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…

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.

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 !

57. Single nanoparticle driven thermoplasmonic tweezer : single-molecule SERS

We have a new paper published in Journal of Physical Chemistry Letters on “Single Molecule Surface Enhanced Raman Scattering in a Single Gold Nanoparticle-Driven Thermoplasmonic Tweezer”

Thanks to the fantastic effort by Sunny Tiwari, and excellent support by Utkarsh Khandelwal (former IISER-P undergrad) and Vandana Sharma from my group, we have been able to combine single molecule Raman scattering with a specialized nanoscale optical tweezer.

The uniqueness of this tweezer platform  is that the optical trapping process is driven by the thermo-plasmonic potential created by a SINGLE, 150nm GOLD NANOPARTICLE. Concomitantly, the same field can be used to perform single-molecule Raman spectroscopy. Kind of  “ek teer mae do shikar” strategy Smile

Using this system, not only we push the limits of optothermal trapping of a single nanoparticle (see video) at low laser powers, but also create a platform for deterministic transport of reversible colloidal assembly in a fluid.

We envisage that our nanometric plasmonic tweezer can be harnessed to trap and tweeze biological entities such as single virus and bacteria. Another possible application of our study is to create reconfigurable plasmonic metafluids in physiological and catalytic environments, and to be potentially adapted as an in vivo optothermal tweezer.

All the videos related to this study can be found on our lab’s Youtube channel :

DoI of the published paper :

preprint version on arxiv :

35. Water droplets on hot tawa

Dosa (dosae in Kannada) is one of the most relished dishes in India. An important prerequisite to prepare a good dosa is a hot pan, usually called as tawa.

Usually, just before the dosa batter is spread on the tawa, a few drops of water is sprinkled on this heated surface.

The video shows the dynamics of water droplets on a heated tawa at around 800 frames per second. Notice how the droplet expands, oscillates and evaporate….all at a very fast pace

Interestingly such fluid dynamics and oscillations can also be realized by heating a metal surface with a laser beam, which we do do in my lab. Of course, in such a situation, the laser heating is more localised and dynamics of the fluid is more complex, and importantly one can trap and optically manipulate colloids, nanoparticles and molecules, in such environments. More on this in a future blog..

31. Fluctuating Sun..on a surface

In front of IISER Pune’s guest house, there is a small, artificial pond which is filled with rainwater. In there are tiny aquatic creatures and some beautiful lotus flowers. Recently, I happened to capture a high speed video (920fps) of the water surface fluctuating in this pond using the reflection of sun’s image (see video above). You will also notice a nice flower in the foreground which adds to the aesthetics.

What is interesting about this oscillation is the way the reflected image of the sunlight fluctuates as a function of time. In physics, there is a wonderful connection between fluctuating surfaces and the light reflected from such a surface. In principle, one can find out a lot about the nature of the fluctuation of the surface, including its topography, spatial frequency etc., by studying the amplitude and phase of the light that is reflected from such a moving surface.

One such example is the way atomic force microscope (AFM) works. In an essence, the topography of the surface an AFM reads, is by recording the fluctuation of light that is reflected from a tiny cantilever close to the surface.

Another fascinating concept related to probing fluctuations using light is the field of cavity optomechanics. The radiation pressure of the optical field couples to a tiny mechanical oscillator, and this interaction leads to a change in the the spectral characteristics of the light in a cavity. By studying this spectrum, one will be able to extract meaningful information about the tiny fluctuations in a cavity. This concept also applies to quantum fluctuations, and is one of the happening subfields in quantum optics and photonics.

Of course there are many such applications of using fluctuation of light to study oscillations in matter.

The model of simple harmonic oscillator that we study in physics is not only of basic relevance to understand any kind of fluctuation, but also applies to a variety of scientific processes in spatial, temporal and spectral domains. Added to this, if we learn about Fourier series and Fourier transforms, then we can go deeper in understanding fluctuations of any kind.

This reminds me of a quote attributed to Sidney Coleman:

The career of a young theoretical physicist consists of treating the harmonic oscillator in ever-increasing levels of abstraction.”

