共查询到20条相似文献,搜索用时 31 毫秒
1.
Hiroshi Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1999,38(2):100-107
Rheo-dielectric behavior was examined for 4−4′−n-octyl-cyanobiphenyl (8CB) having large dipoles parallel to its principal axis (in the direction of the C≡N bond). In the
quiescent state at all temperatures (T) examined, orientational fluctuation of the 8CB molecules was observed as dielectric dispersions at characteristic frequencies
ωc>106 s−1. In the isotropic state at high T, no detectable changes of the complex dielectric constant ɛ*(ω) were found under slow flow at shear rates ˙γ≫ωc. In the nematic state at intermediate T, the terminal relaxation intensity of ɛ*(ω) was decreased under such slow flow. In the smectic state at lower T, the flow effect became much less significant. These results were related to the flow-induced changes of the liquid crystalline
textures in the nematic and smectic states, and the differences of the rheo-dielectric behavior in these states are discussed
in relation to a difference of the symmetry of molecular arrangements in the nematic and smectic textures.
Received: 1 October 1998 Accepted: 13 January 1999 相似文献
2.
H. Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1998,37(6):519-527
Dielectric relaxation behavior was examined for 4-4′-n-pentyl-cyanobiphenyl (5CB) and 4-4′-n-heptyl-cyanobiphenyl (7CB) under flow. In quiescent states at all temperatures examined, both 5CB and 7CB exhibited dispersions
in their complex dielectric constant ε*(ω) at characteristic frequencies ω
c
above 106 rad s–1. This dispersion reflected orientational fluctuation of individual 5CB and 7CB molecules having large dipoles parallel to
their principal axis (in the direction of C≡N bond). In the isotropic state at high temperatures, these molecules exhibited no detectable changes of ε*(ω) under flow at
shear rates . In contrast, in the nematic state at lower temperatures the terminal relaxation intensity of ε*(ω) as well as the static
dielectric constant ε′(0) decreased under flow at . This rheo-dielectric change was discussed in relation to the flow effects on the nematic texture (director distribution)
and anisotropy in motion of individual molecules with respect to the director.
Received: 14 April 1998 Accepted: 29 July 1998 相似文献
3.
Alexander Semenov Aurélia Charlot Rachel Auzély-Velty Marguerite Rinaudo 《Rheologica Acta》2007,46(5):541-568
Dynamics of associating polymer solutions above the reversible gelation point are studied. Each macromolecule consists of
a soluble backbone (B) and a small fraction of specific strongly interacting groups (A or C stickers) attached to B. A mixture
of B–A and B–C associating polymers with 1:1 stoichiometric ratio is considered. As a result of AC association, the polymers
reversibly gelate above the overlap concentration. It is shown that (1) the network strands are linear complexes (double chains)
of B–A and B–C; (2) “diffusion” of the network junction points is characterized by an apparent activation energy, which can
be significantly higher than the energy of one AC bond; (3) most importantly, the randomness of sticker distribution along
the chain can significantly slow down the network relaxation leading to a markedly non-Maxwellian viscoelastic behavior. The
theory elucidates the most essential features of rheological behavior of polysaccharide associating systems (with A = adamantyl
moiety, C = β-cyclodextrin, B = either chitosan or hyaluronan) including similar behavior of G
′ and G
″ in a wide frequency range, strong temperature dependence of the characteristic frequency ω
x
, and an extremely strong effect of added free stickers (fC) on the dynamics.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. 相似文献
4.
Relaxation patterns of long,linear, flexible,monodisperse polymers: BSW spectrum revisited 总被引:1,自引:1,他引:0
Theoretical predictions for the dynamic moduli of long, linear, flexible, monodisperse polymers are summarized and compared
with experimental observations. Surprisingly, the predicted 1/2 power scaling of the long-time modes of the relaxation spectrum
is not found in the experiments. Instead, scaling with a power of about 1/4 extends all the way up to the longest relaxation
times near τ/τ
max = 1. This is expressed in the empirical relaxation time spectrum of Baumgaertel-Schausberger-Winter, denoted as “BSW spectrum,”
and justifies a closer look at the properties of the BSW spectrum. Working with the BSW spectrum, however, is made difficult
by the fact that hypergeometric functions occur naturally in BSW-based rheological material functions. BSW provides no explicit
solutions for the dynamic moduli, G
′(ω), G
″(ω), or the relaxation modulus G(t). To overcome this problem, close approximations of simple analytical form are shown for these moduli. With these approximations,
analysis of linear viscoelastic data allows the direct determination of BSW parameters. 相似文献
5.
