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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.

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The electronic structure of In2O3 has been studied for the first time using a first-principles calculation method based on the density functional theory. Although the complexity of the crystal structure of In2O3 which contained 40 atoms in its unit cell had prevented studies of its electronic structure, we were able to study it using the characteristic of minimum basis sets of the linear muffin-tin orbital method with atomic sphere approximation. The calculated partial density of states (PDOS) showed that the valence bands were composed mainly of oxygen 2p-like states and the conduction bands consisted mainly of indium 5s-like states with free-electron-like character. The results of PDOS analysis were used to analyze the spectra from X-ray photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy. Calculated results were also used to interpret optoelectronic properties of tin-doped indium oxide.