Have you ever witnessed an event where the water starts to boil rapidly at a single point so it looks like an explosion? Buckle up, because the science behind this event will make you fall off your chair.
Let’s start with basics – a simple molecule of water (H2O). Since the molecule is not linear and the oxygen atom has a higher electronegativity than two attached hydrogen atoms, it has a dipole moment, which means it is a polar molecule. Because of its polarity, one molecule of water can form up to four hydrogen bonds in the liquid and solid state (see the image below). Hydrogen bonds keep all molecules of water together in your glass by forming an intermolecular network. This is the reason for such a low-molar-mass molecule is able to stay liquid until the temperature reaches 100 degrees Celsius. The similar molecule hydrogen-sulfide (H2S) is a gas at room temperature because sulfur doesn’t form hydrogen bonds.
Imagine a glass of water is sitting in front of you. Water molecules are casually moving around inside the glass, breaking and forming hydrogen bonds at the same time (assuming that Break/Form hydrogen bond ratio is about 1:1). Nothing interesting, right? However, if we increase the temperature inside the glass, the water molecules start building up kinetic energy and begin to move more rapidly. Molecules are now bumping at one another much faster, breaking a lot more hydrogen bonds than they manage to form new ones. Water is somehow staying liquid, but it’s very hot. When the inside of the glass reaches the point of 100 degrees Celsius, the water starts to boil. One by one, and sometimes even two by two (dimers), water molecules sacrifice themselves and leave the glass taking a lot of energy away from their buddies so they can stay liquid. If the temperature doesn’t drop below 100 degrees, all the water will boil off and the glass will remain empty at some point.
Fun to think about, right? But what if you could warm up a single point in a running water jet with a laser heating pulse?
In the study published in the journal Physical Chemistry Chemical Physics (the year of 2003), Jonathan Hobley, back-then assistant professor at the Tohoku University, Department of Chemistry, now an independent freelance scientist, along with his colleagues created a fantastic experiment in which they forced a heat pulse onto a water jet increasing the temperature rapidly at a single point, causing the water to explode.
The GIF below is the original file from the study filmed with a high-speed camera, showing the explosion of water on a microsecond time-scale.
The heat pulse coming from Nd: YAG (Neodymium-doped Yttrium Aluminum Garnet) laser was so powerful that when applied to the narrow water jet increased the temperature of the small volume of water by about 100 degrees in a very short time. The water molecules suddenly gained too much kinetic energy and became incredibly hot. The only way for the water to cool down is to spread out, so the BOOM happened!
I asked Dr. Hobley to share the experience he gained during his post-doctoral fellowship in Japan so the future generations of scientists can understand what does it mean to live a ‘life of a scientist’.
As a PhD student from the UK we were always starved of funds compared to the USA and Japan. We had to share the laser equipment and we were rarely permitted to do anything fully independent with expensive equipment like picosecond lasers. So when my second post-doc position started in Osaka, Japan, with Japan Atomic Energy Authority I was a bit surprised to be put in charge of my own personal-use picosecond laser with Raman detection. It was all totally not working as a combined unit, all just laying around on the optical bench in bits.
The boss, who worked 800km away in Sendai, interacted with me every day by telephone. It was explained that I was to continue a project aimed at using Stokes-antiStokes Raman Induced Kerr Effect Spectroscopy (RIKES) to measure a temperature rise (time resolved) during laser ablation. RIKES is based on the third order non linear interaction of light with a medium and as such is incredibly complicated mathematically (I sucked at maths at that time) and it is so complicated to achieve experimentally that few papers exist that utilise it for something independently “useful” other than describing the phenomenon itself. So with no self-belief, and knowing that failure would end my career, I began to reconstruct the set-up and align the optics. Every evening my boss would call me and ask “so have you succeeded in getting RIKES signal yet?”. Every evening for the first week I timidly admitted that I had not! By the end of the second week I was still without a RIKES spectrum, so by the Friday, when I had to yet again admit my failure, I was feeling a bit desperate. Beginning of week three, I began going to the Buddhist temple AND the Shinto shrine to pray to whatever god was out there to give me a RIKES signal. But by week three Friday I had to, once again, admit that I had FAILED! I think I was feeling the Japanese sense of honour and failure at that point. It was a very low moment in my life.
Next Monday I started to really focus the 2nd Harmonic (532nm green laser) into the medium (benzene) and was shocked that red laser light was emitted from the sample, not just in the forward direction, but also back towards the laser. What kind of JuJu was this? Green light going forwards and red light comes out forwards and backwards? Impossible! So I studied a bit and realised I had produced “stimulated” Ramen by focusing the green laser into the benzene. That was an important moment, because it gave me the “belief” that strange things really do happen with non-linear optics and Raman. So fortified with this I kept going through week four until by the Friday when the Boss called I could say “Yes Hiroshi, I finally got a RIKES signal. I am sorry it took me so long, but it’s my first time do anything like this on my own with no support”! He replied “What? That’s amazing! The first post-doc that tried failed completely after one year, and it took Tadashi (name altered) nine months to get a RIKES signal even with my help”! I was too happy to hate him for the one month of Hell that I had put myself through! In that one month I had graduated into being a budding scientist from being a recent PhD awardee.
The experiment to measure temperature rise ultimately failed, because we found that under picosecond pulsed irradiation generally you generate plasma before the temperate jump is more than a 1 degree rise. This made the RIKES measurements impossible for picosecond laser ablation. But when the boss invited me to work in Tohoku University (since he was so impressed with my effort) as assistant professor, I devised a way to generate larger temperature jumps to induce ablation. I used non-linear Raman Shifting of an Nd: YAG laser to generate 1.9micron nanosecond laser pulses and probed the temperature using the temperature dependence of Raman spectra of water. We also discovered that we could probe the formation of condensing water droplets using whisper-mode amplification of the Raman signal.
The lessons learn in that one month stay with me throughout my career. There is no better career than science. But it will not make you rich if you do it right (if you do not cheat). It will, however, make you amongst the smartest know-it-all’s at nearly every party. It will also give you a sense of humour that will get you through most difficult situations.