Light-Generated Bubbles Propel Microparticles, Create Optomechanical Force

Energy created from light was converted into kinetic motion using nano-sized laser-generated bubbles. As the bubbles expanded, they acted as a propulsion mechanism for surrounding micron sized particles, propelling the particles at forces many times greater than previously achieved. This technique for creating optomechanical force (OMF) could be useful in the development of micromotors and optical devices for solar cell optics.

“What we ultimately hope to achieve is a highly accurate, passive technology for use in a concentrated solar device that would follow the sun without the need for a mechanical tracking mechanism,” said researcher Avi Niv, Ben-Gurion University of the Negev (BGU).

Ben-Gurion University of the Negev researchers demonstrate a light-generated bubble for microparticle propulsion. Panel (a) shows the 42 µm diameter spherical particle and the 405 nm laser beam as the respective dark and bright patches. Panel (b) shows that 40 milliseconds later the microsphere has traversed a distance roughly 10 times its size. Courtesy of Ben-Gurion U.

The research team demonstrated light-activated motion of microparticles with effective forces in the range of micro-Newtons. Researchers showed that the force resulted from the accumulation of light-generated heat by a micron-sized particle that was converted into motion due to a phase transition in water. They analyzed the ensuing optomechanical force (OMF) using high speed imaging and compared the observed dynamics with a known model of bubbles.

High-speed imagery indicated the role of bubble expansion and subsequent collapse. When researchers compared their observations with known models, they discovered a dynamic behavior controlled by polytropic trapped vapor and the inertia of the surrounding liquid.

To demonstrate the proposed OMF, researchers formed a substance with binary light-activated switching from opacity to transparency, by loading microparticles into the sample until a dense collection was formed. When light was focused onto this substance, the OMF repelled the micro-spheres from that region so that transparency emerged. The researchers measured the transmission of this substance at varying laser intensities.

“In our study, a micron-sized object was propelled at unprecedented speeds of close to one meter-per-second, six times faster than what is common in present devices, while still maintaining motion direction control,” said Niv. “After the bubble initiates movement and bursts, there is no trace of the vapor; the system returns to the original state and the same action can be initiated repeatedly, like a combustion engine.”

The proposed approach could potentially be used to form optical materials for applications ranging from optical processing to imaging and displays.

The research was published in Scientific Reports (doi:10.1038/s41598-017-03114-z).

The video shows the laser driven bubble propulsion of a 40 micron diameter silver coated glass bead. Courtesy of Ben-Gurion University of the Negev.