An appealing design principle for such microcompartments is based on open and, thus, directly accessible surface channels which can be fabricated on solid substrates using available photolithographic methods. The simplest channel geometry which can be produced in this way corresponds to channels with a rectangular cross section. The width and depth of these channels can be varied between a hundred nanometer and a couple of micrometer.
At first sight, it seems rather obvious to use such surface channels as microcompartments. However, if one actually tries to fill these channels with a certain liquid, one observes that the liquid often refuses to enter the channels. In fact, as shown in the new PNAS study, liquids at surface channels can attain a large variety of different wetting morphologies including localized droplets, extended filaments, and thin wedges at the lower channel corners. Examples for these morphologies as observed by atomic (or scanning) force microscopy (AFM) are shown in Figure 1.
When the AFM experiments were first performed, it was not known how to produce a certain liquid morphology since there was no systematic theory for the dependence of this morphology on the materials properties and on the channel design. Such a theory has now been developed. This theory addresses the strong capillary forces between substrate material and liquid and takes the â€˜freedomâ€TM of contact angles at pinned contact lines into account. Such a