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Lasers sort spheres by size

19 Apr 2007

Researchers in the UK use optical tweezers to sort four different sizes of particles.

A breakthrough in the separation of colloidal micro particles could lead to improvements in sorting cells in blood or particles in paint. That's according to Kishan Dholakia and colleagues at the University of St. Andrews who have developed a way to simultaneously sort four different sizes of particles into laterally separated parallel laminar streams. (Optics Letters 32 1144)

"I think this will potentially interest researchers in cell sorting and is an important step to sorting our whole blood where the ability to sort more than two components is needed," Dholakia of St. Andrews' School of Physics and Astronomy told optics.org.

The key component in the system is an acousto-optic deflector (AOD). Although these devices have already been used to generate 2D arrays of discrete optical traps, the team believes that AODs have never created such complex landscapes.

"We are not trying to displace existing techniques, rather this is a method amenable to very small volumes of analyte in micro fluidic environments," commented Dholakia. "The key technical challenge was generating suitable and optimal light patterns to enable both focusing and subsequent separation into laminar streams, this meant intensive work on the acousto-optic system."

A dual-axis AOD, with an efficiency of around 25 %, was incorporated into a standard optical tweezers system at the back aperture of a 100x NA=1.25 oil immersion microscope objective. The optical landscape was created using a 10  W, 1070 nm yttrium IPG fiber laser with output beam diameter of 5 mm and a bandwidth of 5 GHz.

The team used two samples of polydisperse mixtures of silica spheres with diameters ranging from 2.3 to 6.84 µm suspended in deionized water. An optical funnel channeled the colloidal mixture into a single particle stream a few micrometers in width. Dholakia explains that an exit ramp of decreasing intensity guides the particle stream across the flow. While traversing the exit ramp, the spheres experience a hydrodynamic drag force that draws smaller particles out of the optical landscape first.

"We experimented with different configurations and found that inserting discrete gaps of increasing size between the regions of constant intensity produced highly localized laminar particle exit streams," explained Dholakia. "We were able to fractionate particles at flow rates of up to 100 µm/s.

The next steps for the team is to focus on potential applications with blood, other cells and enhancing throughput. With faster electronics the team believes that larger, more complex patterns could be realized, and in principle it should be possible to fractionate more than four species at once.

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