Patterning liquid flow on the microscopic scale

Microscopic fluidic devices, ranging from surgical endoscopes1 and microelectromechanical systems2 to the commercial ‘lab-on-a-chip’ (ref. 29), allow chemical analysis and synthesis on scales unimaginable a decade ago. These devices transport miniscule quantities of liquid along networked channels. Several techniques have been developed to control small-scale flow, including micromechanical3 and electrohydrodynamic4 pumping, electro-osmotic flow5, electrowetting6,7 and thermocapillary pumping8,9,10. Most of these schemes require micro-machining of interior channels and kilovolt sources to drive electrokinetic flow. Recent work8,9,10 has suggested the use of temperature instead of electric fields to derive droplet movement. Here we demonstrate a simple, alternative technique utilizing temperature gradients to direct microscopic flow on a selectively patterned surface (consisting of alternating stripes of bare and coated SiO2). The liquid is manipulated by simultaneously applying a shear stress at the air–liquid interface and a variable surface energy pattern at the liquid–solid interface. To further this technology, we provide a theoretical estimate of the smallest feature size attainable with this technique.