We use optical illumination to generate thermal fields, creating non-reciprocal interactions between passive and active colloids. Active colloids absorb light and produce thermal gradients, driving thermo-osmotic forces that induce propulsion and chiral motion. Our Langevin simulations, backed by experimental observation, reveal how to control colloidal behavior. May have implications in light-driven chiral motion and nonlinear dynamics.
Super effort by Rahul, Ashutosh & Sneha from our group, who combined numerical simulations, analytical theory, with experimental observations.
The 2 anonymous reviewers made us think and work hard, and we thank them!
New ideas are often created by the merging of two old ideas. How often is this true, and how often do we tend to forget this?
Today I visited the Institute of Science Tokyo, formerly known as Tokyo Tech. This is a new avatar of a very interesting institution funded by the government of Japan. By merging the Tokyo Institute of Technology with the Tokyo Medical and Dental University, a very interesting concept has emerged: the Institute of Science Tokyo. These two institutions have been important pillars of the research and educational landscape of Tokyo, and I had the privilege of visiting this new place, which is a result of a new merger.
Thanks to the invitation and fantastic hospitality of Prof. Daiki Nishiguchi, a faculty member in the Physics Department of the Institute of Science Tokyo, I had a memorable experience. I met Daiki a couple of years ago at the University of Tokyo, where he previously held a faculty position. Recently, he has moved to the Institute of Science Tokyo to establish his independent research group as an Associate Professor.
I gave a physics seminar on some of our work on structured light and confinement of soft matter, especially thermally active colloidal matter in optothermal potentials. Since Daiki and his group (see image below) have expertise in topological soft matter, my seminar emphasized structured topological beams, including ring optical beams and optical vortices. I gave an overview of our experimental results and highlighted the prospect of utilizing the topology of light to interact with topological soft matter.
There is much to explore at this interface, and again, it brings me back to the point that new ideas often emerge from the merging of evolving old ideas, such as topological light and topological soft matter.
This is my third visit to Japan, and I always find their calm, focused, and deeply committed research environment inspiring. There is much to learn from their approach to science and technology, and my visit to the Institute of Science Tokyo reinforced this thought.
I thank Daiki and his research group for the wonderful time I had at their laboratory and offer my best wishes to him in his new explorations.
Whereas Sunday was bright, sunny, and clear for outdoor activities, Monday started cloudy with a forecast of rain. I started from my living place to Kyoto University around 10 in the morning. I took the city bus, which shuttles people from the city centre to the university. Within half an hour, I was in a serene, green, and beautiful campus, typical of a Japanese university. Kyoto University has a rich blend of modern and ancient architecture, and I was not surprised to see a large maroon-coloured ark at the entrance of the university.
With Prof. Tetsuro, who hosted me at the Graduate School of Informatics at Kyoto University.
I met Tesuji Tetsuro upon arrival (our previous in-person meeting was in the 2023 Optics & Photonics Congress on optical manipulation at Yokohama). He had just arrived from his run (he is a regular marathon runner), and we had a brief chat. He had arranged an office for me to occupy for the day. We had a short discussion and thereafter went for lunch. Prof. Kazuo Aoki (Tetsuro’s erstwhile advisor at Kyoto University) accompanied us, and I was delighted to meet him. We had a delicious lunch at a small Italian restaurant.
Around 3 pm, we met at the seminar hall where I gave my talk titled Hot Brownian Dynamics Driven by Structured Light. One of the key points I emphasized in my talk was the relevance of structured light in driving Brownian dynamics of colloids. I spoke about various parts of the stochastic differential equation (see equation 1 below) that represent the dynamics of a colloidal system interacting with an external force.
A key element of my discussion was the generalized driving force on the right-hand side of the equation, where the conventional restoring force in an optical trap can be generalized to an external driving force due to structured light. This versatile force is a result of a large set of linear and angular momentum states of structured light. These states can drive soft matter, further resulting in unconventional assembly and dynamics. Furthermore, the generalized driving force can include not only the optical force but also the thermal and hydrodynamic effects initiated by optical illumination. The combination of these forces culminates in a resultant force, offering an unconventional driving mechanism to drive the structure, assembly, and dynamics of colloids and other kinds of soft matter systems, including droplets and fluids. I showed some of our experimental results related to the above-mentioned concepts with emphasis on rotational and orbital degrees of freedom. I also presented our recent results on synchronization in an optothermal trap.
campus mapnear the entrance of Kyoto University at a Japanese izakayaWith Tetsuro and some PhD students
We had a long discussion on how to measure fluid dynamic properties around such colloids, especially when there is an external perturbation force, such as a laser beam, which can itself influence the colloidal dynamics. Tetsuro also mentioned his protocols and certain simulation strategies utilized to study thermo-osmotic flows in such situations. I learned about interesting methods they have been developing to numerically simulate the interactions using differential temperatures. The strategy is interesting and deserves further attention by the community. He also showed his experimental setup and gave a tour of his laboratory facilities.
