Soft Matter Optics – talk at ACS -India

About 2 years ago (22nd May 2020), when all the academic activities were online, I gave a talk on “Soft-Matter Optics: A Cabinet of Curiosities” organized by American Chemical Society as part of India Science Talks. Below is the embedded video of the online talk.

Link to ACS website can be found here.

In there, I give a broad overview of how interesting optical function can emerge from the complex world of soft matter. In addition to this, I have emphasized how optics can be harnessed to study structure and dynamics of soft-matter systems including colloids, liquid crystal and some biological matter. The target audience are new PhD students and anyone who is entering the field of light-soft matter interaction.

58. Optothermal Pulling of colloids using Nanowire Plasmons – my talk at Compflu 2021

Linked is my recorded-talk presented at Compflu 2021 today (13th Dec) in the session : Active and Living Matter.

I discuss our recent work on optothermal pulling, trapping and assembly of micro-colloids under the influence of thermoplasmonic field of a single silver nanowire.

The talk was recorded on 2nd Dec 2021, so the reference on conclusion-slide is not updated.

52. Optothermal pulling and trapping..with nanowire plasmons

We have a new paper published in the journal ‘Soft Matter’ titled : Optothermal pulling, trapping, and assembly of colloids using nanowire plasmons

When a silver nanowire is optically illuminated under certain conditions, they propagate surface plasmons. These surface electromagnetic waves not only propagate light at subwavelength scale, but also generate heat along the nanowire.

A question of interest to us: can we use the quasi one-dimensional optothermal potential of a nanowire-plasmon to trap and assemble soft, microscale matter ?

Motivated by this question – Vandana, Sunny and Dipta from my research group, performed optical trapping based experiments to show an interesting pulling and trapping effect on dielectric colloids (see video). Furthermore, by increasing the concentration of the colloids, an emerging two dimensional crystal was observed. Interestingly, the formation of this two dimensional assembly was found to be sensitive to the optical polarization at the excitation point on the nanowire.

Thanks are also due to other co-authors: my colleague Vijayakumar Chikkadi and his student Rathi for helping us to implement the particle tracking code on python.

Optical trapping and tweezing is a fascinating area of research. By adding plasmons to the mix of things, these optical effects become intriguing. Importantly, they facilitate a platform to explore questions in non equilibrium statistical mechanics including optically driven active matter…

Afterall, more is different…

DOI of article :

Link to arxiv preprint:

All videos here :

27. Question of Life

This link has an interesting article by Paul Davies on an emerging question in science : “Does new physics lurk inside living matter?”
Ever since Schrodinger asked and wrote about “What is Life ?”, biology has always been within the grasp and underneath the metaphoric lens of physicists. Although this question has always drawn attention of physicists, a serious effort to address it was lacking in mainstream physics. This situation has changed, especially in past decade or so, thanks to evolution of physical tools and biology going quantitative and welcoming physics into their life…literally.
In recent times, many of the questions in biology have been re-casted as questions in mainstream physics, which makes it very appealing for researchers who wish to quantitatively measure things in a complex systems, and understand the mechanistic aspects of life and life-like objects (think bio-robots). Importantly, biology has readily offered a spectacular platform by opening itself for quantitative scrutiny. With new experimental tools, and a broad theoretical base of statistical physics, physics of living systems has arrived as a major sub-domain of physics.
From a physical science viewpoint, it is important to know how we go about addressing the questions raised by Davies in his article. The answer may be found by addressing some auxiliary questions at interface of soft matter science, fluid dynamics, statistical physics and information science. This pool of answers may get us closer to the frontiers of biology, and who knows, it may shine light on new questions, which would have otherwise gone unnoticed by the biologists themselves.
In my opinion, the experimental tools to address these questions need to come from various branches of science including chemistry, molecular, organism and evolutionary biology. As you may see, it requires an inclusive effort from various disciplines of science and technology, and mainstream physics has a vital role to play.
Time has also come, especially for the Indian physics community, to take this question seriously, and integrate with the above-mentioned domains, and pursue this fascinating aspect of life-science. A mere glance at any new issue of Phys Rev Lett or Nature Physics clearly says that biology has arrived in physics…big time…
After all, humanity is curious to know : what is life ? Physics may have some interesting answer(s)…

25. Ants@IISER as Active Matter

Nowadays, collective motion in active matter is one of the happening topics in the science of condensed matter, with a motivation in understanding biology at scales spanning from molecules to flock of birds. There is also a lot of contemporary research in active and driven natural systems and soft-robots at various length scales. Of my own interest is to understand how light can drive collective motion in synthetic colloids and other soft-systems in a fluid, and how they can lead to emergence of new assemblies.


Today, when I was walking in the IISER Pune campus, I came across a group of ants carrying food (see video above). It is amazing to see how coordinated is the movement of ants when carrying an object which is much larger than their individual weight (see video). One of the observations you can make is that how ants change their collective direction with minimum communication. How they do it is a fascinating question to explore. Undoubtedly studying such collective motion can lead to deeper understanding of not only the behaviour of ants and non-equilibrium systems, but also in designing adaptable soft-robots for various environments.

IISER Pune campus is quite rich in flora and fauna, and there is a lot to learn just by looking around the natural resources on campus. I hope to explore this rich environment in the context of soft matter systems, and report to you in this blog.