Sci. Adv. 3, e1700555 (2017)

The speed, direction and flow of a liquid suspension of gold nanoparticles can be controlled by a laser beam. That’s the finding of a team of scientists from China and the US. Yanan Wang and co-workers discovered that when a focused spot of laser light is shone onto the window of a glass cuvette containing an aqueous solution of gold nanoparticles a flow will commence after a few minutes. Investigations indicate that the flow is driven by a photoacoustic interaction between the pulsed laser light (150 ns pulse duration, 1 kHz repetition rate and 527 nmwavelength) and the nanoparticles that establishes a long-lasting ultrasound wave. The effect called ‘laser streaming’ appears to have a laser power threshold and disappears for a power below 60 mW. The maximum speed of the flow is estimated to be around 4 cm s–1 for a laser power of 120 mW, which is more than sufficient for many applications in microfluidics. Fluorescent polymer microspheres were added to the solution to visualize the flow. The researchers believe that following optimization and further refinement the laser streaming could be useful for applications such as laser propulsion, surgery and chemical mixing.