Flow of thin liquid films on chemically structured substrates

Chemically patterned surfaces are of significant interest in the context of microfluidic applications. Miniaturization of such devices will eventually lead to structures on the nano-scale. Whereas on the micron scale purely macroscopic descriptions of liquid flow are valid, on the nanometer scale long-ranged inter-molecular interactions, thermal fluctuations such as capillary waves, and finally the molecular structure of the liquid become important. We discuss the most important conceptual differences between flow on chemically patterned substrates on the micron scale and on the nanometer scale. These concern the structure of the triple line, the type of interactions between neighboring liquid flows, and the influence of the molecular structure of the liquid on the flow. We formulate four design issues for nanofluidics related to channel width, channel separation, and channel bending radius, and conclude with a discussion of the relevance of the conceptual differences between the micron scale and the nanometer scale for these issues.