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.

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..

32. Floating bubble…

Optics of bubbles and droplets is always fascinating to study. Above picture is of a floating soap bubble whose surface reflects the illuminating light source (in this case a single tubelight). Interesting questions: what determines the position and number of reflections ?

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 😬

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