Team Quantum 

A chimney releases a cloud of gas with billions of particles. While dredging, massive amounts of solid particles get injected into seawater. Then you have underwater volcanoes, nuclear accidents and a host of other instances where a particle cloud gets released into another (usually denser) medium. 

How the particles distribute themselves in another medium is of interest to researchers because that can give clues on how to control them. You need look no further than Delhi’s smog problem to imagine a potential application for this insight. 

Prof Pallab Sinha Mahapatra of the Fluid Systems Laboratory, Department of Mechanical Engineering, IIT Madras, has studied the distribution of particle clouds in media. The details of the work were recently published in the journal Physics of Fluids. 

He says that when a large number of particles fall as a cloud into another medium, the cloud behaves like a fluid. The dispersion of particles is influenced by many factors, including the liquid-air interface. 

“We have modelled the dropping particles within a liquid and just above the liquid-air interface and used numerical approaches to examine the effect of liquid-air interface on the particle cloud evolution,” says Mahapatra. The team examined whether changes in initial conditions, such as releasing a particle cluster within the liquid and just above the liquid-air interface, has any influence on particle cloud evolution and subsequent deposition. 

The study showed that without the liquid-air interface, the particle cloud is initially cylindrical but progressively transforms into an inverted mushroom, inverted cup, and vortex ring. 

On the other hand, the presence of the liquid-air interface transforms the cloud shape from cylindrical to a distorted cylinder, sphere with trailing stem, inverted mushroom, and vortex ring. More importantly, the researchers found that the viscosity (thickness) of liquid alters the dispersion characteristics in the initial stages, near the liquid-air interface, and alters the particle dispersion mechanisms in the later stages. 

“The understanding of the particle cloud evolution can help us understand the deposition patterns of particles in various real-life scenarios such as pollution and industrial phenomena,” Mahapatra says. Particle clusters falling in viscous liquids have broad applications in industrial and geophysical flows, including dredging, underwater volcanic eruption, and nuclear accidents. For example, the massive quantity of solid particles injected into seawater during dredging operations can contaminate and affect marine life. Hence, care should be taken to reduce the spread of the contaminant particles as much as possible.

In the event of a nuclear accident, the nuclear reactor chambers are designed to improve the spread of molten metal droplets falling in the coolant liquid, which reduces the risk of vapour explosion. Hence, manipulation of cloud evolution is essential in industrial flows. So far in previous research, the influence of the liquid-air interface on particle dispersion was not discussed. The current research evaluates whether the initial conditions can affect the dispersion characteristics of particle clusters or clouds. Simply injecting particle clusters by keeping the nozzle within the liquid or just above the liquid-air interface can manipulate the particle dispersion phenomenon.

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