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Experimental Soft Condensed
Matter Group Rheology of solid-stabilized emulsions
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Emulsions are mixtures of two immiscible fluids, as droplets of one dispersed in the other fluid. They are used in cosmetics, food industry, pharmaceuticals, petroleum industry… In the oilfield they are also present when unwanted, as formation water and oil mix easy due to their different viscosities.
Emulsions are important especially because of their rheological properties. Dilute emulsions are liquid-like and their viscosity can be adjusted by changing the concentration of the dispersed phase. Highly concentrated emulsions, on the other hand, can be solid-like with a high elastic modulus. This is due to droplets, packed together, further deforming upon shear. Additional surface area requires energy, hence the material is elastic. The nature of this elasticity is well understood for the case of surfactant-stabilized emulsions. But what if surfactants were solid particles? How would that affect the rheology?
Our model system is an oil-in-water emulsion, stabilized with colloidal silica particles. We adjust the wetting of precipitated silica by grafting short alkyl chains on their surface. These treated particles adsorb at oil/water interfaces and can thereby stabilize an emulsion. For different oil in water volume fractions we perform oscillatory shear measurements
with results very reminiscent of surfactant-stabilized emulsions (Mason et al., PRL 75 2051, 1995). At small strains, the storage modulus G’ is almost frequency independent and larger than the loss modulus G’’. At higher strains G’’ rises, indicating larger scale restructuring, and yielding as G’ drops.
The graph below shows the concentration dependence of the shear moduli, identified as the values of G’ and G’’
at the frequency where G’’ exhibits a minimum. We compare with data
from surfactant-stabilized emulsions, taken from Mason et al., normalized by the
Scaling with σ/r indicates that elasticity originates from energy stored at the droplet interfaces. But, solid-stabilized emulsions are much stronger. The effective surface tension required to account for the data would be σ≈600mN/m...!
Most puzzling is the early onset of elasticity, at the concentration where drops are not yet fully packed. This is conceivable in an attractive system, that ours seems to be. However in gels G’ usually scales as a power law of concentration, with an exponent of around up to ~4. For solid-stabilized emulsions we observe a significantly different behaviour; the transition is much more abrupt.
This project is done with support and collaboration of Schlumberger-Doll Research.
Kosta Ladavac
McKay Laboratory, Rm 517