Advancing the Understanding of Electrified Jets

A jet is a rapid, columnar stream of liquid that is forced through a small opening, like what happens when you turn the water tap on. A jet does not have a distinct end to it. A filament is similar to a jet, but is much smaller and has a distinct end. Both jets and filaments are found commonly in day to day life, from the aforementioned taps to an ink jet printer.

Generally jets and filaments are not stable, this means that they undergo instability that can change their shape, so when you turn on that tap of water you don’t know exactly where each drop of water will break, land or what size each droplet would be. More viscous (or thicker) liquids tend to be a little more stable. For example, honey is a thicker liquid, so when you pour it you are able to estimate more accurately where it will land. Regardless, fluids of all viscosities are capable of creating jets and filaments.

When an electric field is applied to a jet it can control the way the liquid flows and breaks-up better, this is called an electrified jet. Electrified jets have been exploited in many industrial applications including electrospraying (such as in an ink-jet printer) and electrospinning. Electrospinning is a process that is used to create nanofibres (very fine fibres) that are used in a wide variety of applications from air filtration to artificial organ creation.

Siddharth Gadkari, a graduate research scholar at IITB-Monash Research Academy undertook research on different electrified viscous jet and filament systems. Given the potentially wide application of these systems in various scientific fields, his research has broad application. Overseen by Dr. Rochish Thaokar and Dr. P Sunthar of IIT Bombay, and Dr. Prabhakar Ranganathan and Dr. Ravi Prakash Jagadeeshan of Monash University, Gadkari’s research involved four different theoretical and computational studies.

The first study looked at electrospinning, which is a relatively simple technique used to generate nanofibres. It does this by applying an axial electric field at both ends of a jet of fluid that is flowing through a very fine opening. The electric field creates instabilities on the jet, including a whipping instability, that leads to swirling of the jet in hundreds of circles that increase in diameter in a short distance. This results in tiny fibres (nanofibres) being created. Whilst the proces is easy to perform, it is challenging to model as it brings together three different fields of physics (a multi-physics problem) and has a large number of dependent parameters (such as the type of liquid that is used, viscosity of the liquid, the strength of the electric field and the environmental temperature.

Gadkari’s research looked at whether there was a relationship between the diameter of the resulting fibre and the various process parameters. Being able to predict the diameter of a nanofibre is important when creating them for industrial use; generally the thinner the fibre the more uses it has. As the process of electrospinning is very fast, it can take mere seconds to create nanofibres once an electric field is applied to the liquid jet, making the process difficult to study. This research created a simple equation, taking into account all the relevant parameters, that can then predict the diameter of a nanofibre that would be created. Gadkari reached his solution by creating a non-dimensional equation. This involves effectively removing the specific dimensions of each parameter (for example, you can make a whole number dimensionless by dividing it by itself) and plotting these on a graph to determine if there was any correlation between the parameters and the diameters of the nanofibres produced.

The second study in this research looked at viscosity ratios between two fluids, where one fluid jet was placed inside another fluid, as part of the electrospinning process. For example, a water jet may be placed inside a honey solution. Because honey is more viscous than water, it slows down the electrospining process so that it can be better observed. Gadkari’s study plotted the viscosity ratio between the two fluids and looked at how this ratio affected the stability of the electrospinning process. This study improves the fundamental knowledge of the electrospinning process and will help other scientists detemine what viscosity of fluid to use in other studies.

The third study looks at the concept of capillary thinning. If you put some saliva between your index finger and thumb, you will see a small filament is formed that gets thinner and thinner at the centre as you increase the distance between your finger and thumb. This process is called capillary thinning. If you observe this you can see that, to begin with, the filament is relatively stable, but as it becomes thinner in the centre it becomes more unstable until it eventually breaks. The amount of time it takes a fluid to return to its equilibrium after it has been disturbed is called it’s relaxation time. It has been found that a polymer solution’s relaxation time is a specific property of that fluid, just like viscosity or density of the fluid is.

This study looked at the relaxation time of polymers in dilute solutions. A dilute solution is one in which the molecules are far from one another, but the properties of each individual molecule generally should not change even if concentration changes (in other words, there is only a weak dependency on concentration). As relaxation time is an integral property of fluids, it therefore should not change even if the concentration of the fluid changes.

Gadkari sought to explain why there appeared to be a dependence on relaxation time, when dilute solutions were used. He used a mathematical description of the liquid and created an advanced model for describing the polymer. This built on theoretical models that already existed, and was able to predict results of experiments much better than previous models. It was therefore successful in explaining the atypical dependence of relaxation time on the concentration of the polymer solution.

The final study of this research considered the importance of relaxation time in electrospinning. A viscous liquid will produce thicker nanofibres in electrospining then a thinner liquid, as thinner nanofibres are preferred for industrial applications, it is therefore important to understand how relaxation time impacts electrospinning and therefore the diameter of nanofibres. The diameter of a fibre is also directly proportional to the concentration of the polymer solution being used. The study tested fluids with low concentration and high relaxation time to determine how high relaxation time could help improve the stability of low concentrated polymers when producing nanofibres. Through a series of mathematical equations that include looking at mass, momentum and charge of electric field, Gadkari was able to confirm that a low concentration polymer solution could lead to stable jets where the solution had a high relaxation time. As a result this can be used in future studies to improve the properties of nanofibres produced through electrospinning.

Graduate research scholars of IITB-Monash Research Academy study for a dually-badged PhD from both IIT Bombay and Monash University, spending time at both institutions to enrich their research experience. IITB-Monash Research Academy is a collaboration between India and Australia that endeavours to strengthen scientific relationships between the two countries.

Reflecting on his research Gadkari said “I chose this topic because of its inherent complexities. As a multi-physics problem, few people choose to work on these issues yet we have only just started to scratch the surface of these problems.”

Processes such as electrospinning are critical in developing nanofibres, which are critical in many areas of scientific application. From the material in the sports shoes you wear to artificial joints, nanofibres have been involved in new product development. Understanding the techniques that create them and how they are impacted will help develop better products and medical solutions into the future.

Research scholar: Siddharth Gadkari, IITB-Monash Research Academy

Project title: Viscous liquid jets and filaments in electric fields: Stability analysis and role of viscoelasticity

Supervisors: : Dr. Rochish Thaokar and Dr. P Sunthar of IIT Bombay. Dr. Prabhakar Ranganathan and Dr. Ravi Prakash Jagadeeshan of Monash University

Contact details: sid.gadkari@gmail.com

The above story was written by Ms Rakhee Ghelani based on inputs from the research student and IITB-Monash Research Academy. Copyright IITB-Monash Research Academy.