Category: physics

Osaka and Optical Manipulation

Thanks to the invitation of Prof. Hajime Ishihara, I visited Osaka from 29th April to 2nd May 2023.

I arrived in Osaka on Saturday (29th Apr). On 30th Apr (Sunday) I headed out to visit the famous Kiyomizudera Temple in Kyoto with 2 PhD students from Ishihara’s group : Hideki Arahari and Takao Horai

Kiyomizudera Temple towards the right and beautiful landscape surrounding it.

Apart from the temple, we had a wonderful time exploring the Nishiki Market and Kamo river.

On 1st May, I visited Osaka University to officially meet Prof Ishihara, his group and other research groups in the university. We had a wonderful discussion on optical manipulation and major projects related to it especially in Japan, and undoubtedly Osaka has emerged as a major center in optical manipulation.

with Prof. Ishihara

I was delighted to see so many students and research groups interested in optical trapping/manipulation and related science and technology. I learnt about some very interesting applications of optical manipulation. Also, it was great to see such a great sense of humor in this group of researchers. It was truly amazing.

Science + Humor …..we had a great time..

I also visited labs of Prof. Ashida and Prof Ito, and got a very nice overview of their work including optical manipulation in liquid helium and photochemical reactions in an optical trap.

At 4pm, I gave my talk on “Optical Manipulation based on Opto-Thermal Gradients“. I elaborated on the role of absorption and related thermal gradient in optical manipulation. I presented some of our ongoing work on optical manipulation with structured light. The talk was attended by at least 30 to 40 people, and I was informed that students from various groups in Osaka were present (there are a few universities). The quality of questions and the follow up discussion was very good, and really enjoyed it.

Finally, the day ended with a wonderful dinner at a tofu-themed Japanese restaurant. We had wonderful discussion on history and philosophy in our countries, and was very interesting.

Dinner with Professors Ishihara, Ito, Ashida and Yokoyama

My trajectory in Japan has been Yokohama –> Tokyo –> Okinawa –> Osaka. After 16 days, one conference talk and 4 research seminar across Japan, I am now heading back from Osaka to Tokyo (and writing this blog) on Shinkansen – the bullet train. Tomorrow, I will be leaving to India.

In Japanese, the word ‘Osaka’ also means ‘a large hill’. Japan’s geography has many large hills (see a picture of Mt. Fuji I took from the moving Shinkansen), and metaphorically speaking Japanese like to scale complex landscapes of life by bringing culture with science and technology. There is a lot to learn from this approach to life.

the majestic Mt. Fuji

Osaka, optical manipulation and the perennial ascent towards knowledge. What a memorable trip this has been.

To Japan and to all the people I met here – Doumo Arigatou Gozaimasu !

Two talks in Tokyo

Wonderful Japanese-Gothic architecture at The University of Tokyo

I gave two talks in Tokyo.

First was on 24th April at Department of Applied Physics, Faculty of Advanced Engineering, Tokyo University of Science. My host was Prof. Yutaka Sumino. I spoke about “Soft Matter in Opto-Thermal Gradients“. I gave a short introduction to opto-thermal perturbations and potentials, and discussed some of our work on opto-thermophoretic trapping and Brownian dynamics. The audience contained a few master students too, and I really enjoyed discussing some concepts related to Brownian motion in an optothermal trap, and related experiments. Also, I had a very interesting discussion with Sumino and his students on their experiments on Janus particles.

with Yutaka
with Yutaka”s group

The second talk was on 25th April at Department of Physics, University of Tokyo. My host was Prof. Kazumasa Takeuchi. I spoke about “Soft Matter in Opto-Thermal Gradients : Evolutionary Dynamics and Pattern Formation“.

This talk was also announced on Japan’s statphys mailing list, and also live-casted over zoom. I discussed about the origins of optothermal effects in a laser trap, and how it can lead to some interesting dynamics and pattern formation in soft-matter system. Specifically, I highlighted the concept of Hot Brownian motion, and how it can be influenced using thermo-plasmons. The talk and discussion went on for almost 2 hours, and I really loved it. Also, Takeuchi and his students gave an overview of their work including a live demonstration on turbulence in liquid crystals, and it was fantastic.

with Takeuchi
with Takeuchi’s group

Halina Rubinsztein-Dunlop – History of Optical Manipulation

Below is a video blog featuring Halina Rubinsztein-Dunlop as part of history of optical manipulation. Also pictured in the blog are : Giorgio Volpe (UC, London) and Agnese Callegari (University of Gothenburg). Pictures taken at OMC 2023 in Yokohama, Japan.

Some relevant links at the end :

Notes :

Professor Halina Rubinsztein-Dunlop (2023). Available at: https://smp.uq.edu.au/profile/204/halina-rubinsztein-dunlop (Accessed: 19 April 2023).

Halina Rubinsztein-Dunlop‬ – ‪Google Scholar‬ Available at: https://scholar.google.se/citations?user=4_sqVfYAAAAJ&hl=en (Accessed: 19 April 2023).

‘Halina Rubinsztein-Dunlop’ (2023) Wikipedia. Available at: https://en.wikipedia.org/w/index.php?title=Halina_Rubinsztein-Dunlop&oldid=1145639271 (Accessed: 19 April 2023).

seminal papers :

Friese, M.E.J. et al. (1998) ‘Optical alignment and spinning of laser-trapped microscopic particles’, Nature, 394(6691), pp. 348–350. Available at: https://doi.org/10.1038/28566.