High-frequency viscoelasticity of crosslinked actin filament networks measured by diffusing wave spectroscopy 总被引:2,自引:0,他引:2
We study the short-time relaxation dynamics of crosslinked and uncrosslinked networks of semi-flexible polymers using diffusing
wave spectroscopy (DWS). The networks consist of concentrated solutions of actin filaments, crosslinked with increasing amounts
of α-actinin. Actin filaments (F-actin) are long semi-flexible polymers with a contour length 1–100μm and a persistence length
of 5–15μm; α-actinin is a small 200kDa homodimer with two actin-binding sites. Using the large bandwidth of DWS, we measure
the mean-square-displacement of 0.96μm diameter microspheres imbedded in the polymer network, from which we extract the frequency-dependent
viscoelastic moduli via a generalized Langevin equation. DWS measurements yield, in a single measurement, viscoelastic moduli
at frequencies up to 105Hz, almost three decades higher in frequency than probed by conventional mechanical rheology. Our measurements show that the
magnitude of the small-frequency plateau modulus of F-actin is greatly enhanced in the presence of α-actinin, and that the
frequency dependence of the viscoelastic moduli is much stronger at intermediate frequencies. However, the frequency-dependence
of loss and storage moduli become similar for both crosslinked and uncrosslinked networks at large frequencies, G′(ω)∝G′′(ω)∝ω0.75±0.08. This high-frequency behavior is due to the small-amplitude, large-frequency lateral fluctuations of actin filaments between
entanglements.
Received: 20 January 1998 Accepted: 12 February 1998 相似文献
6.
Linear viscoelastic properties of SiO2/(AP/EP) suspension with various SiO2 volume fractions (ϕ) in a blend of acrylic polymer (AP) and epoxy (EP) were investigated at various temperatures (T). The AP/EP contained 70 vol.% of EP. The SiO2 particles were treated with epoxy silane coupling agent. The effects of the SiO2 particles are more pronounced in the terminal zone: a transition from viscoelastic liquid (ϕ ≤ 30 vol.%) to viscoelastic solid (ϕ ≥ 40 vol.%) was observed which can be interpreted as a critical gelation occurring at a critical particle content and critical
gel temperature. The SiO2/(AP/EP) systems exhibited a critical gel behavior at ϕ ≅ 35 vol.% and T ≅ 100°C characterized with a power–law relationship between the storage and loss moduli (G
′ and G
″) and frequency (ω); G
′ = G
″/tan(nπ/2) ∝ ω
n
. The critical gel exponent (n) was estimated to be about 0.45. The gelation occurred with increasing T. 相似文献
7.
Suspensions consisting of particles of colloidal dimensions have been reported to form connected structures. When attractive forces act between particles in suspension they may flocculate and, depending on particle concentration, shear history and other parameters, flocs may build-up in a three-dimensional network which spans the suspension sample. In this paper a floc network model is introduced to interpret the elastic behavior of flocculated suspensions at small deformations. Elastic percolation concepts are used to explain the variation of the elastic modulus with concentration. Data taken from the suspension rheology literature, and new results with suspensions of magnetic -Fe2O3 and non-magnetic -Fe2O3 particles in mineral oil are interpreted with the model proposed.Non-zero elastic modulus appeared at threshold particle concentrations of about 0.7 vol.% and 0.4 vol.% of the magnetic and non-magnetic suspensions, respectively. The difference is attributed to the denser flocs formed by magnetic suspensions. The volume fraction of particles in the flocs was estimated from the threshold particle concentration by transforming this concentration into a critical volume concentration of flocs, and identifying this critical concentration with the theoretical percolation threshold of three-dimensional networks of different coordination numbers. The results obtained indicate that the flocs are low-density structures, in agreement with cryo-scanning electron micrographs. Above the critical concentration the dynamic elastic modulus G was found to follow a scaling law of the type G (
f
-
f
c
)
f
, where
f
is the volume fraction of flocs in suspension, and
f
c
is its threshold value. For magnetic suspensions the exponent f was found to rise from a low value of about 1.0 to a value of 2.26 as particle concentration was increased. For the non-magnetic a similar change in f was observed; f changed from 0.95 to 3.6. Two other flocculated suspension systems taken from the literature showed a similar change in exponent. This suggests the possibility of a change in the mechanism of stress transport in the suspension as concentration increases, i.e., from a floc-floc bond-bending force mechanism to a rigidity percolation mechanism. 相似文献
8.
