An experimental demonstration of reaction-driven viscous fingering developing when a more viscous solution of a reactant A displaces a less viscous miscible solution of another reactant B is presented. In the absence of reaction, such a displacement of one fluid by another less mobile one is classically stable. However, a simple A + B → C reaction can destabilize this interface if the product C is either more or less viscous than both reactant solutions. Using the pH dependence of the viscosity of some polymer solutions, we provide experimental evidence of both scenarios. We demonstrate quantitatively that reactive viscous fingering results from the buildup in time of nonmonotonic viscosity profiles with patterns behind or ahead of the reaction zone, depending on whether the product is more or less viscous than the reactants. The experimental findings are backed up by numerical simulations. Viscous fingering (VF), also often referred to as the Saffman-Taylor instability, appears when a fluid with a given viscosity displaces another more viscous and hence less mobile one in porous media. This hydrodynamic instability has been largely studied both theoretically and experimentally [1][2][3][4] because of the beauty and generic character of the ramified patterns produced but also because of its practical consequences. VF is indeed observed in applications as diverse as hydrology [5,6] [21,22] has, however, suggested that reactions could even destabilize the classically stable reverse situation of a more viscous fluid displacing a less viscous one. To do so, it has been predicted that the product of the reaction must have a viscosity either larger or smaller than the viscosity of the reactants.In this work we present experimental evidence of such reaction-driven VF destabilization of a more viscous liquid displacing a less viscous one. We show quantitatively that the classically stable interface between a viscous reactant A pushing a less viscous aqueous solution of another reactant B can be destabilized by the buildup through a reaction of nonmonotonic viscosity profiles in time. The experimental study is carried out using aqueous solutions of polymers, chosen mainly because of their viscosity dependence on pH. If a solution of such a polymer A displaces less viscous dyed water, no instability is obtained and the interface remains planar. However, upon addition of a pH changing reactant B in the displaced water, an A + B → C neutralization reaction generates a product C either more viscous than the polymer A or less viscous than the solution of B triggering reactioninduced fingering. We provide experimental realization of both scenarios, and explain the origin of the destabilization by quantitative measurements of viscosities and numerical simulations. We also highlight the difference between VF patterns depending on whether the reaction generates respectively a maximum or a minimum in the spatial viscosity profile.

Experimental studies are conducted to analyze dispersion and miscible viscous fingering of initially circular samples of a given solution displaced linearly at constant speed U by another solution in horizontal Hele-Shaw cells ͑two glass plates separated by a thin gap͒. In the stable case of a dyed water sample having the same viscosity as that of displacing nondyed water, we analyze the transition between dispersive and advective transport of the passive scalar displaced linearly. At low displacement speed and after a certain time, the length of the sample increases as a square root of time allowing to compute the value of a dispersion coefficient. At larger injection speed, the displacement remains advective for the duration of the experiment, with a length of the sample increasing linearly in time. A parametric study allows to gain insight into the switch from one regime to another as a function of the gap width of the cell. In the unstable case of viscous glycerol samples displaced by dyed water, the rear interface of the sample where less viscous water pushes more viscous glycerol is unstable with regard to viscous fingering. The interface deforms into fingers, the number and size of which depend on the viscosity ratio between the two solutions and on the displacement speed. We study the influence of these viscous fingering phenomena on the increased spreading of the sample for various mobility ratios and injection speeds.

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