He, H. et al. (1995) ‘Direct Observation of Transfer of Angular Momentum to Absorptive Particles from a Laser Beam with a Phase Singularity’, Physical Review Letters, 75(5), pp. 826–829. Available at: https://doi.org/10.1103/PhysRevLett.75.826.

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Geo-fractal…somewhere over south India

I am always amazed to see fractal-like patterns….this one at a geographical scale…captured somewhere over south India..

I took the photo on my trip back to Pune from IIT Madras…

Thanks to IIT-M physics department for their invitation for colloquium…

Special thanks to Basudev Roy and Nirmala for hosting…. greatly enjoyed the discussion with many faculties and students..

In my talk, I mainly spoke on topics at the interface of statistical optics, Brownian motion and pattern formation..
Was delighted to see (and meet) Profs. Balki, Suresh Govindarajan Sunil Kumar Arnab Pal and many more in the audience.

The photo, retrospectively, captures the essence of the science discussed…

Hot Brownian Colloids – talk

On 19th Jan 2023, I gave a ~40 min talk on “Hot Brownian Colloids in Structured Optical Tweezers” in a very interesting conference on Frontiers in Non-Equilibrium Physics (FNEP) held at Institute of Mathematical Sciences, Chennai.

I mainly spoke about emergent Brownian dynamics of laser-heated colloids under optical confinement. Below is the link to the talk.

I concluded my talk quoting P W Anderson’s essay “More is Different“

“The constructionist hypothesis breaks down when confronted with the twin difficulties of scale and complexity.  The behavior of large and complex aggregates of elementary particles, it turns out, is not to be understood in terms of a simple extrapolation of the properties of a few particles.

Instead, at each level of complexity entirely new properties appear, and the understanding of the new behaviors requires research which I think is as fundamental in its nature as any other.”

P.W. Anderson  ‘More is Different’
Science, 177, 4047 (1972)

More is not only different, but also wonderful !

Basic advice to undergrads: written assignments

Below is some basic advice I shared with my undergraduate class (Physics majors/Optics). This may be useful to other students here.

  • Do not copy a text verbatim, unless you are quoting the text as it is, with reference at that location.  If you use a source, then write a summary in your own words and cite the original source at the location of your text.
  • In the absence of primary data (which you generate originally), you will be using secondary sources such as research papers, books and internet content. For scholarly purposes (including assignments), it is better to use journal articles and books as references. Wikipedia is essentially a tertiary reference. Although some entries are good, wikipedia content is generally taken from a book or a research paper. Identify that source and use it for your assignment and reference. Directly citing wikipedia (which is usually not peer reviewed by experts) is not a general practice in scientific literature. 
  • When you take a figure or equation from a secondary resource (paper, book, internet), it is important that you cite the source in figure caption/equation location.
  • In an assignment, I am mainly interested in reading what you think about the topic under consideration. Feel free to put down your thoughts and compare and contrast it to the literature you use. This is where the thinking + learning happens.
  • For some general advice on referencing, especially for online communication, have a look at my blogpost.
  • To appreciate importance of written communication in learning, have a look at another blogpost.

New paper :”Optothermal evolution of active colloidal matter in defocused laser trap”

We have a new paper to be published in ACS Photonics on “Optothermal evolution of active colloidal matter in defocused laser trap”

In the context of non-equilibrium statistical mechanics, it is relevant to ask : how does a system of (active) Brownian particles respond to environmental cues ?

Structured light in the form of an optical trap can facilitate a platform to create unconventional, environmental cues in which Brownian particles can dynamically assemble and evolve as a function of space and time.

In this work, we utilize a simple defocused laser trap and study the evolutionary dynamics of thermally active colloids (polystyrene spheres infused with iron oxide). We observe a variety of (nonlinear) dynamical states including hovering of a pair, a kind of synchrony among the assembled colloids in the trap (see video).

Thanks to the great efforts of Dipta and Rahul from my group, we have been able to study and unveil the complex forces at play. The dual contribution of optical potential of the laser and thermophoretic interaction of colloids were revealed by systematic experiments and numerical simulations leading to this elaborate report.

This study further motivates interesting questions in the context of microscopic heat engines where the light and heat created in an optical trap can lead to some interesting nonlinear dynamics of soft matter systems. Another prospect is to characterize structured optothermal fields using Brownian motors as microscopic probes in an optical trap. More on this in the coming month….

all videos related to the work on our lab youtube channel

preprint version of the paper on arxiv : https://arxiv.org/abs/2203.08904

I will post the published version of the article when available

A relevant book and its review

A book review has appeared an important and timely book on a topic which is subject to misinformation:

Are Electromagnetic Fields Making Me Ill? How Electricity and Magnetism Affect our Health, by Bradley J. Roth

reviewed by Robert G. Olsen in the most recent issue of the American Journal of Physics. https://aapt.scitation.org/doi/full/10.1119/5.0107970

As the reviewer says:


“While the focus is on the specific controversies listed in the table of contents, an even more valuable contribution of the book is the description of Roth’s methodology. In describing his approach to evaluating the science (or lack thereof) behind each claim, you will learn some of the thought processes needed to evaluate any new technology. Anyone who adopts these approaches will become a better investigator of new claims”

I have been following book-reviews of American Journal of Physics
There are some really good recommendations and assessments.