Bilayer membranes in micro-fluidics: from gel emulsions to soft functional devices

We outline a concept of self-assembled soft matter devices based on micro-fluidics, which use surfactant bilayer membranes as their main building blocks, arrested in geometric structures provided by top-down lithography. Membranes form spontaneously when suitable water-in-oil emulsions are forced into micro-fluidic channels at high dispersed-phase volume fractions. They turn out to be remarkably stable even when pumped through the micro-fluidic channel system. Their geometric arrangement is self-assembling, driven by interfacial energy and wetting forces. The ordered membrane arrays thus emerging can be loaded with amphiphilic functional molecules, ion channels, or just be used as they are, exploiting their peculiar physical properties. For wet electronic circuitry, the aqueous droplets then serve as the 'solder points'. Furthermore, the membranes can serve as well-controlled coupling media between chemical processes taking place in adjacent droplets. This is shown for the well-known Belousov-Zhabotinski reaction. Suitable channel geometries can be used to (re-)arrange the droplets, and thereby their contents, in a controlled way by just moving the emulsion through the device. It thereby appears feasible to construct complex devices out of molecular-size components in a self-assembled, but well controlled manner.