A research plan for the study of the structure and dynamics of aggregates and
gels of colloidal rods and platelets by means of static and dynamic light scattering is presented.
Studies under both quiescent and flow conditions are envisaged. For purposes of comparison,
preliminary results of the structure and dynamics of spherical colloidal gels are reported.
The materials studied, sterically stabilized spherical colloidal silica suspended in
hexadecane, form stable, reversible gels that can be studied by light scattering in the volume fraction
range 0.01 - 0.10. The gels allow divergences of characteristic times at the gel point to be
sensitively probed by manipulating temperature.
INTRODUCTION AND RESEARCH PLAN
The structure and dynamics of colloidal aggregates and gels have been long
investigated by the fluid physics community; however, most research has focused on suspensions of
spherical particles. Yet, aggregates and gels of anisometric particles - colloidal rods
and platelets - may exhibit structure and dynamics that are quite different from spherical colloids.
For example, suspensions of colloidal rods gel at extremely low volume fractions and form
birefringent sediments and the rheology of solutions and gels of colloidal rods and platelets
differs dramatically from that of colloidal spheres.[1] Scientifically, studies with
anisometric particles offer the opportunity to assess the role of anisotropic excluded volume and
particle orientation in aggregates and gels. Technologically, anisometric colloids find use in a wide
range of materials such as ceramics, polymer nanocomposites, well-bore drilling fluids and magnetic
storage media.
The specific aim of this research is to use small and wide-angle light scattering, flow light scattering, dynamic light scattering and rheology to study the structure and
dynamics of aggregates and gels of anisometric particles. Quiescent aggregates and gels will
be studied, as well as gels subjected to shear deformation. The anisometric particles will be
prepared by two methods. The first, after Ho et al., will allow the production of anisometric
polymer colloids that can be density matched in mixtures of H2O and D2O.[2] Because of the elimination
of sedimentation effects, these materials can be used for ground-based studies at
low volume fraction. The aqueous solutions of anisometric polymer colloids are not suited
for high volume fraction studies because the large refractive index contrast between particle
and solvent causes multiple scattering. At high volume fraction, studies will be performed with
inorganic rods (boehmite) and platelets (gibbsite) that are coated with silica and grafted with
octadecyl chains.[1] These now organophilic colloids can be dispersed in non-polar
solvents (such as hexadecane) in which spherical colloids of similar surface chemistry are known
to gel. Advantages of such thermoreversible gels are that the interparticle potential is
believed to be well characterized and their approximate refractive index matching allows single
scattering studies up to ö = 0.12.[3] However, because inorganic colloids are prone to
sedimentation since they cannot be density matched on the ground, the microgravity environment
presents opportunities for studies that probe a more extensive range of volume fraction,
size and shape. Specific studies that are planned: (1) Effect of aspect ratio on the fractal
dimension and cluster radius of quiescently formed anisometric particle aggregates and gels. (2)
Effect of shear deformation on the structure of anisometric particle gels. (3) The effect of
particle shape on the dynamic structure factor. (4) Comparison of scattering and rheological studies.
PRELIMINARY RESULTS
For purpose of future comparison, the static and dynamic structure factor of high volume fraction gels of spherical colloids have been characterized by means of small and
wide-angle static light scattering and dynamic light scattering, respectively (Figures 1 and 2). The
effect of shear deformation on spherical colloidal gels has also been quantified (Figure 3).
Interestingly, Figure 3 demonstrates that anisotropic scattering is observed even for gels of
spherical particles. The materials studied are silica colloids (a = 40 nm) with surface grafted octadecyl
chains dispersed in the solvent hexadecane. The materials gel below a critical temperature. The
results illustrate that ground-based experiments with inorganic colloidal gels are possible,
provided that the deleterious effects of sedimentation can be ameliorated by means of studies at
sufficiently high volume fraction. Note that the ability to sensitively probe the divergences of
characteristic times associated with the gelation transition (Figure 2) by precise control of
temperature provides a means to study the relationship between colloidal gelation and the glass
transition.
Solomon, M.J., Varadan, P., Aggregation and Gelation of Anisometric Colloidal Particles: Preliminary Results and Research Plan, Fifth Microgravity Fluids Physics and Transport Phenomena Conference, NASA Glenn Reserach Center, Cleveland, OH, CP-2000-210470, pp. 1143-1145, August 9, 2000.