March 2022, ScientificAmerican.com 19FLUID DYNAMICSUnusual Flow
A strange fluid effect revealed
For most fluids, an increase in pressure
should lead to a burst of speed, like squeez-
ing ketchup from a tube. But when flowing
through porous materials such as soil or
sedimentary rock, certain liquids slow down
under pressure. Pinpointing the cause of this
slowing would benefit industries such as
environmental clean-up and oil extraction,
where pumping one liquid into the ground
forces another out; however, such move-
ment is challenging to observe directly.
Princeton University chemical engineer
Christopher Browne and physicist Sujit
Datta offer a solution to this puzzle. By
tweaking a special liquid to be transparent
and pumping it through the pores of an
equally transparent artificial rock, they doc-
umented how the liquid’s movement
becomes chaotic, causing swirling eddies
that gum up the pores and slow the flow.
The fluids of interest, called polymersolutions, are dissolved
versions of large stretchy
molecule chains common
in biology as well as the
cosmetics and energy indus-
tries. Theoretical studies have
suggested that when the chains
stretch through a nearly flat channel and
then recoil, they generate forces that stir up
eddies. But whether that turbulence “arises
in realistic 3-D soils, sediments and porous
rocks has been hotly debated,” Datta says.
To resolve the controversy, the research-
ers pumped a synthetic polymer solution
into a simulated “sedimentary rock” built
from a box filled with tiny glass beads. They
tweaked the polymer solution’s precise
chemistry by diluting it slightly to change
how light refracts, rendering the “rock” fully
transparent even when saturated.
The scientists laced the polymer with flu-
orescent chips and tracked its movement
through the pores under a microscope,
recording patchy regions of eddies and mea-
suring how the solution flowed under differ-
ing pressure. This confirmed that the mac-roscale slowing had its
microscopic origins where
researchers had suspected:
polymer chains stretching
out and then coiling back as
they passed through pores. The
findings appeared in Science Advances.
“Visualizing flow inside a 3-D porous
media literally gives a window into some-
thing that was impossible to see,” says Uni-
versity of Pennsylvania biochemical engineer
Paulo Arratia, who was not involved in the
study. As a next step, “if you could actually
see the molecules stretching and recoiling,
that would be wonderful [to] connect the
molecular point of view to the microscopic.”
Industrial applications require knowing
which specific pressures are needed to push
a polymer solution through a porous mate-
rial at a given flow rate. The study provides
a physical model describing that relation and
could predict, for example, how much con-
taminant can be retrieved from a chemical
site by injecting a solution. “Without predict-
ability,” Datta says, “injection operations are
trial and error.” — Rachel BerkowitzFLUID DYNAMICSUnusual Flow
A strange fl uid eff ect revealed
For most fl uids, an increase in pressure
should lead to a burst of speed, like squeez-
ing ketchup from a tube. But when fl owing
through porous materials such as soil or
sedimentary rock, certain liquids slow down
under pressure. Pinpointing the cause of this
slowing would benefi t industries such as
environmental clean-up and oil extraction,
where pumping one liquid into the ground
forces another out; however, such move-
ment is challenging to observe directly.
Princeton University chemical engineer
Christopher Browne and physicist Sujit
Datta off er a solution to this puzzle. By
tweaking a special liquid to be transparent
and pumping it through the pores of an
equally transparent artifi cial rock, they doc-
umented how the liquid’s movement
becomes chaotic, causing swirling eddies
that gum up the pores and slow the fl ow.
The fl uids of interest, called polymersolutions, are dissolved
versions of large stretchy
molecule chains common
in biology as well as the
cosmetics and energy indus-
tries. Theoretical studies have
suggested that when the chains
stretch through a nearly fl at channel and
then recoil, they generate forces that stir up
eddies. But whether that turbulence “arises
in realistic 3-D soils, sediments and porous
rocks has been hotly debated,” Datta says.
To resolve the controversy, the research-
ers pumped a synthetic polymer solution
into a simulated “sedimentary rock” built
from a box fi lled with tiny glass beads. They
tweaked the polymer solution’s precise
chemistry by diluting it slightly to change
how light refracts, rendering the “rock” fully
transparent even when saturated.
The scientists laced the polymer with fl u-
orescent chips and tracked its movement
through the pores under a microscope,
recording patchy regions of eddies and mea-
suring how the solution fl owed under diff er-
ing pressure. This confi rmed that the mac-roscale slowing had its
microscopic origins where
researchers had suspected:
polymer chains stretching
out and then coiling back as
they passed through pores. The
fi ndings appeared in Science Advances.
“Visualizing fl ow inside a 3-D porous
media literally gives a window into some-
thing that was impossible to see,” says Uni-
versity of Pennsylvania biochemical engineer
Paulo Arratia, who was not involved in the
study. As a next step, “if you could actually
see the molecules stretching and recoiling,
that would be wonderful [to] connect the
molecular point of view to the microscopic.”
Industrial applications require knowing
which specifi c pressures are needed to push
a polymer solution through a porous mate-
rial at a given fl ow rate. The study provides
a physical model describing that relation and
could predict, for example, how much con-
taminant can be retrieved from a chemical
site by injecting a solution. “Without predict-
ability,” Datta says, “injection operations are
trial and error.” — Rachel Berkowitzsad0322Adva3p.indd 19sad0322Adva3p.indd 19Untitled-1 1Untitled-1 1 1/20/22 4:40 PM1/20/22 4:40 PM1/20/22 6:03 PM1/20/22 6:03 PM
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