Scientists Levitate Water Droplets Using Sound Waves To Detect Contaminants

  • Scientists used high sound waves to levitate water droplets to identify impurities in water. 
  • This technique helped them to detect small scale heavy elements like cadmium and barium, within few minutes. 
  • It could be used by pharmaceutical, agricultural and other industries for water analysis.

Researchers at the Instituto de Ciencias Físicas UNAM, Mexico have levitated water droplets in midair using sound waves to detect harmful contaminants present in the water.

Since these contaminants are injurious to both human and environment, it’s crucial to identify tiny amounts of metals (like mercury and lead) in water. We already have several water monitoring systems, but they require sending samples to a lab for in-depth analysis, which is a time consuming process.

On the other hand, this new technique of levitating water droplets can help scientists to carry out on-site contaminant analysis in real time to prevent future lead contamination issues or detect wastewater impurities from industrial areas. It could be used by pharmaceutical, agricultural and other industries for water analysis.

How This Technique Works?

Levitating droplets of water makes it easier to perform laser induced breakdown spectroscopy (LIBS) –a sensitive technique that analyzes heavy elements in the drops — because the water drop in air can be evaporated in a controlled manner, increasing the contaminant mass concentration.

LIBS

The laser induced breakdown spectroscopy provides a simple and quick way to detect different elements in droplets concurrently. It vaporizes the substance and produces a plasma by projecting a high-power laser pulse onto a sample.

Since the light beam ejected by plasma consists of atomic fingerprints of the substance, the sample’s chemical composition can be detected by observing this beam of light.

Image credit: Jairo Peralta and Victor Contreras, Instituto de Ciencias Físicas UNAM

This new technique combined with LIBS can help researchers identify small-scale heavy elements such as cadmium and barium, within few minutes. For instance, in this experiment, researchers detected 200 microgram per liter of barium and 700 microgram per liter of cadmium.

Reference: OSA Publishing | doi:10.1364/OL.43.002260

Advantages Over Other Existing Techniques

There are already dozens of commercial handheld devices available for LIBS analysis. But it isn’t easy to monitor liquid samples because the plasma generated in these samples lasts for a very short period – it cools down rapidly. Also, because plasma rests on splashes of water, it slightly changes the spectroscopy reading.

In order to provide a better signal to detect contaminants in liquid, one needs to have high-powered laser equipment to produce a plasma, which are generally heavy and non-portable.

Schematic representation of the setup on the xz and xy plane | Courtesy of researchers 

That’s why liquid drops are placed on a substrate, which gradually dries, leaving contaminants present in the sample. The process is simple, but involves 2 major problems –

  1. High-powered laser excites atoms present in the sample as well as in the substrate.
  2. Liquid evaporation results in nonuniform distribution of contaminants on the substrate, damaging signal quality.

Therefore, researchers came up with a new, inexpensive technique [acoustic levitation] that involves levitating single droplet of water using high sound waves, which generates enough [stable] force to counterbalance gravity and hover a droplet in the air without any support.

The best thing is, unlike other levitation mechanisms, the sample doesn’t need to have any magnetic or electric response to be levitated in the air.

Read: Acoustic Tractor Beam Levitates Light Material In SoundLess Void

What’s Next?

Scientists plan to further enhance their technology – they will improve the design of acoustic trap to levitate samples more stably. This will refine the readings of LIBS. Also, they will try to make samples more sensitive by stably hovering tinier drops in the air. Overall, the technology will enable compact and less sensitive detector to find impurities in water in real time.

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