Masataka Sugimoto Hirokazu Hida Takashi Taniguchi Kiyohito Koyama Yuji Aoki 《Rheologica Acta》2007,46(7):957-964
Poly(vinyl chloride) (PVC)/di-isononyl phthalate systems with PVC content of 45.5 (PVC8) and 70.4 wt% (PVC6) were prepared
by a hot roller at 150 °C and press molded at 180 °C. The dynamic viscoelasticity and elongational viscosity of PVC8 and PVC6
were measured in the temperature range from 150 to 220 °C. We have found that the storage and loss shear moduli, G′ and G″, of PVC8 and PVC6 exhibited the power-law dependence on the angular frequency ω at 190 and 210 °C, respectively. Correspondingly, the tan δ values did not depend on ω. These temperatures indicate the critical gel temperature T
gel of each system. The critical relaxation exponent n obtained from these data was 0.75 irrespective of PVC content, which was in agreement with the n values reported previously for the low PVC concentration samples. These results suggest that the PVC gels of different plasticizer
content have a similar fractal structure. Below T
gel, the gradual melting of the PVC crystallites takes place with elevating temperature, and above T
gel, a densely connected network throughout the whole system disappears. Correspondingly, the elongational viscosity behavior
of PVC8 and PVC6 exhibited strong strain hardening below T
gel, although it did not show any strain hardening above T
gel. These changes in rheological behavior are attributed to the gradual melting of the PVC crystallites worked as the cross-linking
domains in this physical gel, thereby inapplicability of the of time–temperature superposition for PVC/plasticizer systems. 相似文献
9.
The rheological properties of rennet-induced skim milk gels were determined by two methods, i.e., via stress relaxation and dynamic tests. The stress relaxation modulusG
c
(t) was calculated from the dynamic moduliG andG by using a simple approximation formula and by means of a more complex procedure, via calculation of the relaxation spectrum. Either calculation method gave the same results forG
c
(t). The magnitude of the relaxation modulus obtained from the stress relaxation experiments was 10% to 20% lower than that calculated from the dynamic tests.Rennet-induced skim milk gels did not show an equilibrium modulus. An increase in temperature in the range from 20° to 35 °C resulted in lower moduli at a given time scale and faster relaxation. Dynamic measurements were also performed on acid-induced skim milk gels at various temperatures andG
c
(t) was calculated. The moduli of the acid-induced gels were higher than those of the rennet-induced gels and a kind of permanent network seemed to exist, also at higher temperatures.
G
storage shear modulus,N·m–2;
-
G
loss shear modulus,N·m–2;
-
G
c
calculated storage shear modulus,N·m–2;
-
G
c
calculated loss shear modulus,N·m–2;
-
G
e
equilibrium shear modulus,N·m–2;
-
G
ec
calculated equilibrium shear modulus,N·m–2;
-
G(t)
relaxation shear modulus,N·m–2;
-
G
c
(t)
calculated relaxation shear modulus,N·m–2;
-
G
*(t)
pseudo relaxation shear modulus,N·m–2;
-
H
relaxation spectrum,N·m–2;
-
t
time,s;
-
relaxation time,s;
-
angular frequency, rad·s–1.
Partly presented at the Conference on Rheology of Food, Pharmaceutical and Biological Materials, Warwick, UK, September 13–15, 1989 [33]. 相似文献
10.
The dynamic flow behavior of polyamide-6 (PA-6) and a nanocomposite (PNC) based on it was studied. The latter resin contained
2 wt% of organoclay. The two materials were blended in proportions of 0, 25, 50, 75, and 100 wt% PNC. The dynamic shear rheological
properties of well-dried specimens were measured under N2 at T=240 °C, frequency ω=0.1–100 rad/s, and strains γ=10 and 40%. At constant T, γ, and ω the time sweeps resulted in significant
increases of the shear moduli. The γ and ω scans showed a complex rheological behavior of all clay-containing specimens. At
γ=10% the linear viscoelasticity was observed for all compositions only at ω>1 rad/s, while at γ=40% only for 0 and 25 wt%
of PNC. However, the effect was moderate, namely decreasing G′ and G′′ (at ω=6.28 rad/s; γ=50%) by 15 and 7.5%, respectively.
