Have you ever boiled water in a pot and noticed swirling patterns that seem to rise from the bottom and eventually vanish? Those fluid patterns are called convection cells, a type of circular fluid flow set up by differences in temperature, pressure, or other forces. Convection cells are everywhere, from our coffee cups, oceans and atmosphere to the Sun’s atmosphere and even the clouds and dust in space.
Convection cells can sometimes turn into a different flow pattern known as a shear flow, where the fluid (or plasma) moves in distinct layers, each flowing faster or slower. Like layers of cards sliding against each other, a slipping motion causes one layer to slide faster than another, causing a horizontal flow. Scientists at the Institute for Plasma Research at Homi Bhabha National Institute have studied such flow patterns to understand their behaviour. In a new study, they are trying to figure out what causes convection cells to flip to a shear flow state.
The researchers examined the particles’ behaviour on a tiny (microscopic) scale to understand how convection cells can transform into a shear flow. By following the velocities of each particle, they could spot where and how the flow starts to tilt or change its structure. They used a system known as a Yukawa liquid, a collection of charged dust particles immersed in plasma, to study particle behaviours.
A Yukawa liquid is a system of charged “dust” grains interacting with electrons and ions in a background plasma. Plasma, the stuff stars are made of, is an ionised gas containing free electrons and ions. When micron-sized dust particles get charged, they can repel or attract each other and interact with the background plasma. Yukawa liquids are excellent for studying fluid behaviours since they act like a fluid at a small scale, yet the particles are big enough to be seen under a microscope so scientists can track them directly.
The researchers used classical molecular dynamics simulation software, multi-potential molecular dynamics code (MPMD-v2.0), to simulate the behaviour of the Yukawa liquid. They also defined a 2D box where the “dust” particles are placed. The box has length and height dimensions, representing the space in which the particles can move.
To kick-start, the researchers introduce forces so that the dust grains form convection cells. This can be done by applying a temperature gradient or a similar drive. Real systems always have minor disturbances, like a slight bump or a bit of random motion. To mimic this, the researchers introduced small velocity perturbations to destabilise the convection cells. As the simulation runs, they record information about the velocity field, i.e., how fast and in which direction every particle moves. They also calculate what’s known as the “Reynolds stress,” a key quantity indicating how the sideways or diagonal components of particle velocity contribute to the overall flow patterns. It is a factor that describes how fluctuating velocities within a fluid can transfer momentum. In simpler words, if particles are swirling around unpredictably, they can push the fluid in specific directions more strongly than in others. This pushing can tilt convection cells, turning them from neat circular loops into slanted cells and, eventually, into a shear flow where layers of fluid move side-by-side at different speeds.
In their study, the team found that the aspect ratio of the box where the liquid is confined matters. When the length of the box is considerable compared to its height, the convection cells can combine into a larger vortex or remain stable. But when the length is relatively small, the system is more likely to create that layered, side-by-side movement—a bi-directional shear flow. The team found that the reason behind this shift is the self-generated Reynolds stress. Once amplified, the random microscopic motions of particles tilt the convection cells, eventually causing the streaming flow that we call shear flow.
The team also found that If they include friction or collisions with neutral atoms, the growth of these shear flows is slowed down or suppressed. If the particles lose energy to collisions, it’s harder for them to maintain big-scale flows.
The study provides a first-principles representation of how the flow changes from convection into shear, with the system represented by a 2D square box and disturbances by small periodic velocity changes. However, to gain a deeper understanding, a more complex 3-dimensional system with more randomly changing variables will have to be studied, which better represents a real-world system. Observing real-world dust grains forming and losing convection cells in favour of a shear flow would also provide a substantial test of the simulation predictions.
Although a fundamental science investigation, shear flows offer insights into the behaviour of fluids that could have immediate applications. In specific fusion devices, like Tokamaks, shear flows help stabilise the plasma. Understanding how shear flows form can help engineers control them. Many atmospheric and oceanic patterns also involve convection cells, which can turn into a layered shear flow, like the gasses in Jupiter's atmosphere. While the systems are not identical, insights into how flows become layered could influence simplified modelling approaches.
The study shows that even the tiniest particles can exhibit behaviours that mirror large-scale geophysical phenomena when allowed to move freely and interact in a charged environment. This discovery clarifies some of the most basic ideas in fluid dynamics and has potential implications for fields as diverse as fusion research, atmospheric science, and future plasma studies.
This research news was partly generated using artificial intelligence and edited by an editor at Research Matters