Researchers find junction is key in how pore space geometry impacts transport of substances through fluids

The junction is the key
Pore-space heterogeneity affects transport dynamics. a Experimental setup. A two-dimensional porous medium is constructed inside a microfluidic chip. The effect of pore space heterogeneity on transport dynamics is investigated as a fluorescent solute traverses the chip. The inlet design is optimized to create initially sharp fronts. Parallel pores, perpendicular pores, and pore junctions are depicted in the cartoon to the right. b Measurement of the dispersive front width. Cartoon explaining the front width measures employed, see also ‘Methods’. The concentration profiles are averaged perpendicular to the pressure axis. The deviation from a step function measures the efficiency of transport. The distance between relative solute concentrations of 25% and 75% is the experimental measure for the front width. Each experimental run is corrected for an offset caused by diffusive spreading in the tubing. The average offset is added as a constant to match with the order of magnitude of visually obtained front widths in microscopy images. c Change of the dispersive front width with disorder in simulation and experiment. The dispersive front width is measured as a function of disorder in experiments (rhomb) and simulation (circle) for Pe ≈ 30. Error bars show the standard deviations computed over a sample size of n = 6 for the simulation and n = 6 for the experimental data points χ = 1%, 11%, 16% and n = 5 for χ = 6%, 26%, 56%. The front width is evaluated when a relative solute concentration of 50% reaches the middle of the porous medium. The analytically derived functional form of Eq. (3) captures the non-monotonic dependency of the front width up to a disorder of χ ≈ 20%, as demonstrated by a fit over the simulation data points in the range χ ∈ [1%, 20%] with R2 = 0.97, see also supplemental theory sections 1.7 and 1.8. The non-monotonicity can be explained using pore junction statistics, but not by pore space statistics. Credit: Nature Communications (2022). DOI: 10.1038/s41467-022-33485-5

What laws govern how chemicals pass through filters? How do droplets of oil move through layers of stone? How do blood cells travel through a living organism? A team of researchers led by the Technical University of Munich (TUM) and the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) has discovered how pore space geometry impacts transport of substances through fluids.

Concentration takes energy. As you read this article, the in your brain expand and the speed of the blood stream slows so that your "reading neurons" receive more glucose and oxygen molecules from the .

"This all happens without any action on our part. Through evolution, nature has developed the ability to adapt the exactly to suit the changing requirements of organisms," explains Karen Alim, TUM Professor of Theory of Biological Networks and Max Planck Research Group Leader at the MPI-DS. "Our objective is to understand the physics underlying this adaptive network."

The research team, which included scientists from Nottingham Trent University, is now a large step closer to this goal: For the first time, their new model describes how the transport of substances through complex porous media is controlled by the microscopic structures of the media.

Porous media—in organisms just as in technically produced materials—are characterized by a complex system of cavities which can be penetrated by fluids transporting certain substances: In living organisms billions of cells are supplied by small and even miniscule blood vessels; water and oil can circulate though pores in sandstone, and bioreactors and filters contain porous catalyst materials which increases the reactive surface.

The secret of microflows

To investigate the flow-physical principle underlying these movements, Alim and her team chose a new, experimental approach: as their model for , the researchers chose microchips which they then equipped with tiny, pillar-shaped obstructions before letting a colored fluid flow through the chips.

They investigated three different obstruction geometries: In the first variation the tiny pillars were positioned on a perfectly symmetrical basic pattern, in the second case there were slight deviations from this symmetry and in the third case the pillars were arranged in a chaotic pattern. The researchers then measured how evenly the colored liquid dispersed through the entire space.

"The result was a complete surprise," lead author Felix Meigel recalls. The researchers expected that the liquid would penetrate the chip with the symmetrical pattern most efficiently. But in fact the color transport was only mediocre here: The color always dispersed out along the direction of flow, but didn't move into the neighboring pore spaces.

The best performer was however the chip with the slightly irregularly arranged obstructions: Here the liquid marked with color meandered back and forth and thus quickly filled up the entire pore space. The worst results came from the chip with the randomly arranged obstructions: Here areas formed which the colored liquid failed to reach at all; the transport efficiency of the liquid was correspondingly poor.

Branching is the key

The scientists have now been able to use calculations to explain the phenomenon: "The key to understanding what was going on was the network that the pores form," says Alim. "Previous research concentrated on the individual pores, which made it impossible to examine the complex overall system. We were able to show that the decisive factor is the immediate surroundings of the pores."

Thus the way the liquid disperses depends primarily on the branching of the pore spaces, the pore-junction units. Like the junctions in a system of water pipes, they control the direction and speed of flow.

"The results can now help in the development of materials in which liquids can optimally disperse," Alim predicts, adding that this could help optimize the transport of ions in batteries or increase the efficiency of catalysts and filters which depend on how well liquid reagents circulate around a catalyst or absorber material.

And last but not least, she says, the new findings will help make it possible to better understand the dynamics of vein networks in living organisms. In her next project the physicist plans to investigate how neurotransmitters control the optimization of blood transport.

The findings are published in the journal Nature Communications.

More information: Felix J. Meigel et al, Dispersive transport dynamics in porous media emerge from local correlations, Nature Communications (2022). DOI: 10.1038/s41467-022-33485-5

Journal information: Nature Communications

Citation: Researchers find junction is key in how pore space geometry impacts transport of substances through fluids (2022, November 30) retrieved 28 May 2024 from
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