3D imaging makes for better filtration membranes

Materials World magazine
6 Aug 2019

A new 3D imaging technique is furthering knowledge of fouling with the aim of producing more advanced filtration materials. Ceri Jones finds out how.

Materials used in fluid filtration processes can quickly lose their efficacy due to fouling. During separation, for example removing oil from water following an environmental spill, membranes become clogged as oil emulsifies within the material, reducing its performance so that it requires cleaning or replacement. This increases the cost and the time of the separation process.

Recent research from MIT, USA, suggests this can be overcome with a greater understanding of the fouling process in order to develop better performing filters. To this end, a team of graduates, led by Professor of Chemical Engineering, Gregory Rutledge, used confocal laser scanning microscopy (CLSM) to examine separation membrane materials more acutely.

‘CLSM is an optical imaging method in which laser light is used to illuminate a sample at a very precise point within it. By adjusting the position of the sample and the depth of focus of the laser, the point of illumination can be moved throughout it, to build up a 3D reconstruction,’ Rutledge told Materials World.

Rutledge explained that, unlike existing methods that measure fouling on the surface at low-resolution, this new technique is non-destructive, and produces a high-resolution 3D image of both the membrane and the foulant at resolutions of between 200-600nm.

‘Such resolution is essential to study fouling during separation of emulsions, since the emulsion droplets themselves are usually about 10 micrometres in diameter, or smaller. The ability to image both the membrane and the foulant simultaneously allows us to see how the two components interact,’ Rutledge said.

Research is at the proof-of-concept stage and while the main goal is not commercialisation, Rutledge hopes to attract early adopters who can help advance the method. ‘We believe the things we learn using this technique on oil-in-water emulsions will have broader impact on a variety of such liquid-liquid membrane separations,’ he said. ‘As we learn more about how liquids interact with membranes using this tool, manufacturers will be empowered to apply the knowledge gained to new product designs. Conceivably, the method could eventually find its way into the analytical and quality control labs of manufacturers.’

How it works

To test the capabilities of the CLSM imaging technique, the team submersed a separation filter in oil to replicate fouling. The membrane fibres and oil had been labelled using dyes that fluoresce at different wavelengths, or colours. Two lasers were scanned over the material and where the beams crossed they caused the dyes to fluoresce, illuminating areas of material.

‘The technique allows the material to be scanned not only across the area of the membrane, but also into the depth of the material, layer by layer, to build up a full 3D image of the way the oil droplets are dispersed in the membrane, which in this case is composed of an array of microscopic fibres,’ according to MIT.

In this way, the team was able to construct a 3D picture of the membrane, oil droplets ranging from 10-20 micrometres in size, or both.

‘Being able to “see” the build-up of oil on a membrane as it occurs gives us physical insight into what is going on, allowing us to understand the mechanisms by which fouling occurs. Once we know the mechanisms, we can begin to design membranes to counter these,’ Rutledge explained.

The ability to measure both the geometry and chemistry of filtration materials – whether they prompt liquid droplets to coalesce or spread out – is a new step in the development of membrane technology. This makes it possible to choose the most suitable material for the task, examine its performance and characterise a new type of membrane.

As well as environmental applications, advanced separation membranes could be used for industrial operations including metal-working, food processing, or treating wastewater. Conversely, it would also help to remove emulsified water from oils, such as in the fuel lines of high-performance engines.

The technique relies upon CLSM equipment, which is widely available, but it also requires specialist knowledge of dyeing and sampling methods, which Rutledge believes will become more common as the technique is standardised.

‘We are very interested in gaining deeper understanding of the mechanisms of interaction between porous media – membranes – and immiscible liquids – oil and water – brought into contact with them. I believe there are a number of very fundamental chemical engineering questions in this field that can be tackled through a combination of new data acquisition, modelling and materials design,’ he said. ‘We will explore the limits of the method to determine what is possible and what is not. We will continue to develop tools and protocols needed to facilitate more widespread adoption of the technique.’