Picture Credit: Wooj Han, Flickr
The world of science and technology has been in hot pursuit of the nanoparticle for the last few decades Bridging the gap between the atomic and the bulk levels, nanoparticles of all materials possess unique properties. These open up an almost endless array of new possibilities for applications in practically every field of human endeavour. The sustained efforts of the scientific community, driven by enormous investments in time and resources, has yielded great advances in the knowledge of the how and why of nanoparticle properties and behavior.
However, for all the possibilities that this new body of knowledge throws open, it would be next to impossible to employ any of it in practical applications without the ability and the tools to select, sort, arrange and manipulate specific particles as needed. Much work has been done to identify, develop and refine methods for this purpose.
For example, nanoparticle manipulation (in the range of 100nm) has been achieved through the use of external magnetic and electric fields to position and sort selective particles. These have also been integrated with electrostatically actuated micro-grippers to ‘pick and place’ particles. In microfluidic systems, methods such as di-electrophoresis, optical trapping and magnetic tweezers have been used.
Each of these methods has their merits and demerits in terms of technique and implementation for particle manipulation in microfluidic systems. For example, in the case of optical tweezers, the diffraction which occurs at the interfaces between different media will complicate the production of highly focused laser beams. For di-electrophoresis, the force field produced diminishes rapidly as distance from the electrodes is increased.
In contrast, employing mechanical vibration in microfluidic systems is simpler and more effective. When a fluid in an open container is vibrated at certain frequencies, standing waves are formed at the fluid-air interface. These capillary waves cause the fluid to flow within the boundaries of the container.This movement causes sediment particles to move to specific locations within the container.
Particle collection using this method has been experimentally demonstrated in the recent works but the mechanism behind the particle movement had not been understood and characterized. One of the methods that have been tried out is using acoustic radiation forces in the forms of Bulk Acoustic Waves (in the range of 100 kHz) and Surface Acoustic Waves (in the range of 10 MHz). But again, the use of acoustic radiation forces to manipulate nano-sized particles has been very limited.
At the IITB-Monash Research Academy in Mumbai, Research Scholar Prashant Agrawal has been studying this phenomenon. Guided by Dr. Prasanna S. Gandhi (IITB) and Dr. Adrian Neild (Monash), Prashant has been working to understand this movement of particles towards the collection region using an open rectangular chamber undergoing vertical and horizontal vibrations.
These shallow chambers are vibrated at low frequencies (in the range of 100 Hz) to produce wavelengths of the order of 1mm. The collection due to these first order fields and the effect of second order phenomena (referred to as streaming) is studied by simulating the flow field and characterizing the particle’s motion in it. Simulating the flow field generated due to such a vibration, understanding the particle motion in this flow field and further, characterizing its motion against various chamber and particle parameters are challenging and, at the same time, intriguing.
There is a huge gap between the operating frequencies of the existing acoustic wave technologies and the ones used by the team at IITB-Monash. According to Prashant, understanding particle behavior in the intermediate grey region, establishing a link between these two methods via simulations and then exploring any potential practical applications will provide a complete view of these collection mechanisms.
Prashant’s work has revealed that it is possible to collect very small particles (below the size of 1 micron) in well-defined lines at certain locations in an open rectangular chamber by simply shaking the chamber. The number of lines that the particles are collected in can be changed by simply changing the frequency with which we vibrate the chamber.
Picture Credit: Prashant
The collection of micron sized particles at low frequencies in an open rectangular chamber depends on the gradients in the flow field generated due to the resonant capillary waves at the water-air interface. The collection mechanism is primarily driven by the particle’s inertia and the phase difference between its motion and the fluid’s, as opposed to the second order pressure and velocity fields at the higher acoustic frequencies.
The ability to manipulate particles in a fluidic environment is crucial in many fields to accomplish tasks such as enhanced sensing, collection, concentration, and sorting. For example, this work will be primarily beneficial in biological sampling and can be used for extracting and concentrating cells and target particles to certain locations for analysis.
These systems provide the advantage of increased automation in manipulating particles and reduction in the reagent and the used sample. Simultaneous manipulation of nanoparticles in solutions within microfluidic systems has important analytical and sensitivity benefits, as it allows concentration and collection at sensing locations.
(Image of Prashant) The work done by Prashant tries to explain and highlight a phenomenon which had not been explored and therefore not understood in the past. He not only presents and explains the mechanism in this technique but also shows that it has an edge over the existing techniques. Says he, “The idea of exploring a field not understood previously in depth intrigued me. The link between this technique and the state of the art methodology is particularly exciting to explore.”
The IITB-Monash Research Academy operates a graduate research program in Mumbai. The IITB-Monash Research Academy is a Joint Venture between the IIT Bombay and Monash University. It fosters research partnerships between Australia and India. Research is conducted by scholars in both countries, whilst studying for a dually-badged PhD from the two organisations.
Research scholar: Prashant Agrawal
Project title: Microparticle manipulation using capillary effects through low frequency mechanical vibrations: Experiments and Simulations
Supervisors: Dr. Prasanna S. Gandhi (IITB) and Dr. Adrian Neild (Monash)
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