Particles don’t always go with the flow (and why that matters)
It is commonly assumed that tiny particles just go with the flow as they make their way through soil, biological tissue, and other complex materials. But a team of Yale researchers led by Professor Amir Pahlavan shows that even gentle chemical gradients, such as a small change in salt concentration, can dramatically reshape how particles move through porous materials. Their results are published in Science Advances and featured on the journal’s cover.
Why you should care how colloids move
How small particles known as colloids, like fine clays, microbes, or engineered particles, move through porous materials such as soil, filters, and biological tissue can have significant and wide-ranging effects on everything from environmental cleanups to agriculture. It’s long been known that chemical gradients—that is, gradual changes in the concentration of salt or other chemicals—can drive colloids to migrate directionally, a phenomenon known as diffusiophoresis. But it was often assumed that this effect would matter only when there was little or no flow, because phoretic speeds are typically orders of magnitude smaller than average flow speeds in porous media. Experiments set up in Pahlavan’s lab demonstrated a very different outcome.
“We report that even if you have a very strong flow in the background, salt gradients can drive the particles to migrate across the fluid streamlines, from fast to slow or vice versa,” said Mobin Alipour, a post-doctoral researcher and the lead author of the study.
Porous media are everywhere, from soil to biological tissues, so the applications of this science are vast. And because colloids and chemical gradients are found in filtration, wastewater treatment facilities, chromatography, agriculture, and drug delivery, the researchers note that a better understanding of the physics behind them could benefit these fields. In wastewater treatment facilities, for instance, chemical gradients could potentially be controlled to increase filtration efficiency. In drug delivery, they could help ensure that drug-carrying colloids reach their intended sites with enhanced precision.
How they did it
Since you can’t see through soil, the researchers created their own porous media out of transparent polymers. On a chip, they created a maze-like environment with channels about 100 microns wide (roughly equal to the width of a human hair). They filled this porous medium with a suspension of micron-sized colloids in salty water and then flushed it with another solution at a different salt concentration. Under a microscope, they followed the trajectories of fluorescent colloids as they made their way through the pathways under constant flow.
What they found was that, despite the relatively strong force of the flow, the chemical gradient from the salt had an unexpectedly significant effect on the movement of the particles. The mechanism is counterintuitive but simple: diffusiophoresis creates small cross-streamline migration events. When particles drift into faster streamlines, their net speed increases; when they drift into slower regions, they linger. Those microscopic choices accumulate into large macroscopic differences in how quickly, and how broadly, particles travel.
“We expected the influence of salt gradients on the overall transport of colloids to be negligible when there’s strong flow,” said Amir Pahlavan. “But those tiny sideways phoretic nudges due to salt gradients are persistent, and add up, changing the global picture of transport drastically.”
Going forward
Beyond the immediate result, the study points to a broader takeaway: chemical gradients can modulate the influence of geometric disorder in porous materials. In other words, the pore architecture still matters, but chemical landscapes can amplify or damp its effect, offering a potential route to steering transport without changing the structure itself.
These findings could influence how scientists think about particle transport in settings where gradients naturally arise, such as salinity intrusion in coastal aquifers, fertilizer and nutrient gradients in soils, or chemical microenvironments in biological tissues.
The researchers say a key next step is to understand how the effect depends on particle surface chemistry and multi-ion environments, and how to translate controlled microfluidic insights into real materials such as membranes and soils. Another frontier is turning the phenomenon into design rules: when do gradients help flush particles out, and when do they enhance trapping?
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Published Date
Feb 16, 2026
