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John O | October 2017

Researchers unexpectedly discover new theory of liquid streaming

university of houston (texas) researchers unexpectedly discovered a new optofluidics principle while studying the nonlinear transmission of light through an aqueous suspension of gold particles, according to a report from the school.


yanan wang, a post-doctoral researcher at uh, is co-first author on a paper describing the discovery of a new principle of optofluidics. (university of houston)


the researchers noticed that a pulse laser appeared to be forcing the movement of a stream of liquid in a glass cuvette, but further investigation showed that it was more than just momentum of laser photons passing to the liquid.


“light usually passes straight through water without any absorption and scattering,” the report explained. “strong momentum from the photons wouldn’t generate a liquid stream. the gold nanoparticles turned out to be key – researchers found that the nanoparticles were initially needed to create the stream because they reacted to focused laser pulsing to create a plasmonic-acoustic cavity…that formed on the inner wall of the cuvette.”


as nanoparticles were heated and cooled with each laser pulse, their expansion and contraction released ultrasound waves that moved the liquid. this was captured on video by the researchers (see below).


“once a cavity is created, the nanoparticles can be removed,” the article said. “streaming can be induced in any fluid.”


this new optofluidics principle combines the well-known principles of acoustic streaming and photoacoustics, which have both been used by scientists for years.


the research was recently published in science advances. the abstract read:


“transforming a laser beam into a mass flow has been a challenge both scientifically and technologically. we report the discovery of a new optofluidic principle and demonstrate the generation of a steady-state water flow by a pulsed laser beam through a glass window.


“to generate a flow or stream in the same path as the refracted laser beam in pure water from an arbitrary spot on the window, we first fill a glass cuvette with an aqueous solution of au nanoparticles. a flow will emerge from the focused laser spot on the window after the laser is turned on for a few to tens of minutes; the flow remains after the colloidal solution is completely replaced by pure water.


“microscopically, this transformation is made possible by an underlying plasmonic nanoparticle-decorated cavity, which is self-fabricated on the glass by nanoparticle-assisted laser etching and exhibits size and shape uniquely tailored to the incident beam profile.


“hydrophone signals indicate that the flow is driven via acoustic streaming by a long-lasting ultrasound wave that is resonantly generated by the laser and the cavity through the photoacoustic effect. the principle of this light-driven flow via ultrasound, that is, photoacoustic streaming by coupling photoacoustics to acoustic streaming, is general and can be applied to any liquid, opening up new research and applications in optofluidics as well as traditional photoacoustics and acoustic streaming.”


watch the video below to learn more:

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