For compositions containing >25 wt% PNC two types of non-linearity were detected. At ω≤ωc=1.4 ± 0.2 rad/s yield stress provided evidence of a 3-D structure. At ω > ωc, G′ and G′′ were sensitive to shear history – the effect was reversible. From the frequency scans at ω > ωc the zero-shear relative viscosity vs concentration plot was constructed. The initial slope gave the intrinsic viscosity from
which the aspect ratio of organoclay particles, p=287 ± 9 was calculated, in agreement with the value calculated from the
reduced permeability data, p=286.
Received: 24 May 2001 Accepted: 27 August 2001 相似文献
11.
In a previous work, we have shown that chitosan true physical gelation occurs in some organic and inorganic acids (Hamdine et al. 2004). Two systems presenting similar gelation mechanisms were characterized furthermore in order to investigate the sol–gel transition: the chitosan–phosphoric acid and the chitosan–oxalic acid systems. By performing rheological measurements in the framework of linear viscoelasticity, we have investigated the effect of time, temperature, and polymer concentration on the gelation evolution. For both acid-based systems, gelation occurred above a critical polymer concentration around 5% w/v (g/100 ml) of chitosan. Isothermal time sweep experiments showed that the gelation occurs in three stages: (i) incubation; (ii) rapid increase of G′; and (iii) a last stage where G′ slowly reached its equilibrium value due to slow molecular diffusion. At the gel point, G′ and G′′ scaled with ω
n
, with n=0.55 for both acid-based systems and a fractal dimension d
f of 1.9. Cooling–heating cycles revealed that the gels showed thermoreversibility after one sequence, but became permanent during subsequent cycles.This revised version was published online in October 2005 with corrections to the author's name. 相似文献
12.
V. F. Shumsky I. Getmanchuk T. Ignatova Yu. Maslak P. Cassagnau G. Boiteux F. Melis 《Rheologica Acta》2010,49(8):827-836
The viscoelastic characteristics of the blends of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) (PMMA/SAN) were investigated at various temperatures below, near, and above the phase separation temperature.
The investigated polymer system is characterized by a lower critical solution temperature. Rheological behavior of the blends
in the region of a phase separation was compared with change of the light scattering intensity. The presence of nanofillers
in the blend results in that the phase separation occurs at a higher temperature. At the isothermal conditions, the phase
separation begins earlier and proceeds with a higher rate as compared with the same blend without filler. The results of the
study show the considerable change of the viscoelastic characteristics of PMMA/SAN when the polymer system passes from the
homogeneous state to the heterogeneous one. Such characteristics as the dependence of the storage modulus (G
′) on the loss modulus (G
″), the dependence of the loss viscosity (η
″) on the dynamic viscosity (η
′), the dependences of the complex viscosity (η*), and the free volume fraction (f) on the blend composition are the most sensitive to the phase separation. The phase separation affects the characteristics
G
′(ω), where ω is the frequency only in a low-frequency range. Temperatures of phase separation were estimated using dependence G
′(T) at ω, which is the constant in the range of low frequencies. 相似文献
13.
Soumitra Choudhary Joseph C. White Whitney L. Stoppel Susan C. Roberts Surita R. Bhatia 《Rheologica Acta》2011,50(1):39-52
We report the preparation and rheological characterization of interpenetrating polymer network (IPN) hydrogels made from alginate
and hydrophobically modified ethyl hydroxyl ethyl cellulose (HMEHEC). To our knowledge, there have been no studies of the
gelation behavior of IPNs. We found that the rheology of these systems can be easily tuned, with the elastic modulus of the
IPN strongly dependent on the relative ratio of HMEHEC to alginate. The sol–gel transition of these systems was found to satisfy
the Winter–Chambon criterion for gelation at various crosslinker densities. From the power law relationship of the dynamic
moduli (G
′ ~G
″ ~ω
n), the exponent n appears to be dependent on both the crosslinker density and relative amount of two polymers. The value of n was found to be ~0.5 for all samples for stoichiometric amounts of crosslinker. The effect of molecular weight of HMEHEC
on the gel point and viscoelastic exponent has also been reported. Alginate seems to dominate the kinetics of the process
but the effect of high molecular weight HMEHEC on the gel point, especially at lower proportion was also evident. 相似文献
14.
