Gravitational Stability of Suspensions of Attractive Colloidal Particles
Edited by Qichao Hu
December 5th, 2010
Gravity tends to cause sedimentation and creaming in colloidal suspensions, and cause phase separation of the particles. There are several techniques to stabilize the suspension, including density matching the particles to the fluid, restricting the particle size, increasing the fluid viscosity, and increasing the attraction between the particles leading to gel network to support the buoyancy. In the last option, nonadsorbing particles or polymers are added to the suspension to induce depletion interaction between the interacting particles. In this technique, the polymers increase the viscosity of the suspension, and the gel network can support the buoyant weight of the particles through its compressional modulus characteristics.
To fully understand the workings of the stabilization, it is important to investigate the relationships between the compressional modulus and depletion attraction, and changing volume fraction. In this work, they measure the volume fraction dependence of the compressional modulus in a depletion attraction induced emulsion network.
The colloidal suspension used consists of a monodisperse emulsion of paraffin oil in water, and stabilized with nonionic surfactant (Lutensol T08). A nonabsorbing polymer, polyvinylpyrrolidon (PVP) or other surfactant are added above the critical micells concentration to induce the depletion interaction.
Static light scattering is used to measure the osmotic compressibility of the polymer and micelles as functions of concentration, and thus to determine the strength of the depletion attraction. Images of the creaming emulsions are recorded and positions of the interface between the emulsion and the clear fluid is measured, from which the gravitational stability can be monitored.
Changing the initial height of the emulsion gel leads to variation in the buoyant stress in the emulsion. Since the sample is a gel, the emulsions at the top and bottom of the sample are subjected to the same buoyant stress. Upon creaming, the volume fraction at the top of the sample increases from its initial value. This leads to compressive strain, and the ability of the sample to withstand this compression is determined by the compressional modulus.
In the figure above, the time evolution of the sample height is monitored. We observed when the depletion attraction is not too large, the taller samples initially cream slowly, but then show collapse. Whereas for less tall samples, they cream monotonically, reaching new steady-state height, and have no delayed collapse.
In the figure above, the gravitational stress as a function of volume fraction in the steady state at the top of the emulsion sample is plotted.
In the figure above, the steady state volume fraction at the top of the emulsion sample is plotted as a function of the height profile. The compressional modulus is used to calculate the evolution of the height of the sample as it begins to cream. Assuming there is no sudden collapse, the evolution is described by the theory for poroelasticity, which accounts for the relative flow of fluid through the network.
The behavior of the emulsion gel is characterized by the compressional modulus, which determines the stress they can support. A direct analogy to hard sphere colloidal particles can be drawn. However, in hard sphere colloidal particles, the equation of state can determine the volume fraction dependence of the osmotic pressure, whereas in particle emulsion gel, they are not in equilibrium, and the height dependence does not determine the equation of state.
Overall this work demonstrates the ability to predict the stability of depletion attraction colloidal gels against gravitationally induced phase separation.