We propose to drive open microfluidic systems by shear in a covering fluid layer, e.g., oil covering water-filled chemical channels. The advantages as compared to other means of pumping are simpler forcing and prevention of evaporation of volatile components. We calculate the expected throughput for straight channels and show that devices can be built with off-the-shelf technology. Molecular dynamics simulations suggest that this concept is scalable down to the nanoscale.Progressive miniaturization and integration of chemical and physical processes into microfluidic systems, so-called "chemical chips", is expected to permit cheap mass production and to facilitate the handling of much smaller quantities than standard laboratory equipment [1,2,3]. Most currently built microfluidic systems contain the liquids in closed pipes and use body forces (e.g., gravity or centrifugal forces) [4], electroosmosis, capillary wicking, or simply applied pressure to generate flow. Driving these systems is relatively straightforward, but there are major drawbacks. Small pipes get easily clogged (in particular while processing biological fluids which contain large molecules such as proteins or DNA), driving gets increasingly difficult with reduced cross section, and fabrication more challenging.Open microfluidic systems are a second line of development which can overcome these problems. As demonstrated experimentally [5,6] and theoretically [7,8], fluids can be guided on flat solid surfaces by wettability patterns, e.g., hydrophilic stripes on an otherwise hydrophobic substrate. Several methods to generate flow in such "chemical channels" have been discussed: capillary forces (i.e., wicking into channels or motion due to wettability gradients) [5], electrowetting [9,10], thermally generated Marangoni forces (e.g., in arrays of local heaters) [11,12], body forces, surface acoustic waves [13], or combinations of these. While capillary forces can only induce flow over relatively short distances, electrode and heater arrays require complex sample structures. Surface acoustic waves can only drive drops which are large compared with the wavelength (in general on the micron scale), and high rotation velocities are required to obtain sufficiently strong centrifugal forces.As illustrated in Fig. 1 we propose to use shear, generated in a covering fluid layer (e.g., oil on water), to induce flow over long length scales, e.g., across the whole device. A top plate moving with respect to the bottom patterned substrate generates shear in the oil which fills the gap between the two plates and, in turn, the oil will induce flow in a chemical channel covered with, e.g., water. This principle can be realized in a device with two circular plates rotating relative to each other and the chemical patterning can be implemented with standard printing techniques [14,15]. Design and scalability of shear driven systems is similar to body force driven devices and the same type of surface tension driven pearling instabilities occur in straight channels [8]. In b...