and hydrodynamic interactions. Three grades of these
near-hard structured particles are developed from fumed silica. These
particles have the same structure as measured by the mass fractal
dimension. Stress dependent viscosities and diffusivity measurements are
reported as a function of particle concentration, providing an
opportunity to determine the scaling behavior of these properties with
the two characteristic sizes: the primary particle and aggregate sizes.
Except at stresses exceeding the critical value demarking the onset of
shear thickening, flow curves superimposed with each other and with hard
sphere suspensions compared at the same effective packing fraction,
f/fm.
a) Corresponding author. E-mail:
czukoski@uiuc.edu
Return to top. |
