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 ?
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.
Generally, when the Nobel prizes are announced in October, it is an occasion to celebrate science. The people who get the prize are shot into the limelight, and deservingly so. It is also an occasion where the subjectivity behind these prizes get exposed.
One such case is the invention of laser and the Nobel prize related to it. People who work with lasers and who are aware of its history will readily recognise that Theodore Maiman, the person who actually created the first working laser at optical wavelength, did not receive the Nobel prize.
It has been around 60 years since the invention of lasers, and John Dudley, in a recent commentary, has briefly summarised the context in which black body radiation science was initiated and its evolution towards lasers and the subsequent nomination and award of Nobel Prize. Some of the numbers related to nomination, that Dudley furnishes, is very interesting:
“Maiman, despite being the first to see laser emission, never won the Nobel Prize, and neither did Jim Gordon. Whilst it is natural to consider these omissions as major oversights by the Nobel Committee, the available Nobel Prize archives reveal that the lack of any Nobel recognition for Maiman and Gordon may simply be linked to the fact that they were not strongly supported by the broader physics community at the time. In particular, starting as early as 1958, Charles Townes had been nominated 75 times for the Nobel Prize, including 29 nominations for the year in which he won. In contrast, based on what we know of the nomination archives (which are accessible until 1966), Gordon was nominated only once in 1963 and Maiman only once in 1964.”
Interestingly, I also learnt the initial reason why black body radiation was studied in late 1800s. To my surprise, it seems it was initiated purely for an economic and practical purpose, which turned out to be a trigger point for the revolution in quantum mechanics. As Dudley says:
“In fact, it is not widely appreciated that these studies were not initially motivated by questions of fundamental scientific curiosity, but were rather stimulated by a very practical and economic problem. In particular, the city of Berlin at the time was choosing between gas and electric lighting, essentially the same problem as we have had in recent years in switching from incandescent and fluorescent lights to LEDs. Naturally, when making such a decision, standardizing the spectral content of the different light sources was a critical first step, and it was this that drove experiments to measure precision radiation curves of sources at different temperatures. Theoretical work by Wien was able to connect the peak emission wavelength and the source temperature, but explaining the shape of the emission curve was only possible with the introduction of energy quantization by Max Planck in 1900.”
This highlights how nonlinear the evolution of ideas are, and how new directions in science can be motivated and triggered by something which is purely practical. In some other cases, great science has also evolved from “blue sky” curiosity driven research, as in the case of laser, which has enormous practical utility.
Perhaps there lies the beauty of science: if you pay attention, everything is an inspiration…
Internet is a funny thing. I started searching for some research paper but I ended up with a totally unrelated news feature from Nature 1, which ended up as an interesting read. It is about two Japanese crystal growers from Japan : Kenji Watanabe2 and Takashi Taniguchi 3 who work at National Institute of Materials Science (NIMS) in Tsukuba. They have come into the limelight for their outstanding skills of growing high quality crystals of hexagonal boron nitride or more commonly called as hBN in the research community. The news article gives a very nice overview of how they go about growing their crystals with an element of human touch 1 :
“The two researchers have contrasting styles. Taniguchi is known for his parties, blasts the music of Queen through the lab as he runs the press late at night and, even at the age of 60, still plays soccer with his colleagues at lunchtime. Watanabe, three years younger, is soft-spoken, detail-oriented and prefers tennis. But the scientists worked well together and published their first paper on cBN crystals in 2002.”
In scientific research, an important aspect of lab-based experiments is that it critically depends on the quality of the sample that one is interrogating, and more so in research on condensed matter, where quality of the material is paramount. After the emergence of graphene as a remarkable 2D material 4, various researchers across the globe (including IISER-Pune) have been intensely studying graphene and other 2D materials. In order to probe graphene in the lab, first you need to place the one atom thick material on a substrate. Generally, silicon (with or without silicon oxide) is used for this purpose. The electronic mobility of graphene, which is to be maximized for its magic work, critically depends on the quality of the substrate (and superstrate) on which it is placed on, and hBN is the best choice for maximum mobility. This is where quality of hBN comes into picture, and Watanabe and Taniguchi are considered the masters of growing high purity hBN crystals, exclusively for this purpose, as the news feature highlights 1. Of course, hBN is not limited to be just a substrate for graphene based nano-electronic devices. It has also emerged as an interesting nanophotonic material, which can potentially function as a hyper lens. All these applications critically depend on the quality of the sample one can produce, and hence, growth of high quality crystal is so important.
