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Holograms trap airborne water droplets

10 May 2006

UK researchers unveil a new way of using holograms to manipulate arrays of liquid droplets and test their physical and chemical properties.

Researchers in the UK say that they have used holographic optical tweezers to trap and coagulate arrays of micron-sized droplets in mid-air for the first time. The result could have applications ranging from airborne sampling and sensing right through to microfluidics (Optics Express 14, 4175).

"The main difference between our experiment and previous work using holographic traps is the fact that our particles, in this case droplets, are trapped in air," David McGloin of the University of St. Andrews told optics.org. "By controlling the ambient conditions and the vapor pressure of the droplets, we are able to move and coagulate them in a very controlled way."

According to the researchers, the technique could open up the way for a range of potential studies, including looking at droplet gas uptake rates, evaporation, condensation, freezing, nucleation and coagulation. Spectroscopy can also be used to determine the particles' composition.

The technique, say the researchers, has a range of fundamental and practical applications. "There are the fundamental applications looking at how airborne particles interact in external potentials and single particle dynamics. There are also applied applications like airborne sampling, probing chemical contaminants and bioaerosols. Also, one of the drivers of the work is in creating optically controlled chemical micro-reactors for microfluidics," added McGloin.

The team created 4 µm diameter water aerosol droplets using an ultrasonic nebuliser and fed them into a sealed glass cube, which acted as a trapping chamber. Green laser light at 532 nm illuminated a spatial light modulator (SLM) and the resulting hologram was projected into the glass cube.

The SLM allowed for the real-time x-y planar translation of the droplets to within 230 nm in accuracy, at speeds of up to 85 µm/sec. In addition to this precise manipulation, the researchers have also shown the ability to coagulate multiple droplets.

Due to the uncontrollable motion of the nebuliser aerosol, McGloin and colleagues found it difficult to fill all the holographic trapping sites - even with a relatively simple chamber pattern. The team is now looking at ways of using "droplet-on-demand" single droplet maker devices (such as ink jet printer-heads) to inject and position the drops more precisely.

Giving further cause for investigation, the team found that small droplets cannot be trapped easily at high laser powers, which would not be true with comparable experiments trapping colloidal particles in fluid. "This is a puzzle and indicates that there are lots of new things to discover when looking at airborne trapping," said McGloin. "We are currently looking at the solution to this problem."

Darius Nikbin is Science/Technology Reporter on Optics.org and Opto & Laser Europe magazine.

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