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An experimental study on immiscible viscous fingering (VF) with chemical reaction is described, whereby a surfactant produced in a radial Hele-Shaw cell results in a decrease in interfacial tension. The surfactant is formed at the interface between alkaline solution (sodium hydroxide) and a long-chain fatty acid (linoleic acid). This topic is closely related to alkaline flooding, which is an enhanced oil recovery method. The reaction was found to have two opposing effects on VF depending on the flow rate, namely, narrowing and widening of the fingers. Moreover, the influences of the reaction on VF evolution can be categorized into five different types based on the effects appearing in VF evolution and the fingering width and area at the maximum observation region. Possible mechanisms for each type are proposed, and an argument based on scaling of the VF properties using dimensionless numbers gives support to the proposed mechanisms. This dual role of the reaction in immiscible VF evolution may contribute to establishing optimal conditions for alkaline flooding. Moreover, the finding that one chemical reaction has two opposing effects on flow is of value from the standpoint of fundamental studies of reacting flow dynamics.

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