Thomas G. Mason 《Rheologica Acta》2000,39(4):371-378
We obtain the linear viscoelastic shear moduli of complex fluids from the time-dependent mean square displacement, <Δr
2(t)>, of thermally-driven colloidal spheres suspended in the fluid using a generalized Stokes–Einstein (GSE) equation. Different
representations of the GSE equation can be used to obtain the viscoelastic spectrum, G˜(s), in the Laplace frequency domain, the complex shear modulus, G
*(ω), in the Fourier frequency domain, and the stress relaxation modulus, G
r
(t), in the time domain. Because trapezoid integration (s domain) or the Fast Fourier Transform (ω domain) of <Δr
2(t)> known only over a finite temporal interval can lead to errors which result in unphysical behavior of the moduli near the
frequency extremes, we estimate the transforms algebraically by describing <Δr
2(t)> as a local power law. If the logarithmic slope of <Δr
2(t)> can be accurately determined, these estimates generally perform well at the frequency extremes.
Received: 8 September 2000/Accepted: 9 March 2000 相似文献
15.
Yuji Aoki Kentaro Hirayama Koji Kikuchi Masataka Sugimoto Kiyohito Koyama 《Rheologica Acta》2010,49(10):1071-1076
A poly(vinyl chloride) (PVC, Mw = 102×103)(\mbox{PVC,}\;{\rm M}_{\rm w} =102\times 10^3) di-octyl phthalate (DOP) gel with PVC content of 20 wt.% was prepared by a solvent evaporation method. The dynamic viscoelsticity
and elongational viscosity of the PVC/DOP gel were measured at various temperatures. The gel exhibited a typical sol–gel transition
behavior with elevating temperature. The critical gel temperature (Tgel) characterized with a power–law relationship between the storage and loss moduli, G′ and G″, and frequency ω, G¢=G¢¢/tan ( np/2 ) μ wn{G}^\prime={G}^{\prime\prime}{\rm /tan}\;\left( {{n}\pi {\rm /2}} \right)\propto \omega ^{n}, was observed to be 152°C. The elongational viscosity of the gel was measured below the Tgel. The gel exhibited strong strain hardening. Elongational viscosity against strain plot was independent of strain rate. This
finding is different from the elongational viscosity behavior of linear polymer solutions and melts. The stress–strain relations
were expressed by the neo-Hookean model at high temperature (135°C) near the Tgel. However, the stress–strain curves were deviated from the neo-Hookean model at smaller strain with decreasing temperature.
These results indicated that this physical gel behaves as the neo-Hookean model at low cross-linking point, and is deviated
from the neo-Hookean model with increasing of the PVC crystallites worked as the cross-linking junctions. 相似文献
16.
Electro-rheological suspensions (ERS) are known to undergo liquid-to-solid transition under the application of an electric
field. Long-range interaction between neighboring particles results in sample-spanning particulate structures which behave
as soft solids. Here, we studied the rheological expression of this field-induced transition which has many similarities with
chemical gelation. This similarity shows in mechanical spectroscopy on a suspension of monodisperse silica in PDMS as model
ERS. Upon application of the electric field, dynamic moduli G′, G′′ grow by orders of magnitude and evolve in a pattern which is otherwise typical for gelation of network polymers (random
chemical or physical gelation). At the gel point, the slow dynamics is governed by power-law relaxation behavior (frequency-independent
tan δ). A low field strength is sufficient to reach the gel point and, correspondingly, the percolating particle structure
at the gel point is still very fragile. It can be broken by the imposition of low stress. For inducing a finite yield stress,
the field strength needs to be increased further until the long-range electrostatic interaction generates string-like particle
alignments which become clearly visible under the optical microscope. The onset of fragile connectivity was defined experimentally
by the tan δ method. The ERS was probed dynamically at low frequencies where the transition is most pronounced, and also in
steady shear where the rate of structure formation equals the rate of internal breaking.
Received: 1 May 2001 Accepted: 11 August 2001 相似文献
17.
Self-consistent modeling of entangled network strands and linear dangling structures in a single-strand mean-field slip-link model 总被引:1,自引:0,他引:1
Linear viscoelastic (LVE) measurements as well as non-linear elongation measurements have been performed on stoichiometrically
imbalanced polymeric networks to gain insight into the structural influence on the rheological response (Jensen et al., Rheol
Acta 49(1):1–13, 2010). In particular, we seek knowledge about the effect of dangling ends and soluble structures. To interpret our recent experimental
results, we exploit a molecular model that can predict LVE data and non-linear stress–strain data. The slip-link model has
proven to be a robust tool for both LVE and non-linear stress–strain predictions for linear chains (Khaliullin and Schieber,
Phys Rev Lett 100(18):188302–188304, 2008, Macromolecules 42(19):7504–7517, 2009; Schieber, J Chem Phys 118(11):5162–5166, 2003), and it is thus used to analyze the experimental results. Initially, we consider a stoichiometrically balanced network,
i.e., all strands in the ensemble are attached to the network in both ends. Next we add dangling strands to the network representing
the stoichiometric imbalance, or imperfections during curing. By considering monodisperse network strands without dangling
ends, we find that the relative low-frequency plateau, G0/GN0G_0/G_N^0, decreases linearly with the average number of entanglements. The decrease from GN0G_N^0 to G
0 is a result of monomer fluctuations between entanglements, which is similar to “longitudinal modes” in tube theory. It is
found that the slope of G′ is dependent on the fraction of network strands and the structural distribution of the network. The power-law behavior of
G
″ is not yet captured quantitatively by the model, but our results suggest that it is a result of polydisperse dangling and
soluble structures. 相似文献
18.
