Carnegie Mellon University
August 14, 2014

Sorting Microparticles for Cleaner Water

Sorting Microparticles for Cleaner Water

Detail showing a section of 10μm diameter polystyrene particles lining up in a 500μm wide  and 500μm deep PDMS channel. The surface acoustic waves were excited at 19.74MHz with a peak to peak voltage of 12.4V on a 200μm wavelength interdigitated transducer.Detail showing a section of 10μm diameter polystyrene particles lining up in a 500μm wide and 500μm deep PDMS channel. The surface acoustic waves were excited at 19.74MHz with a peak to peak voltage of 12.4V on a 200μm wavelength interdigitated transducer.

In 1993, 400,000 people in Milwaukee were infected with an intestinal parasite known as Cryptosporidium. At that time, it was impossible to identify pathogens in drinking water before people became ill, so the city could only manage the aftermath of what became the largest documented waterborne disease outbreak in United States history. Today, researchers like CEE Professor Irving Oppenheim and CEE Associate Professor Kelvin Gregory are developing tools with the objective of detecting pathogens much earlier.

Bacteria are just one of many types of microparticles that occur in water sources. With help from the National Science Foundation, Oppenheim, Gregory and ECE Professor David Greve are working to identify and sort microparticles based on properties like size, density, and stiffness.

They do this with a surface acoustic wave (SAW) device that sends surface acoustic waves across a microfluidic channel-a tiny groove that is carved into a piece of clear plastic. As water flows through the channel, waves are sent from both sides, producing standing waves and a series of nodes and anti-nodes in the water.  "The microparticles suspended in the water then move to align at the nodes or anti-nodes," explains Oppenheim. 

The speed at which the particles move depends on factors like their size and density; larger particles generally move more quickly than their smaller counterparts. Because of this, particles get pulled to different places within the channel and are separated from each other. Placing outlets at strategic points can allow certain particles, like harmful pathogens, to be separated

Though the SAW method of sorting microparticles is well established, CEE PhD student and project member Erin Dauson says that there are several ways that this research is unique. For example, the microfluidic channels she builds and studies are made with a material similar to plexiglass. 

"Most other groups use a softer silicone, called PDMS, that needs to be molded," explains Dauson. "We use a harder material-PMMA-that is easier to make on a larger scale." In addition to the manufacturing benefits, PMMA is also advantageous because it doesn't dampen, or reduce the strength of, the surface acoustic waves as quickly.  "With traditional PDMS, you have to use more energy to create the same amount of waves.  Eventually, we hope that PMMA will be more energy efficient," she says. 

The SAW method also has the potential to save time because it can be calibrated to particles with certain properties. "The ability to target certain pathogens for separation from the large numbers of cells in drinking water will reduce the number of samples analyzed," Gregory says. "This will enable real-time detection of pathogens for a proactive approach." Someday, he and the research team hope that this technology will help authorities identify and remove disease-causing agents well before they get to our faucets.  


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