From blood to paper:
Fundamental knowledge in the field of particle flows

Researchers from Graz, Princeton and Zurich observe crucial phenomenon in T junctions

Everything flows in fluid dynamics. A researcher from Graz University of Technology, together with colleagues from Princeton University and ETH Zurich have hit upon a previously undiscovered phenomenon in the flow behaviour of particles. Particles, such as blood cells, gas bubbles or pulp fibres accumulate under certain conditions in T-shaped junctions. The decisive factors are flow velocity, particle density and particle size. By means of the description of these parameters, particle accumulations at these T junctions may not only be prevented, but also in the future deliberately brought about. This could lead to far-reaching consequences – for instance in the paper industry. This fundamental finding has been published in the current issue of the prestigious journal PNAS.

From blood vessels to large industrial plants, T-shaped junctions play an important role in both nature and technical applications as a universal geometric unity and can be found, for instance, in our blood vessels. Stefan Radl of the Institute for Process and Particle Engineering at Graz University of Technology together with “fluid-mechanics luminary” Howard Stone of the University of Princeton and Daniele Vigolo of ETH Zurich observed for the first time that particles are “trapped” and accumulate in T junctions under certain conditions. “As so often in research, the observation occurred by accident – we were actually focusing on a different aspect of particle flow,“ explains Stefan Radl. Then, using simulations and experiments, the three researchers investigated when and why the particles remain at the T junction. “Three factors play a role: the flow rate, particle density and particle size. We were able to derive limits theoretically for all three parameters and back them up with experimental data,” continued Radl.

Allowing particles through or “trapping” them

The observed phenomenon is regarded as a fundamental piece of the puzzle in classical fluid mechanics and was published by the prestigious journal PNAS in the current issue. By means of the described three parameters responsible for allowing particles through or trapping them, accumulations of particles could be deliberately prevented in the future. To give a practical example in medicine: gas embolism. “If a diver comes up to the surface too quickly, gas bubbles can accumulate in the branches of the blood vessels, clog them up and lead to the diver’s death. Gas bubbles also behave like particles and through our observations, we can better understand the formation of the gas embolism and how to avoid it,“ explains Stefan Radl. But not only that: “In other cases, it can be desirable to hold back specific particles and to separate them from the fluid, for instance in the paper industry, “ continues Radl. In the framework of the Austrian Research Promotion (FFG) project “FLIPPR - Future Lignin and Pulp Processing Research“, researchers from the Institute of Paper, Pulp and Fibre Technology and from the Institute for Process and Particle Engineering at Graz University of Technology together with colleagues from the University of Graz and the University of Natural Resources and Life Sciences (BOKU) are researching the possibilities of targeted particle separation. Well-known partners in the paper industry are also supporting FLIPPR and are hoping for an early implementation of Stefan Radl’s research findings.

The next steps

Up to now, the researchers have carried out the experiments of particle flows in T junctions on a small scale, where the earth’s gravity is negligible. “Now we have to raise the investigations to the next level and repeat them at a larger scale. In the future, we want to investigate whether other parameters influence particle flow behaviour in industrial plants,“ says Stefan Radl, with an eye on the future.

Full bibliographic details: Daniele Vigolo, Stefan Radl, Howard A. Stone: Unexpected trapping of particles at a T junction. PNAS Early Edition, March 2014.

Further Information on FLIPPR