Spatial structures generated by chemical reactions at interfaces

Far-from-equilibrium networks of physicochemical processes may exhibit structuring¹, in time (for example, chemical oscillations ²), in space (stationary patterns³) or as a combination of both (chemical waves⁴). Much experimental effort has been invested during the past two decades in temporal and spatio-temporal phenomena ^2,4; and except for periodic precipitation processes⁵ no general chemical system was available for a wide experimental study of the evolution of spatial structures. Recently, however, we have revealed a remarkably wide family of processes which produce spatial structures: chemical reactions at liquid interfaces. These include reactions at gas/liquid interfaces⁶, and photochemical reactions at liquid/air^7,8 and liquid/liquid⁸ interfaces. We now report yet another important class: reactions at membrane surfaces, as shown in Fig. 1. Although hydrodynamic slow currents are apparent at a mature stage of the structure development, the actual mechanism leading to the initial structuring is as yet not clear. We show, however, that these structures are not a visualizing method of patterns caused by evaporative cooling^9,10: structures are formed also in the absence of such patterning and hydrodynamic currents start only after chemical structures are formed. Some of our observations suggest that chemical reaction is necessary for structure evolution, and that gravity apparently plays no role in this phenomenon.