Ensuring smooth fluid flow at a micro scale

Have you ever experienced how much energy is needed to push fluid out of a needle attached to a syringe? Replace the needle with a tube of slightly larger diameter, and you exert much less energy. When the diameter of the passage shrinks, the pumping energy requirement amplifies.

So what happens when the passage shrinks? What governs this energy amplification? The answer: Ratio of surface area to volume.

Simply put, a smaller diameter tube has more surface-area-to-volume ratio than a larger one, thus requiring more energy to pump out the fluid. To use fluid mechanics terminology—more the surface area, greater the resistance due to no-slip boundary condition at the tube wall.

This concept of surface-area-to-volume ratio has been exploited at a microscale in applications such as electronics cooling, where an increase in surface area leads to an increase in heat transfer. This has contributed significantly to the growth of an academic discipline called microfluidics.

A microchannel is a channel with a hydraulic diameter below 1 millimetre. Microchannels are used in fluid control and heat transfer.

Flow passages are miniaturised in order to develop portable devices in healthcare, microelectronics and chemical analysis. These devices usually require large pumping machines, to push the fluids through microchannels, which affects portability. This is why it has been extremely difficult to exploit microfluidics commercially, although it holds great promise.

One possible solution is to coat the microchannel walls with a low-energy material that does not allow the fluid to stick to the walls (this is called the classic no-slip boundary condition in fluid mechanics). Instead, it makes the liquid "slip" on the wall.

However, the amount of slip generated by the low-energy coating is ususally not sufficient. If it is possible to reduce the fluid-solid contact area by introducing gas cavities on the microchannel walls, a greater reduction in resistance can be achieved.

One such surface already exists in nature—the lotus leaf. The surface of a lotus leaf surface consists of gas cavities coated with low-energy wax material. This makes the leaf's surface repel the water, giving it self-cleaning and anti-wetting properties. Such surfaces are termed 'super hydrophobic'.

Can we mimic this on a microchannel wall? If so, how much reduction in energy is possible? What causes the gas cavities to vanish? Can they exist for a longer period? Finally, what happens at the liquid-gas interface?

Anvesh Gaddam, a research scholar at the IITB-Monash Research Academy, is trying to address these issues. Supervised by Dr Suhas S Joshi, Dr Amit Agrawal, and Dr Mark Thompson, his approach is two-fold—computational and experimental.

The IITB-Monash Research Academy—a Joint Venture between IIT Bombay and Monash University—operates a graduate research program in Mumbai that fosters research partnerships between Australia and India. Research is conducted by scholars in both countries, whilst studying for a dually-badged PhD from both organisations.

Anvesh's research is broadly divided into three aspects.

The first one involves an examination of the nature of air-water interface. Theoretically, the resistance across the air-water interface depends on the viscosity ratio. But, in practical conditions this might be compromised by both geometrical, flow parameters and to some extent on impurities. In this study, he tries to ascertain the parameters that affect the quality of air-water interface.

The second aspect involves finding the critical parameters that affect the stability of the air-water interface. In general, the gas cavities disappear due to dissolution of the gases into the liquid and the pressure difference acting across the liquid-gas interface. After detailed investigation, he proposed a possible mechanism to stabilise the liquid-gas interface in microchannels. He manipulated the position of micro-vortices in gas cavities to delay the collapsing of the air-water interface at relatively low pressures.

The third part of the study comprises usage of the low viscous fluids in gas cavities to avoid collapsing of the fluid-fluid interface at high pressures. Here, a few correlations are being proposed to predict the reduction in resistance across the microchannel. These correlations, once developed, can be used by designers to predict the amount of pumping power required to design a particular microfluidic device.

Says Prof Murali Sastry, CEO of the IITB-Monash Research Academy, "A large number of multi-national companies have set up R&D centres in India. It is important that IIT Bombay and Monash University are connected to the research agenda that is being crafted in these R&D centres. Given its strong industry-facing intent, the Academy is an important vehicle that will help achieve this connection."

Anvesh is quick to agree.

"Given the immense benefits of manipulation of fluids at microscale in reducing the amount of sample and analysis time in medical diagnostics," says Anvesh, "it is disheartening to see just a handful of microfluidic devices in the market, due to some of the factors mentioned above. By the end of this decade, I hope to see commercial development of microfluidic devices at a much larger scale."

Given his painstaking work in this field and his commitment, we are confident that things will turn around soon.

Research scholar: Anvesh Gaddam, IITB-Monash Research Academy

Project title: Design and Fabrication of Patterned Surfaces

Supervisors: Design and Fabrication of Patterned Surfaces

Contact details: anvesh.gaddam@gmail.com

This story was written by Mr Krishna Warrier based on inputs from the research student and IITB-Monash Research Academy. Copyright IITB-Monash Research Academy.