When a reactive and miscible less-viscous liquid displaces a more-viscous liquid in a Hele-Shaw cell, reactive miscible viscous fingering takes place. We succeed in showing experimentally how a reactive miscible viscous fingering pattern in a radial Hele-Shaw cell changes when the viscosity of the more-viscous liquid is varied owing to variation in chemical species concentration induced by an instantaneous chemical reaction. This is done by making use of a polymer solution's dependence of viscosity on pH. When the viscosity is increased by the chemical reaction, the shielding effect is suppressed and the fingers are widened. As a result, the ratio of the area occupied by the fingering pattern in a circle whose radius is the length of the longest finger is larger in the reactive case than in the non-reactive case. When the viscosity is decreased by the chemical reaction, in contrast, the shielding effect is enhanced and the fingers are narrowed. These lead to the area ratio being smaller in the reactive case than in the non-reactive case. A physical model to explain this change in the fingering pattern caused by the chemical reaction is proposed.

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.

We experimentally demonstrate that a precipitation reaction at the miscible interface between two reactive solutions can trigger a hydrodynamic instability due to the buildup of a locally adverse mobility gradient related to a decrease in permeability. The precipitate results from an A þ B → C type of reaction when a solution containing one of the reactants is injected into a solution of the other reactant in a porous medium or a Hele-Shaw cell. Fingerlike precipitation patterns are observed upon displacement, the properties of which depend on whether A displaces B or vice versa. A mathematical modeling of the underlying mobility profile confirms that the instability originates from a local decrease in mobility driven by the localized precipitation. Nonlinear simulations of the related reaction-diffusion-convection model reproduce the properties of the instability observed experimentally. In particular, the simulations suggest that differences in diffusivity between A and B may contribute to the asymmetric characteristics of the fingering precipitation patterns. DOI: 10.1103/PhysRevLett.113.024502 PACS numbers: 47.55.P−, 47.15.gp, 47.56.+r, 47.70.Fw Chemical reactions are able to influence and even more strikingly induce hydrodynamic fingering instabilities of a frontal interface when a high mobility fluid displaces a less mobile one in a porous medium. This occurs in viscous fingering if a less viscous fluid displaces a more viscous one [1]. Fingering can also result from a change in permeability in a porous medium as in reactive dissolution instabilities [2][3][4][5][6][7][8][9][10]. In these cases, the invading fluid contains chemicals which dissolve the solid matrix of the porous medium, leading to a related increase in porosity behind the reaction front. As a result, the resistance to flow decreases in these higher mobility reactive zones, which favors further dissolution, giving, thus, a positive feedback leading to instability. Dispersion of reactants is the stabilizing factor counteracting the growth of fluid channels in order to provide a fingered pattern with a given characteristic wavelength [2,5,7,10]. The reverse case of precipitation is not expected to destabilize an interface as the related decrease in permeability and, hence, in mobility behind the front is expected to block the flow rather than destabilize it. There is, however, increased interest to understand the effect of precipitation reactions during flow displacements in porous media in the context of CO 2 sequestration techniques [11][12][13][14]. Mineralization by which CO 2 injected in a porous medium could undergo precipitation reactions (to yield carbonates, for instance [13][14][15][16]) is indeed promising in view of a permanent safe storage of CO 2 in geological strata. Understanding the conditions in which precipitations can affect the stability of the spreading CO 2 plumes [12] is, thus, particularly important.In this context, we demonstrate experimentally and explain theoretically how a precipitation reaction localized at th...

Effects of reactant concentrations on the characteristics of reacti®e miscible