Granular materials such as sand, sediment, grains and tiny particles may look simple, but when they begin to flow, they can display surprisingly complex behaviour. Add flowing water to the system, and things become even more interesting.
In a new experimental study, Miles Morgan and his team investigated what happens when water is used to drive a dense bed of grains downward through a narrow vertical silo. They discovered several different flow patterns—including finger-like structures, porous flow and conventional silo flow—but one behaviour stood out: the sudden formation of straight, wormhole-like channels through the granular material.
Once these channels appear, they can rapidly grow toward the outlet, creating a preferred pathway through which water and grains move while much of the surrounding material is bypassed.
The discovery could help scientists better understand fluid-driven sediment transport and processes occurring in environments ranging from industrial systems to underground fractures and geological formations.
Why Granular Flow Is So Complicated
Granular materials behave differently from ordinary liquids. Sand, for example, can act like a solid when packed tightly, but it can also flow like a liquid under the right conditions.
The behaviour depends on factors such as grain size, gravity, friction, packing density and the speed of the flow. Researchers study granular systems under several different flow regimes, including slow, viscous and highly energetic flows.
A familiar example is a silo. When grains leave a silo through an opening at the bottom, material near the centre moves downward first. Grains farther away gradually move toward the centre to replace them.
This creates a depression at the top of the grain bed. Eventually, the sides become steep enough for grains to slide or avalanche downward. These avalanches continually feed material toward the centre.
In a conventional dry silo, the overall discharge rate can often be described using the Beverloo equation, which relates the flow of grains to factors such as outlet size and grain properties.
But things change dramatically when water is introduced.
What Happens When Water Drives the Grains?
Morgan and his colleagues created a submerged, quasi-two-dimensional silo using a narrow setup similar to a Hele-Shaw cell.
The researchers filled the system with grains of different sizes and then allowed water to flow through the material at controlled rates. The grains were eventually transported toward an outlet at the bottom.
Instead of producing just one predictable flow pattern, the experiments revealed several distinct behaviours.
Depending on the grain size and water flow rate, the system could develop:
Finger-like patterns
Porous flow through the granular bed
Classical silo-like flow
Straight, wormhole-like channels
The last of these was particularly unusual.
The Formation of Granular “Wormholes”
The researchers observed that, under certain conditions, a narrow central channel could suddenly develop inside the granular material.
This channel resembles a wormhole—not because it is an actual hole through empty space, but because it forms a relatively narrow pathway through which water and grains can move much faster than through the surrounding material.
Once the channel appears, it can rapidly propagate toward the outlet.
The result is a highly concentrated flow path. Instead of grains throughout a large region gradually moving toward the outlet, much of the flow becomes focused inside this central channel.
In effect, the channel acts as a bypass through the wider granular packing.
This is important because such a pathway can dramatically change how material is transported through a granular system.
A Battle Between Gravity and Water
So why does the wormhole appear?
The researchers suggest that the instability results from a competition between two different processes.
The first is gravity-driven flow at the surface.
As grains move toward the centre of the silo, the surface develops a V-shaped depression. Grains on the sides then avalanche downward, replenishing material near the centre.
This surface avalanching normally helps maintain a stable flow.
The second process is fluid-assisted flow inside the silo.
As the water flow increases, the central region can begin drawing grains downward more rapidly. Eventually, the demand for grains from this central region becomes greater than the amount that surface avalanches can supply.
This creates an imbalance.
And then something unusual happens.
If the central region is demanding more grains than the surface can provide, water begins occupying the space where grains are missing.
That can further encourage flow through the central region, causing a positive feedback effect.
A narrow channel develops, allowing water and grains to move through the packing much more easily.
The result is the formation of a wormhole-like structure.
A Critical Flow Rate
The instability does not happen at every water flow rate.
At lower flow rates, the gravitational avalanches at the surface can keep up with the material being removed from the central region. The interface therefore remains relatively stable.
