When you purchase loose rice at the supermarket, you instinctively measure it out in a scoop and pour it into your bag. It flows smoothly, like a liquid, even though it is actually just a collection of many solid particles. Scientists say that it belongs to the family of complex fluids.
Complex fluids are often mixtures of particles and liquids, whose response to applied forces is more intricate than that of simple fluids like air and water. Sand flowing in an hourglass, or blood flowing in our bodies are evocative examples. Despite their widespread occurrence in nature and industry, their flowing motion is poorly understood.
Over 50% of materials processed in many large industries today such as mining, agro, pharmaceuticals and food processing, all handle large quantities of particulate materials. They need to be transported, processed and distributed. All these actions involve controlling the flow of these complex fluids – a critical operation in some industries such as pharmaceuticals. Unfortunately, we do not yet fully understand how these complex fluids flow or deform under applied loads. Prof. Prabhu Nott and his research group at the Department of Chemical Engineering at Indian Institute of Science (IISc), Bangalore, are working towards solving this mystery.
“We were essentially trying to build simple theoretical descriptions for flowing grains,” recollects Nott about his early days as a theorist in this field, 22 years ago. Today, his team performs an equal mix of theoretical, computational and experimental studies on different types of complex fluids. Dense, slow, granular flows are one such type – perfect to represent processed food and grains that are stored in tall silos and distributed from them. The team performs flow imaging and stress measurements on rice a variety of grains using their unique, home-grown apparatus.
“I don’t believe such an apparatus exists elsewhere,” says Nott while demonstrating the machine.
Based on the classical Couette geometry for measuring viscosity of a fluid, it consists of two cylinders that rotate around the same axis. Grains are placed in the annular gap between the two cylinders. Only the inner cylinder rotates at a fixed speed, while the outer one remains stationary. The grains begin moving along with the inner cylinder and flow patterns develop in them. What makes Nott’s set-up unique is a sensor that can be moved along the outer cylinder, which measures the force at different heights and times of the granular flow, giving an insight into the spatial variation of the forces during motion.
Recently, their experiments and computations revealed an unexpected flow pattern. In addition to rotating with the inner cylinder, the grains also rolled up and down the entire length, forming in a single vortex! This spectacular secondary flow is in stark contrast to the multiple, small vortices that are observed during pure liquid flow. In this case, the flowing grains exert a net frictional force on the outer cylinder wall, to which the wall responds back with an opposite force. The team tested flows of glass beads, sand grains, rice grains and even mustard seeds. All of them behaved similarly.
Doesn’t the shape and size of the grain matter? Indeed, different grains will exert a different amount of net frictional force. Nott adds that there are many other properties that will change in their net value based on the collection of grains being used. Quantifying them is an important challenge.
His student, Peter Varun Dsouza, is tackling this challenge by first trying to miniaturize their unique experimental apparatus. A smaller version will make it mobile enough to perform on-site tests for different type of particles – from powders in the food processing industry to wet coal in the mining industry. It will also be more suited to study very fine granular powders of different chemical components that go into the manufacture of medicines today. To complement the experiments, another student K P Krishnaraj performs computations that use simple grain interaction models to gain insight on the flow, and understand how forces are transmitted in granular systems.
The ultimate aim is to design better processing equipment for various types of particle flows, and maybe even customise them for different types of particles. In the food industry, it can lead to accurate control on grain flow, intermixing of grains, grinding and size reduction, as well as coatings. In medicine, Nott’s research can take us closer to precise pharmaceutical dosing. And in our offices, it can lead to long-lasting toner cartridges that perfectly control the amount of carbon used for printing.
Nott sums it up perfectly - “If you can control powder flow accurately, the applications are endless.”
About the lab
Prof. Prabhu Nott is a Professor at the Department of Chemical Engineering at Indian Institute of Science (IISc), Bangalore.