Credit: © 2008 AIP

The idea of using laser beams as 'optical tweezers' to pick up and move solid microparticles is well known, especially in the fields of physics and biology, but researchers have now shown that laser beams can direct and control the flow of fluids as well.

A team of researchers from Ludwig-Maximilians University Munich in Germany has recently reported optically driven fluid flow along user-defined paths as narrow as 2 μm in a film of water (J. Appl. Phys. 104, 104701; 2008).

Dieter Braun and Franz Weinert used an infrared laser scanning microscope to trace a light path over a film of water. The laser beam locally heats the water reducing its viscosity. When combined, the effects of thermal expansion and a viscosity gradient cause the water to flow along the light path in the opposing direction to that of the moving focal spot.

Put simply, the laser beam creates a moving hot spot around 10 K higher in temperature than its surroundings. Near the laser spot, in the region of increasing temperature, the fluid expands; in the cooling volume already passed by the laser beam, the fluid contracts. If nothing else enters into the equation, the expansion and contraction are equal and opposite — no net flow results. However, as the viscosity of the fluid (water in this case) is temperature-dependent, this process can become asymmetric resulting in a net fluid flow.

An advantage of the laser-induced fluid flow is that it negates the need for physical, micromachined channels, enabling the prospect of user-defined channels that can be reconfigured and could transport nanoparticles and dissolved molecules.

To investigate the process, Braun and Weinert used 20 nm fluorescent beads that were guided by the flow on a millimetre length scale without significant diffusion into the surrounding fluid. The researchers also show mixing of 'DNA hairpins' in dynamically created gel pockets, demonstrating controlled molecule mixing capabilities.

The fluid velocities of 150 μm s−1 achieved are an order of magnitude slower than those possible using conventional microfluidics; however, the optically driven technique used here removes the need for bulky, physical connections and external pumps. Although the team shows the effect in two-dimensional fluids, the researchers noted that the method — a valveless, contactless and pumpless one — could be applied to three-dimensional fluids in the future.