At IISER-Pune, right in front of my lab is the lab of my colleague – Surjeet Singh5. In his lab, they grow some fascinating crystals of quantum magnets, superconductors etc. Furthermore, many of my other colleagues, including chemists and biologists, grow and study crystals made of inorganic, organic and biological materials. I have to mention that, in India, there are many researchers across the country who are excellent crystal-growers. In fact, India has a rich history in crystal growth research ( for example: GN Ramachandran school 6), and I hope this legacy will continue with the support of academia and funding agencies. After all, high quality materials need high quality skills to grow and characterize them, and I have repeatedly heard that crystal growth is as much as an art as it is a science.
Speaking of science and art, Nobel prizes for 2020 has been announced. Great to see a good representation of women among the laureates this year. It is befitting to end this blog with a poem7 by Louise Glück (the literature laureate of 2020):
October (section I) by Louise Glück
Is it winter again, is it cold again,
didn’t Frank just slip on the ice,
didn’t he heal, weren’t the spring seeds planted
didn’t the night end,
didn’t the melting ice
flood the narrow gutters
wasn’t my body
rescued, wasn’t it safe
didn’t the scar form, invisible
above the injury
terror and cold,
didn’t they just end, wasn’t the back garden
harrowed and planted—
I remember how the earth felt, red and dense,
in stiff rows, weren’t the seeds planted,
didn’t vines climb the south wall
I can’t hear your voice
for the wind’s cries, whistling over the bare ground
I no longer care
what sound it makes
when was I silenced, when did it first seem
pointless to describe that sound
what it sounds like can’t change what it is—
didn’t the night end, wasn’t the earth
safe when it was planted
didn’t we plant the seeds,
weren’t we necessary to the earth,
the vines, were they harvested?
1. Zastrow M. Meet the crystal growers who sparked a revolution in graphene electronics. Nature. 2019;572(7770):429-432. https://www.nature.com/articles/d41586-019-02472-0
2. WATANABE, Kenji | SAMURAI – National institute for Materials Science. WATANABE, Kenji | SAMURAI – National Institute for Materials Science. Accessed October 9, 2020. https://samurai.nims.go.jp/profiles/watanabe_kenji_aml
3. TANIGUCHI, Takashi | SAMURAI – National institute for Materials Science. TANIGUCHI, Takashi | SAMURAI – National Institute for Materials Science. Accessed October 9, 2020. https://samurai.nims.go.jp/profiles/taniguchi_takashi
4. The Nobel Prize in Physics 2010. NobelPrize.org. Accessed October 9, 2020. https://www.nobelprize.org/prizes/physics/2010/press-release/
5. Surjeet Singh. Accessed October 9, 2020. http://www.iiserpune.ac.in/~surjeet.singh/
6. G. N. Ramachandran. In: Wikipedia. ; 2020. Accessed October 9, 2020. https://en.wikipedia.org/w/index.php?title=G._N._Ramachandran&oldid=976314830
7. Poets A of A. October (section I) by Louise Glück – Poems | Academy of American Poets. Accessed October 9, 2020. https://poets.org/poem/october-section-i
Michael V. Berry is a distinguished theoretical physicist. He has made outstanding contribution towards classical and quantum physics, including optics (Pancharatnam-Berry phase, caustics, etc.). Berry is also a prolific writer and commentator on science and its pursuit. Recently, I came across a foreword published on his webpage, that I think is provocative but worth reading..here is a part of it :
“At a meeting in Bangalore in 1988, marking the birth centenary of the Nobel Laureate C V Raman, I was asked to give several additional lectures in place of overseas speakers who had cancelled. During one of those talks, I suddenly realised that underlying each of them was one or more contributions by Sir George Gabriel Stokes. Understanding divergent series, phenomena involving polarized light, fluid motion, refraction and diffraction by sound and of sound, Stokes theorem (I didn’t know then that he learned it from Kelvin)…the list seemed endless.
