Yield stress thixotropic clay suspension: Investigations of structure by light, neutron,
and x-ray scattering
Fre
´
de
´
ric Pignon,
1
Albert Magnin,
1
Jean-Michel Piau,
1
Bernard Cabane,
2
Peter Lindner,
3
and Olivier Diat
4
1
Laboratoire de Rhe ´ologie, Universite ´ Joseph Fourier, Grenoble I, Institut National Polytechnique de Grenoble, CNRS UMR No. 5520,
Boı ˆte Postale 53, 38041 Grenoble Cedex 9, France
2
Equipe Mixte CEA-RP, Rho ˆne-Poulenc, 93308 Aubervilliers, France
3
Institut Laue-Langevin, Boı ˆte Postale 156, F-38042 Grenoble Cedex 9, France
4
ESRF, Boı ˆte Postale 220, F-38043 Grenoble, France
Received 5 December 1996; revised manuscript received 29 May 1997
The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal suspension of
synthetic clay were studied by using a combination of small-angle neutron and x-ray scattering and static light
scattering. At the same time, macroscopic mechanical behavior at rest was characterized by means of rheo-
metric measurements. Two characteristic length scales were detected in these yield stress suspensions of
discotic texture. The first, measuring several tens of nanometres, is linked to a fractal dimension of 3. The
second, of the order of 1 m, is linked to a fractal behavior of dimension D that increases with the particle
volume fraction. Consequently, it is suggested that the structure of the dispersions at rest is composed of
subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about 1 m. At
larger length scales, these micrometer-sized aggregates are rearranged to form a continuous three-dimensional
isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The increase
of this fractal dimension with the particle volume fraction, the ionic strength, and the gelation time is correlated
to a hardening of the mechanical properties of the gels at rest. The gel state is reached above a volume fraction
v
* for a given ionic strength and gelation time. In the gel phase, a critical volume fraction
vc
separates two
domains. Gels belonging to the domain
v
*
v
vc
have a fractal behavior of dimension D=1 0.05,
suggesting an alignment of the micrometer-sized aggregates that leads to the formation of a mechanically weak
fibrous structure. Gels belonging
v
vc
have a fractal dimension D=1.80.01, corresponding to a me-
chanically stronger structure consisting of zones of high and lower particle density. A scaling law enabled these
fractal dimensions to be correlated with the effect of the volume fraction on the yield stress. In contrast to what
is commonly assumed in relation to clay suspensions, it is suggested here that it is the large length scales, of
the order of 1 m, associated with a fractal arrangement that governs the macroscopic mechanical behavior.
S1063-651X9707209-7
PACS numbers: 82.70.Kj, 83.70.Hq
I. INTRODUCTION
The suspension studied here was made with Laponite, a
synthetic clay of the hectorite type. In an aqueous medium,
this forms a transparent thixotropic gel above a certain vol-
ume fraction, at a low ionic strength of I
s
10
-3
M and
p H of 9.5. The clay consists of discotic particles of uniform
size, about 300 Å in diameter and about 20 Å in thickness
1.
Owing to its high purity and very small crystal size, this is
perfectly suited to light-scattering measurements as, unlike
natural clays at the same concentration, it does not produce
any multiple scattering. In addition, it has macroscopic vol-
ume properties yield stress and thixotropy that need to be
linked with the mesoscopic properties of the system, i.e., its
structure, in order to be better understood and controlled.
Many structural models relating to this clay suspension
already have been proposed by different authors in order to
try to explain the mechanism of swelling gel in time and its
thixotropic behavior. Certain authors have described the for-
mation of a so-called ‘‘house of cards’’ network, where elec-
trostatic attraction appears to occur between the edges of the
positively charged platelets and the negatively charged faces
2,3. Others have demonstrated the existence in the sus-
pending medium of tactoids, i.e., piles of two to four indi-
vidual platelets separated by a few layers of water 4,5.
Ramsay and Lindner 6 concluded from small-angle
neutron-scattering measurements that the formation of a bal-
anced gel structure is due essentially to repulsion between
the isolated microcrystals of clay. They detected short-range
correlations and alignments between the particles in a struc-
ture that is isotropic over longer ranges. Morvan et al. 7
have shown a Q
-3
power-law decay by ultrasmall and small-
angle x-ray scattering SAXS, in a region of modulus of
scattering vectors of about 1 10
-3
Å
-1
. This power-law
decay is attributed to a large-scale organization, i.e., a vari-
ably dense network and a heterogeneous structure. They pro-
posed two possible microstructures: i an edge-face contact
leading to a card-house structure and ii a locally parallel
interaction between first neighbors due to osmotic repulsion
between adjacent platelets. Most recently, Mourchid et al.
8 have studied the structure and sol-gel transition of these
Laponite suspensions by means of cryofracture, TEM,
SAXS, and rheometric measurements. They have put forth a
hypothesis that tallies with their own observations and may
explain the sol-gel transition. This hypothesis considers the
PHYSICAL REVIEW E SEPTEMBER 1997 VOLUME 56, NUMBER 3
56 1063-651X/97/563/32819/$10.00 3281 © 1997 The American Physical Society