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Chemically-driven viscous fingering
Viscous fingering is a hydrodynamic instability that occurs when a fluid of given viscosity displaces another more viscous fluid in a porous medium. The interface between the two fluids does not remain flat but deforms into "fingers". The reverse situation of a more viscous fluid displacing a less viscous one is classically understood as stable.
This is typically the case when a viscous solution of polymer displaces water for instance : the interface remains stable and hence planar.
We will show that this intuitive picture breaks down if the solutions at hand are reactive. Indeed, if a solution of a reactant A is injected into a more viscous solution of another reactant B, viscous fingering can be triggered if a simple A+B->C reaction generates a product C of different viscosity.
We will first present a theoretical study giving the conditions under which such a chemical reaction can influence or even trigger viscous fingering.
We will next give an experimental demonstration of reaction-driven viscous fingering developing when a more viscous solution of polymer displaces a less viscous miscible aqueous solution of another reactant B. We demonstrate that, in this case, viscous fingering results from the build-up in time of non-monotonic viscosity profiles with patterns behind or ahead of the reaction zone respectively depending on whether the product is more or less viscous than the reactants.
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