130 research outputs found
Drainage in a model stratified porous medium
We show that when a non-wetting fluid drains a stratified porous medium at
sufficiently small capillary numbers Ca, it flows only through the coarsest
stratum of the medium; by contrast, above a threshold Ca, the non-wetting fluid
is also forced laterally, into part of the adjacent, finer strata. The spatial
extent of this partial invasion increases with Ca. We quantitatively understand
this behavior by balancing the stratum-scale viscous pressure driving the flow
with the capillary pressure required to invade individual pores. Because
geological formations are frequently stratified, we anticipate that our results
will be relevant to a number of important applications, including understanding
oil migration, preventing groundwater contamination, and sub-surface CO
storage
Confinement and activity regulate bacterial motion in porous media
Understanding how bacteria move in porous media is critical to applications
in healthcare, agriculture, environmental remediation, and chemical sensing.
Recent work has demonstrated that E. coli, which moves by run-and-tumble
dynamics in a homogeneous medium, exhibits a new form of motility when confined
in a disordered porous medium: hopping-and-trapping motility, in which cells
perform rapid, directed hops punctuated by intervals of slow, undirected
trapping. Here, we use direct visualization to shed light on how these
processes depend on pore-scale confinement and cellular activity. We find that
hopping is determined by pore-scale confinement, and is independent of cellular
activity; by contrast, trapping is determined by the competition between
pore-scale confinement and cellular activity, as predicted by an entropic
trapping model. These results thus help to elucidate the factors that regulate
bacterial motion in porous media, and could help aid the development of new
models of motility in heterogeneous environments
Elastic turbulence generates anomalous flow resistance in porous media
Diverse processes rely on the viscous flow of polymer solutions through
porous media. In many cases, the macroscopic flow resistance abruptly increases
above a threshold flow rate in a porous medium---but not in bulk solution. The
reason why has been a puzzle for over half a century. Here, by directly
visualizing the flow in a transparent 3D porous medium, we demonstrate that
this anomalous increase is due to the onset of an elastic instability. We
establish that the energy dissipated by the unstable flow fluctuations, which
vary across pores, generates the anomalous increase in flow resistance through
the entire medium. Thus, by linking the pore-scale onset of unstable flow to
macroscopic transport, our work provides generally-applicable guidelines for
predicting and controlling polymer solution flows
Harnessing elastic instabilities for enhanced mixing and reaction kinetics in porous media
Turbulent flows have been used for millennia to mix solutes; a familiar
example is stirring cream into coffee. However, many energy, environmental, and
industrial processes rely on the mixing of solutes in porous media where
confinement suppresses inertial turbulence. As a result, mixing is drastically
hindered, requiring fluid to permeate long distances for appreciable mixing and
introducing additional steps to drive mixing that can be expensive and
environmentally harmful. Here, we demonstrate that this limitation can be
overcome just by adding dilute amounts of flexible polymers to the fluid.
Flow-driven stretching of the polymers generates an elastic instability (EI),
driving turbulent-like chaotic flow fluctuations, despite the pore-scale
confinement that prohibits typical inertial turbulence. Using in situ imaging,
we show that these fluctuations stretch and fold the fluid within the pores
along thin layers (``lamellae'') characterized by sharp solute concentration
gradients, driving mixing by diffusion in the pores. This process results in a
reduction in the required mixing length, a increase in
solute transverse dispersivity, and can be harnessed to increase the rate at
which chemical compounds react by -- enhancements that we rationalize
using turbulence-inspired modeling of the underlying transport processes. Our
work thereby establishes a simple, robust, versatile, and predictive new way to
mix solutes in porous media, with potential applications ranging from
large-scale chemical production to environmental remediation
Controlling the Morphology of Polyurea Microcapsules Using Microfluidics
We use microfluidics to continuously produce monodisperse polyurea microcapsules (PUMCs) having either aqueous or nonaqueous cores. The microcapsule shells are formed by the reaction between an isocyanate, dissolved in oil, and an amine, dissolved in water, at the surface of oil-in-water or water-in-oil drops immediately as they are formed. Different microcapsule morphologies can be generated using our approach. The thickness of the microcapsule shell increases with an increase in the amine solubility in the oil; this finding provides a simple mechanism by which the PUMC shell thickness can be controlled
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