Optical Vortex Traps Tiny Droplets
For Daniel T. Chiu, a chemistry professor at the University of Washington in Seattle, all but the smallest drops of fluid are too big and more than one, too many. Therefore, he developed a method to generate single femtoliter droplets, using an optical vortex to push them around, as well as tweezers and refractive index changes to fuse them together.
This effort is driven, Chiu said, by his group’s long-term goals. “That really is to try to use a droplet as a platform to analyze single cells and single cellular structures,” he said.
For such research, standard microfluidic systems generate too many droplets. Chiu often has only a single cell or part of a cell, and a multitude of drops is overwhelming. A second problem is that the drops, although only picoliters or nanoliters in volume, are too large.
Exposing an aqueous droplet to the beam from a 1064-nm Nd:YAG laser changes the droplet’s refractive index, allowing it to be trapped (A-D). Trapping individual droplets that have reagents enables long-duration chemical reactions that begin with the fusing of droplets — here, creating silver chloride (E-H). With prolonged exposure to light, the silver is reduced, and silver granules form, demonstrating the stability of the generation/reaction chamber.
To get around these issues, the researchers fabricated a microfluidic chamber of the transparent rubber polydimethylsiloxane, as reported in the ASAP edition of Analytical Chemistry on July 27. They connected microinjectors to the 2.5-μm-diameter chamber inlets, using sudden changes in pressure to produce single droplets of one fluid inside another. The two fluids were immiscible, with the droplets being water-based and the surrounding fluid based on oil.
They mounted the microfluidic chamber atop a microscope objective, with dichroic mirrors directing the beam from a 488-nm argon-ion laser and a 1064-nm Nd:YAG laser, both from Spectra-Physics of Mountain View, Calif., into the chamber. They used CCD cameras from Cohu Inc. of San Diego and Princeton Instruments of Trenton, N.J., for imaging.
The investigators generated fluorescence in the sample with the first laser while using the second laser to manipulate the droplets. Common optical tweezers would not work because the refractive index of the droplets was lower than that of the surrounding fluid. They thus developed an optical vortex, sending the laser through a computer-generated hologram to create a helical wavefront with a dark core. The droplets were trapped in the vortex, enabling the researchers to position the droplets as they saw fit.
Chiu pointed out that the vortex did not allow complete manipulation. “We cannot fuse two droplets using the vortex because the vortex works by repelling the droplets,” he said.
The researchers overcame this problem by heating the droplets, thereby changing their volume and their refractive index. With the refractive index temporarily higher than the surrounding fluid, the droplets could be manipulated with optical tweezers for fusion and other tasks. They have since discovered a way to accomplish these activities using the vortex alone.
With their setup, they generated single droplets of a few microns in diameter and successfully manipulated them by, for example, fusing two droplets so as to make them fluorescent. They also reduced silver ions to metallic silver granules. In these experiments, they controlled and monitored a single droplet for as long as an hour and a half.
As for the future, Chiu said the technique could be used to fuse droplets with specific chemicals inside. What has reacted, or not, could then be analyzed, shedding light on the chemical process. He said that the applications would be primarily biological, which suits his research.
“You know, 99.9 percent of our focus is biological — mostly single cells and cellular structures,” he pointed out.
Contact: Daniel Chiu, University of Washington department of chemistry; e-mail: firstname.lastname@example.org.
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