Overall, it was a long, thoughtful day with wonderful discussions on topics of common scientific interest. We ended with a delicious dinner at a Japanese izakaya, and I thank Tetsuro for his invitation and hospitality. Kyoto University has a wonderful atmosphere for research, and I hope to visit again.
The central circle indicates anchored gold nanoparticles stuck to the glass, and the two moving circles are gold colloids that are trapped synchronously due to the optothermal potential.
We have an Arxiv preprint on how a mixture of colloids (thermally active + passive particles in water) can lead to the emergence of revolution dynamics in an optical ring trap (dotted line). Super effort by our lab members Rahul Chand and Ashutosh Shukla.
Interestingly, the revolution emerges only when an active and a passive colloid are combined (not as individuals) in a ring potential (dotted line)
the direction (clock or anti-clockwise) of the revolution depends on the relative placement of the colloids in the trap
This revolution can be further used to propel a third active colloid
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 !
First, the geography : located in the midst of the ocean(s), away from the mainland Japan, Okinawa is the southern most part of Japan. I took a 2.5 hrs. flight from Tokyo and reached the destination when it was dark.
In the morning, I woke up and opened the window to be pleasantly surprised to see the beautiful ocean in my field of view. Below is a photo I took from the balcony of the room.
At the outset, I had a fabulous discussion. Mahesh and I had a excellent free-wheeling discussion on various topics including science, arts and history. Also, I visited his lab to see very creative experiments on non-equilibrium physics including some fascinating experimental designs to probe dynamics at liquid-air interface. He has a fabulous set of students working on interesting problems. It was both refreshing and stimulating.
with Mahesh Bandi
In the afternoon, I gave my seminar and below is an interesting announcement (because it spells out my full name :-)). I discussed about some non-equilibrium aspects of our work, and towards the end emphasized some interesting prospects of statistical optics in an optothermal-assembly.
After my talk, I had illuminating discussion with Prof. Pinaki Chakraborty on thermal diffusion, fluid dynamics and history of science. I enjoyed visiting his lab, in which they have built one of the biggest Taylor-Couette flow set up I have ever seen (see the frame behind Pinaki in the picture below).
with Pinaki
Next day, I had an elaborate lab tour of Prof. Sile Nic Chormaic, who is an expert in various topics related to optics/photonics including optical trapping. As she was travelling, her post-doc Dr. Souvik Sil, hosted me and took me around to show various aspects of their lab. It was a great learning experience.
with Souvik
On the same day, I visited the cutting-edge lab of Prof. Keshav Dani. Since he was travelling, his team members gave me a brief tour and I was amazed by some of their experiments.
Another major highlight of my trip was to meet many IISER students. Specifically, I met Amit Bhunia (post-doc) and Saurav Raj (PhD student) who are IISER Pune alumnus. I also met many IISER Kolkata students (Souvik being one of them).
Overall, I was mighty impressed by the research infrastructure at OIST and the people working there. It is quite remarkable how they have been able to build such a great facility in such a short time (their campus started around 2010).
a corridor in OIST
Today, I have reached Osaka, and on Monday will be giving a talk and visiting a few groups at Osaka Unviersity. Prof. Hajime Ishihara is my host, and I look forward to plenty of interaction on optical manipulation.
Fluorescent video imaging of trapping a single fluorescent nano-diamond on a gold nanoparticle
We have a new paper appearing in Optics Letters on optical manipulation of fluorescent nano-diamonds. This experimental work (with some simulations) was performed by our group member Ashutosh Shukla in collaboration with the group of Prof. Kasturi Saha (IIT-Bombay). Our alumni, Sunny Tiwari, gave vital inputs to perform the experiments.
Fluorescent nano-diamonds have emerged as important (quantum) imaging agents in biological applications. It remains a challenge to manipulate them in complex fluidic environments.
Herein, we have come up with an optical trapping method based on opto-thermoelectric effect (see Fluorescence imaging video above). By using opto-thermal potentials created by a single gold nanoparticle (~200nm) on a glass surface, we have been able to trap individual nano-diamonds, and capture their spectral signatures. The Brownian motion in the trap can be tracked, and this leads to the measurement of effective trap stiffness. Furthermore, we extrapolate this thermo-plasmonic trapping method to trap and track individual nano-diamonds on silver nanowires.
As mentioned in the abstract of the manuscript, we envisage that our drop-casting platform can be extrapolated to perform targeted, low-power trapping, manipulation, and multimodal imaging of FNDs inside biological systems such as cells.
VISMAYA - History & Philosophy of Science 