By manipulating colloidal microspheres within customized channels, we have created micrometer-scale fluid pumps and particulate valves. We describe two positive-displacement designs, a gear and a peristaltic pump, both of which are about the size of a human red blood cell. Two colloidal valve designs are also demonstrated, one actuated and one passive, for the direction of cells or small particles. The use of colloids as both valves and pumps will allow device integration at a density far beyond what is currently achievable by other approaches and may provide a link between fluid manipulation at the macro- and nanoscale.

Separation of equivalently sized polystyrene, n=1.59, poly(methylmethacrylate), n=1.49, and silica, n=1.43, beads has been accomplished using optical chromatography. The optical separations were performed using a glass flowcell that permits the optical trapping laser to be lightly focused into the fluid pathway against the fluid flow. Separation occurs due to the balance of fluid and optical forces; particles come to rest when the force due to the fluid flow equals the radiation pressure force. The ability to optically separate particles based upon their refractive index opens avenues for the characterization of colloidal samples based upon chemical characteristics, in addition to size.

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In recent years, single-beam optical traps have been used to manipulate individual colloids and biological objects such as cells. We have implemented a rapidly scanning laser optical trap with rates as high as 1200 Hz where a single laser beam is used to trap multiple colloids simultaneously. The optics are optimized to achieve a small laser focus size and a large scanning pattern in the sample. This approach provides great pattern flexibility and, because of the use of piezoelectrics, small particles (1 μm in diameter) in low-viscosity solvents, such as water, can be readily manipulated.

In this letter, an optical microfabrication and actuation method for the creation of microfluidic structures is described. In this approach, an optical trap is used to position and polymerize colloidal microspheres into linear structures to create particle or cell directing devices within microfluidic channel networks. To demonstrate the utility of these structures, two microscale particulate valves are shown, a passive design that restricts particulate flow in one direction and another design that directs particulate flow to one of two exit channels.