Figure 1. Sequence of photographs showing the evolution of the pink diffusive boundary layer formed by the reaction of phenolphthalein in MIBK (top layer) with aqueous sodium hydroxide (bottom layer) in a Hele-Shaw cell. The phenolphthalein mimics carbon dioxide in a saline aquifer. From Cardoso S.S.S. and Andres J.T.H., 2014, Nature Comm. 5:5743.