The MIT researchers' system shows the fibers of the filter membrane in red, and the oily droplets accumulating on it in green.MIT
It is easy to see how water and oil don’t mix well. You can see an oil slick in a puddle, for example, or battle a layer of grime while washing dirty dishes.
But as hard as they are to mix, it is also very difficult to separate completely which would be useful when cleaning up an oil spill, for example. Attempting to separate these two elements often results in membranes that get clogged up or “fouled.”
New research from MIT might provide a tool for developing better membrane materials that can resist or prevent fouling.
Cleaning up water that has been affected by oils is an essential part of many industrial processes including petroleum refining, food processing, and metal finishing. Water that is untreated and that leaves residue can be highly damaging to ecosystems.
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Filtration materials easily fouled
Cleaning this water can take many forms but changes depending on the type of oil-contamination, the amount of wastewater and the sizes of the oil droplets. When the oil has been emulsified or combined with the water, one efficient way to clean it, is to pass the water through membranes that filter out the oil droplets.
However, the membranes quickly get snagged full of the oil droplets and then, they themselves require difficult and time-consuming cleaning.
The new imaging research by MIT graduate students, Yi-Min Lin and Chen Song and Professor of chemical engineering, Gregory Rutledge, could make looking for materials with low-fouling properties much quicker and more effective. Filtration membranes have previously been notoriously difficult to examine.
“There’s a lot of effort to develop new types of membranes, but when they get put in service, you want to see how they interact with the contaminated water, and they don’t lend themselves to easy examination. They are usually designed to pack inasmuch membrane area as possible, and being able to look inside is very hard,” Rutledge says.
Lasers build 3D image
The new technique uses confocal laser scanning microscopy. This is achieved by two lasers, scanning across the material and where the two beams cross, a material marked with a fluorescent dye, will glow.
The research team used two different fluorescent dyes, one to mark the oily material in the fluid; the other to mark the fibers in the filtration membrane.
This allowed the materials to be thoroughly scanned not only across its surface but into its depth. The result is a full 3D image of the way the oil droplets are dispersed in the membrane.
This presents a huge breakthrough in the way materials can be analyzed. Until now, Rutledge says, “methods for imaging pore spaces in membranes were pretty crude.”
Previously, a material's ability to filter was based only on flow rates and pressure changes through the material, ignoring any information about the buildup of oily material in the material's pores.
Material design improves
With the new process, he says, “Now you can actually measure the geometry, and build a three-dimensional model and characterize the material in some detail. So what’s new now is that we can really look at how separation takes place in these membranes.”
The research will provide better knowledge about the use and development of filtering materials that can be applied in other scenarios, outside of oily water cleanup.