My enthusiasm thus ignited, I acquired Stokes’s collected works and explored the vast range and originality of his physics and mathematics (separately and in combination). Paul Dirac was certainly wrong in his uncharacteristically ungenerous assessment (reported by John Polkinghorne), dismissing Stokes as “… a second-rate Lucasian Professor”. On the contrary, in every subject he touched his contributions were definitive, and influenced all who followed. Perhaps Dirac failed to understand, as we do now, that discovering new laws of nature is not the only fundamental science: equally fundamental is discovering and understanding phenomena hidden in the laws we already know…………..”
An important takeaway is that fundamental science can also evolve as a consequence of existing laws applied to new boundary conditions or systems. In an essence, Berry’s comment also resonates with PW Anderson’s argument on emergence, which laid a philosophical foundation and integrated science of condensed matter. Undoubtedly, Stokes made some profound discoveries in physics, and a recent book illustrates his science and life (Berry’s foreword is from the same book).
post script: in the year 2000, Berry shared the IgNobel prize, with Andre Geim, for magnetic levitation of frogs. As you may know, Geim went on to win the Nobel prize in Physics (2010) for his groundbreaking work on graphene.
Will Berry get a Nobel prize in 2020 ? He is certainly a deserving candidate…we will see on 6th Oct…
When I come across any book, I do two things : first, I take a glimpse at table of contents, and second, I read the preface/foreword to the book. The second part is generally revealing in its own way, as I get to learn not only about the content of the book, but also about the human side of the topic under study. Recently, I was reading a technical book. In there, I came across a foreword written by Jacques Friedel, in which he quotes his grandfather Georges Friedel, and a part of the quoted text is reproduced below :
…none of the three approaches – the naturalist, the physicist, and the mathematician – should be neglected and that a healthy balance must be preserved amongst them !……
The text in bold is my emphasis. This quote resonates with what I think is a good way of doing science. Let me elaborate a bit on this “trinocular” view of science.
Image courtesy : Pexels – Creative Commons License
Naturalist: In this approach, one can cater to the curiosity of the self by absorbing and observing nature. In a way, this approach helps you to connect with a phenomenon at a personal level with a touch of imagination of ones own. The feeling of wonder is what plays a critical role to be a naturalist, and a naturalist approach is to take this grasp seriously, and wonder about why nature behaves the way it does. In a way, most of the children are naturalist. Also, this approach, in my view, is one of the fundamental aspect of what makes us human : the ability to wonder and question.
Physicist (scientist to be more general) : Once you observe a phenomenon or intrigued by a fact, the questions to ask are: why and how such a thing happens? To answer these questions, you need to bring in the existing knowledge of science and look into the problem at hand through this metaphorical lens. You will have to ask to what framework of concepts does your observation belong to, and try to cast your naturalist observation in this light. This helps you to identify the scientific parameters of the problem, i.e., the dependent and independent variables. With this knowledge about parameters, you can not only probe the system under study, but also control it in a systematic way (first step to engineering). Such a control gives us an intellectual platform to construct hierarchical structures, which can further serve as foundation to new phenomena and structures.
Mathematician : This viewpoint brings in the analytical framework to the observations at hand. From the scientific thought – via hypothesis, experiments and models, we would have obtained some insight into a problem. These building blocks can be further refined and articulated in a precise language, such that we can generalize the problem to a larger set of questions which can go beyond the system under study. This transfer of real to abstract picture is what make mathematics so powerful. It catches the essentials of the problem, and facilitates a framework for generalization, which can be further applied to a new problem.
What I have discussed above is a way (not the only way) to approach research in natural science. Interestingly, the above 3 approaches need not be considered in chronological order. The inspiration to study a natural phenomenon or anything for that matter can be initiated from any of the 3 approaches. A question or an observation in any one framework can be cast as a query in any other framework, and that is what makes pursuit of science so wonderful.
Perhaps, the most important lesson from the Friedel’s quote is to keep a healthy balance of all the three approaches while studying a natural process. Importantly, this triangulation and extrapolation of approaches is how you build knowledge : be it engineering, medicine, public policy or any facet of epistemology. At the heart of all these approaches is to look at a problem from multiple viewpoints and be open to adaptation, criticism, and revision.
After all, depth in view needs more than one cue !
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.
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”
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 ), 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.