Researchers at the University of Houston were investigating the nonlinear transmission of light through an aqueous suspension of gold nanoparticles when they saw something unexpected. A pulse laser seemed to have forced the movement of a stream of liquid in a glass laboratory cuvette.
As they explored, they realized something more complicated was a work than a transfer of momentum from the laser photons to the liquid. Their observation resulted in a novel optofluidics principle, which is explained in a paper published in the Science Advances journal on September 27th, 2017.
It was not so simple. The momentum from a laser isn’t strong enough to activate the movement.
Jiming Bao, Associate Professor of Electrical and Computer Engineering, the University of Houston and Lead Author of the paper
Usually, light passes straight through water without any scattering and absorption, so Bao said that even strong momentum from the photons would not produce a liquid stream. The gold nanoparticles turned out to be key – Researchers identified that the nanoparticles were originally required to create the stream since they reacted to focused laser pulsing to produce a plasmonic-acoustic cavity, a structure Bao explained as a “bowl” that formed on the inner wall of the cuvette, a kind of glass test tube.
The moving stream of liquid is induced by ultrasound waves produced by the expansion and contraction of the nanoparticles, which occurs when nanoparticles on the cavity surface heat up and cool down with each laser pulse. The stream was recorded on video.
The nanoparticles can be removed as soon as a cavity is made. Bao said that streaming can be triggered in any fluid.
The discovery has the prospective to drastically enhance work in several fields, including lab-on-a-chip experiments involving moving liquids, such as a droplet of blood, at a microscopic scale.
The driving of flow by acoustic wave known as acoustic streaming was found by British Scientist Michael Faraday in 1831; it is now extensively used in microfluidics. The generation of ultrasound by gold nanoparticles, known as photoacoustics, is also well known and is employed in biomedical imaging.
This innovative optofluidics principle couples acoustic streaming with photoacoustics. “(It) can be used to generate high-speed flows inside any liquids without any chemical additives and apparent visible moving mechanical parts,” the Researchers wrote. “The speed, direction and size of the flow can be controlled by the laser.”
Besides Bao, Yanan Wang and Qiuhui Zhang, Zhuan Zhu, Feng Lin, Shuo Song, Md Kamrul Alam and Dong Liu, all of UH; Jiangdong Deng of Harvard University; Geng Ku of the University of Kansas; Suchuan Dong of Purdue University; and Zhiming Wang of the University of Electronic Science and Technology of China are the other Co-first Authors involved in the project. In addition, Bao, Wang and Lin have appointments at the University of Electronic Science and Technology of China.
Bao said additional work is required to better understand the formation of plasmonic-acoustic cavity by gold nanoparticles and to establish better ways to produce a liquid stream, among other things. But there will be a number of applications for the newly discovered principle.
“Laser streaming will find applications in optically controlled or activated devices such as microfluidics, laser propulsion, laser surgery and cleaning, mass transport or mixing,” the Researchers conclude.