U. Narusawa 《Heat and Mass Transfer》2001,37(2-3):197-203
The rate of entropy generation, S˙
′
G[W/mK], is examined both theoretically and numerically for forced and mixed convection in a rectangular duct heated at the
bottom. Under fully-developed flow conditions S˙
′
G is expressed in terms of relevant non-dimensional hydrodynamic and thermal parameters. Numerically, it is demonstrated that
S˙
′
G is a single, effective parameter to examine both thermal and hydrodynamic fields and their variations.
Received on 22 November 1999 相似文献
19.
H. Henning Winter 《Rheologica Acta》2009,48(3):241-243
A slight rearrangement of the classical Cox and Merz rule suggests that the shear stress value of steady shear flow, , and complex modulus value of small amplitude oscillatory shear, G ∗ (ω) = (G′2 + G″2)1/2, are equivalent in many respects. Small changes of material structure, which express themselves most sensitively in the steady
shear stress, τ, show equally pronounced in linear viscoelastic data when plotting these with G ∗ as one of the variables. An example is given to demonstrate this phenomenon: viscosity data that cover about three decades
in frequency get stretched out over about nine decades in G ∗ while maintaining steep gradients in a transition region. This suggests a more effective way of exploiting the Cox–Merz rule
when it is valid and exploring reasons for lack of validity when it is not. The τ −G ∗ equivalence could also further the understanding of the steady shear normal stress function as proposed by Laun. 相似文献
20.
Maria del Mar Ramos-Tejada Alfonso Ontiveros Rosario del Carmen Plaza Angel V. Delgado Juan D. G. Durán 《Rheologica Acta》2003,42(1-2):148-157
Steady-state, oscillatory, and transient rheological determinations were used to assess the stability of homoionic sodium
montmorillonite (NaMt) suspensions at constant ionic strength (10–2 mol/l NaCl) and different pH values, after adsorption of humic acid (HA) on the particles. The adsorption of the latter was
first spectrophotometrically determined, at pH 3 and 9. While at pH 9 adsorption saturation was observed, at pH 3 the adsorption
density continued to grow up to the maximum equilibrium HA concentration reached (∼200 mg/l). Considering the similarity between
the structure of edge surfaces of NaMt particles and the surfaces of silica and alumina, the adsorption of HA was also investigated
on the latter solids. The results suggest that at pH 3 humic acids adsorb preferentially on edge surfaces, mainly through
electrostatic attraction with positively charged aluminol groups. This hypothesis is indirectly confirmed by zeta potential,
ζ, values: while HA concentration has little effect on ζ for silica, the addition of HA yields the zeta potential of alumina
increasingly negative for all pH values. Using shear stress vs shear rate plots, the yield stress of NaMt was determined as
a function of particle concentration, C, for pH 3, 5, 7, and 9, with and without addition of 50 mg/l HA. The yield stress, σy, was fitted with a power law σy∝C
n
; it was found that n values as high as 12 are characteristic of NaMt suspensions at pH 9 in the presence of HA. This indicates a strong stabilizing
effect of humic acid. This stabilization was confirmed by oscillometric measurements, as the storage modulus G′ in the viscoelastic linear region also scales with C, displaying large n values at neutral and basic pHs in the presence of HA. The modulus (in the viscoelastic linear region, for a frequency ν=1 Hz)
was found to increase with time, but G′ was lower at any time when HA was added, a consequence of the stabilization provided by HA. Similarly, creep-recovery experiments
demonstrated that NaMt suspensions containing HA displayed a less elastic behavior, and a permanent deformation. Modeling
the results as a Kelvin-Voigt model allowed one to establish a new scaling law of the reciprocal instantaneous deformation
with C. As before, high values of n were found for suspensions at pH 9 in the presence of HA. 相似文献