R Sri Muthu Mrinalini & G R Jayanth | IISc, Bangalore
As more uses are found for nano-sized particles, there is an increasing need to ‘see’ and ‘feel’ them in order to understand their properties and behaviour better. For decades, electron microscopes have been used to observe atomic-sized particles. These days, however, Atomic Force Microscopes (AFM) are gaining popularity as they have certain advantages over the electron microscope.
Unlike electron microscopes that send electron beams to ‘illuminate’ the object under observation, and thus ‘see’ it, AFMs use a very sharp needle, with dimensions of the order of a few nanometres, to ‘feel’ the object and develop its geometry. AFM takes a blind man’s approach to sense the shape of an object, touching and feeling it with fingers (needle) to understand what it looks like. Only, it does it with a lot more precision. Human fingers would be unable to get around all the curves and crevices of every object, but the AFM’s needle can. By sensing the variations in the mechanical forces during its interaction with the object, and subsequently controlling the force, it is able to very accurately ‘see’ how the small particles look like and what exactly their structure is.
AFMs have another big advantage over electron microscopes. They do not necessarily require vacuum conditions to operate. AFMs can work in air as well as in liquid medium.
But AFMs have a problem in their needles. Their tip is very sharp and it gets blunted or broken very frequently, and needs to be changed. The way AFMs are currently built, replacement of the tip is not a very smooth process. The tip is attached to a spring-like structure that has integrated sensors to measure the mechanical forces. Right now, replacing the tip means replacement of the needle as well as the spring. It is done manually and one needs to wait for about half-an-hour after the replacement, to resume operation. In industrial set-ups, this is a big hassle.
New instruments have emerged in the market that allow automatic replacement of needles. But for industries that have been using older AFMs, buying entirely new instruments can be a costly affair.
It is in this area that our work can have some significance. We have developed a method to replace the needle without changing the spring. More importantly, existing AFMs can be retrofitted with this technology, so buying new instruments won’t be necessary. This alone can potentially bring down the cost of automatic replacement by a factor of 100.
What we have done is to introduce a micro-gripper between the spring and the needle. This micro-gripper is actually a tiny liquid droplet. It is about a few picolitres (10 -12 litres) in volume. It acts like an adhesive to which the needles can get attached. A liquid droplet might not seem to be an ideal adhesive to hold anything firmly in place, but the relative magnitudes of forces operating at the micro-scale are very different from what we experience. At that scale, the surface tension of the liquid is very strong and good enough to hold the needle.
We have also developed a mechanism for detaching the needle from the micro-gripper without disturbing any other part of the instrument. This ability to attach and detach the needle to and from the micro-gripper presents an effective way of replacing the needle without needing to change the spring.
We are not the first ones to attempt replacement of needles in the AFM without disturbing its other parts. But we are the only ones to have shown that it can be done in ambient conditions — others have been able to achieve this only in vacuum — and can also be retrofitted to older instruments. The replacement process is still manual and we are currently working on its automation.