But as the imposed water flow increases, the central flow becomes stronger.
Eventually, a critical condition is reached where surface avalanching can no longer supply enough grains.
Beyond this point, the system can transition into the wormhole regime.
The researchers found that this behaviour depends on both grain size and fluid flow rate.
Using empirical models describing surface grain motion and internal silo flow, they were able to predict the approximate height at which the instability develops.
Their predictions showed agreement with experimental observations across different grain sizes and flow conditions.
From Finger Instabilities to Wormholes
The experiments also connect the behaviour of the submerged silo to other fluid-driven granular instabilities.
When fluids move through granular materials, they can produce structures known as viscous fingers. These are narrow pathways that develop because fluid finds easier routes through the material.
Similar phenomena are known in other physical systems, including situations where gases or liquids are injected into granular materials.
However, the vertical silo configuration introduces an important additional factor: gravity.
Gravity causes grains at the upper surface to avalanche downward. This additional grain movement can stabilize the interface under conditions where finger-like instabilities might otherwise develop.
But when the fluid flow becomes strong enough, that stabilizing mechanism is no longer sufficient.
The system can then switch into a very different mode—the formation of a rapidly growing central channel.
Why This Discovery Matters
Although the experiments were conducted in a controlled laboratory system, the underlying physics could be relevant to many real-world environments.
Granular materials are found almost everywhere in nature and industry. Sand, sediments, soil particles and crushed materials are routinely transported by fluids.
Understanding how water creates preferential pathways through these materials could improve our understanding of sediment transport, hydraulic processing and erosion.
The findings may also be relevant to geological processes.
For example, underground water moving through sediment can gradually remove fine particles from a packed material. This process, known as suffosion, can contribute to changes in underground structures and is associated with certain sinkhole-forming processes.
Similar fluid-driven grain movement can occur inside fractures and faults. In fault zones, granular material known as fault gouge can be transported by fluids. Changes in how that material moves could potentially influence the mechanical behaviour of faults.
In industrial systems, understanding these flow patterns could also help engineers control how granular materials are transported through fluid-filled equipment.
A New Map of Granular Flow Behaviour
One of the important outcomes of the study is a phase diagram showing transitions between different flow regimes.
Instead of treating fluid-driven silo flow as a single phenomenon, the researchers show that several distinct behaviours can emerge depending on the relationship between grain size and fluid flow rate.
At one set of conditions, the material behaves more like a conventional silo. Under other conditions, fingers or porous flow can develop. At sufficiently strong fluid forcing, wormhole-like channels can appear.
This provides researchers with a framework for predicting when a particular type of flow may occur.
What Comes Next?
The researchers point out that more work is needed to fully understand the process.
One major challenge is developing a more accurate model of how grains move along the free surface while submerged in water. Such a model could then be coupled with the internal flow of grains inside the silo.
The researchers also suggest that friction between the grains and the silo walls may play an important role in determining when wormholes appear.
Future experiments using silos with greater internal spacing could help determine how strongly wall effects influence the instability.
A Hidden World Inside Granular Materials
The study demonstrates that a simple combination of grains, gravity and flowing water can produce remarkably complex behaviour.
The most striking discovery is that increasing fluid flow does not simply make all the grains move faster. Instead, beyond a critical point, the system can reorganize itself and create a narrow, rapidly developing pathway.
These wormhole-like channels reveal how small changes in the balance between fluid forces and gravity can produce completely different patterns of material transport.
From laboratory silos to underground fractures, such hidden pathways may play an important role wherever fluids interact with densely packed grains. Understanding them could ultimately help scientists predict—and engineers control—how granular materials move through natural and industrial systems.
Reference: Morgan, M.L., James, D.W., Monloubou, M. et al. Wormhole formation in fluid-driven granular flow. Commun Phys 8, 468 (2025). https://doi.org/10.1038/s42005-025-02366-w
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