This is also true of experimental physics or for that matter most of the aspects of measurement science and technology. After all, fluctuations are ubiquitous, and harmonic oscillators are the windows into this beautiful world.

24. Bubbles in nimboo soda

Bubble assembly at a curved lemon-soda interface

One of the fascinating things about liquid-solid interface is that it gives a platform for fluids to assemble in a variety of geometries that can be tailored by changing the properties of the interface. Among the formations, bubble generation and assembly are intriguing aspects. If you observe the bubbles at the interface of a lemon slice dipped in soda(image above), they are almost spherical in shape, indicating a large contact angle.

How fluids interact on a solid surface depends on an important concept called as wetting. Associated with this wettability is the contact angle between a droplet/bubble and the solid beneath it. Based on the measure of this contact angle, one can classify how well or otherwise a drop/bubble can wet on a solid.

For a water droplet resting on a solid surface, larger contact angles, close to 90 degree, indicates that the surface is hydrophobic in nature. A lotus leaf is an excellent example of a hydrophobic surface. If the angle happens to be, say around 10 degrees, then the liquid spreads very easily on the surface and hence it is called as hydrophilic surface.

This kind of classification of surfaces based on wetting has a huge implication in studying liquid-solid interfaces including blood flow, capillary phenomena in plants, and of course in paint and printing industry, and many more.

Recently, I came across a research paper-highlight which connects the formation of bubbles to the energy problem. It always amazes me how simple concepts in science can inspire research problems and lead to fundamental questions and applications.

Let the bubbles rise..

ps: thanks to wordpress app, I have been able to write and post this blog directly from my mobile phone. That makes it quick and easy 😬

Continue reading “24. Bubbles in nimboo soda”

23. Falling droplets….

Below is a video I took on falling water droplets from a tap at my home. Observe how a large drop detaches itself from the tap and falls down, not as a single drop, but as a series of droplets with certain degree of periodicity associated with it. The video was shot at around 960 frames per second.

Why does this happen ? A simple answer is : to minimize surface energy. Interestingly, the transition of a large drop to smaller droplets is mediated via formation of a liquid tread, which further breaks up into smaller droplets. This tread (not evident in my video) takes the form of an instability, and facilitates the process of minimizing the free energy. The nature of this breakup depends on parameters such as surface tension, viscosity, density and geometry of the liquid thread. The initial conditions, such as the opening of the tap and pressure of the flow, too play a critical role in determining the droplet formation.

Actually, the problem of falling droplets has a rich history, which dates back to the times of Leonardo da Vinci (who else Hot smile), who made innumerable observations on the fluid flow (see some comments from his notebook here). There are many other people who have contributed towards our understanding of this problem. In the current literature, this instability problem is generally know as Plateau-Rayleigh instability, name after the two who played a vital role in quantifying this phenomenon and generalizing it to fluid jets.

In recent times, thanks to high speed photography, our visualization and hence deeper understanding of this instability problem has enormously increased. This understanding is fantastically communicated in a public lecture titled “The life and death of a drop” (see embedded video below) given by Sidney Nagel. This video has some spectacular movies captured by high speed camera ( > 10,000 frames per second) and looks at the falling droplet problem from the viewpoint of basic physics. 

Why is this interesting problem ? Apart from the aesthetic and curiosity, the problem of fluid jets and their evolution is of great relevance in understanding fundamental processes of fluid dynamics, including astrophysical situations.  Also, the problem of fluid droplets, their instability and splashing is of huge relevance in applications such as ink jet printing, wall painting, water reservoir management, blood flow analysis and many other problems in physiology and biomedicine. 

What strikes me about the falling droplets is its simplicity and universality. It reminds me of a poem by Emily Dickinson:


How happy is the little stone
That rambles in the road alone,
And doesn’t care about careers,
And exigencies never fears;
Whose coat of elemental brown
A passing universe put on;
And independent as the sun,
Associates or glows alone,
Fulfilling absolute decree
In casual simplicity.