We present an experimental investigation of an instability triggered by a fast chemical reaction in a low inertia parallel flow in a small channel. Two fluids evenly injected in a straight channel react creating a strongly stratified distribution of viscosity near their interface, which destabilizes the flow. Depending on the flow rates and the aspect ratio of the flow channel, several unstable regimes are observed: mixing regime, weakly stratified regime and a stable stratified regime. In channels of 1:1 aspect ratio we find most efficient mixing (and highest flow resistance) at an intermediate pressure drops, as the flow transitions between a fully 3D regime and more stratified regime. For channels of small aspect ratio the non-monotonicity is less evident: higher pressure drops lead to increased mixing. (C) 2011 Elsevier B.V. All rights reserved.