Explosive boiling of water after pulsed IR laser heating Atsushi Takamizawa, a Shinji Kajimoto, a Jonathan Hobley,* a Koji Hatanaka, a Koji Ohta b and Hiroshi Fukumura* a a Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. E-mail: jonathan@orgphys.chem.tohoku.ac.jp b Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Kansai Centre, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received 29th October 2002, Accepted 17th January 2003 First published as an Advance Article on the web 30th January 2003 By focussing 1 J of 1064 nm Nd : YAG beam into 30 atmospheres of hydrogen we could Raman shift to produce a 10 ns, 300 mJ, 1.9 mm laser pulse. This pulse can directly heat water by more than 100 K (average), during the laser pulse, inducing vaporisation. Vaporisation was studied using time-resolved shadowgraphy and Raman spectroscopy to obtain macro and molecular level information. The O–H stretching Raman bands of water are sensitive to temperature allowing us to measure the average temperatures during the boiling process. After the T-jump, explosive boiling occurred within 100 ns during which time the bulk temperature decreased, indicating that the vaporising water molecules deprived heat from their surroundings. Shadowgraphs confirmed the timescale for this phenomenon visually. After 10 ms, vaporised gas molecules condensed and formed droplets, which were observed by a morphology-dependent resonance (MDR) Raman. Introduction The everyday processes that we take for granted about impor- tant molecules such as water are often rather complex. For example, water boils due to the collective movement of mole- cules after the intermolecular network of hydrogen bonds becomes sufficiently weakened for the liquid structure to meta- morphose into the gas phase. This phenomenon of changing phase from a liquid to a gas involves a variety of factors. As well as the chemical aspects of breaking the hydrogen bonds in the water, bubble nucleation is often also required. Many papers report the boiling of water. In one report, rapid laser induced temperature jumps were induced near the liquid–solid interface and bubble formation was observed using surface reflectivity changes where the growth of bubbles was said to cause light scattering. 1 Boiling of water at surfaces has also been dealt with theoretically. 2 The transition from liquid to gas after laser irradiation has been theoretically discussed in terms of various possible mechanisms 3 and bubble nucleation has been discussed theoretically in terms of the change of Gibbs free energy. 4 It has been shown that extreme changes of temperature induced by laser irradiation of a water droplet causes spinodal phase change. 5 Spinodal phase change was also induced by a micro heater and observed photographically on a timescale of ms–ms. 6 Further, even below the critical boil- ing point when the bulk temperature is changed by an intense nanosecond laser pulse, a pressure wave is generated by photo- acoustic effects causing bubble generation inside the bulk. 7–10 Laser ablation of liquid water doped with various absorbers has been observed by imaging the phase explosion process 11–13 although we note that the absorbers used in these studies will change the H-bond network due to the chemistry of disso- lution. Recently, femtosecond pump probe experiments have been reported on liquid water, pumping the O–H stretching region and probing the same region. 14–17 These studies have greatly enhanced our understanding of the very rapid dissipa- tion of vibrational energy in water although the T-jumps are below the boiling point and the timescale in such femtosecond pump-probe experiments is often inappropriate for observing a slower process such as boiling. Despite these numerous excel- lent studies on this fundamental subject, spectroscopic studies on the boiling process itself are conspicuously lacking. In the present work we describe the first use of spectroscopic methods to probe the explosive boiling of bulk water. In particular, we can spectroscopically determine the temperature of the bulk liquid as phase change occurs. Temperature and pressure increases within the bulk are important for causing boiling. In our work we produce tem- perature jumps in the bulk by direct vibrational excitation of water molecules with a near infrared laser pulse. Laser T-jump is already important in studies of protein and DNA folding dynamics 18–24 since this is the most natural method possible to achieve rapid heating, because photo absorbers don’t need to be added to sensitise heat generation. As a subject in its own right, bulk water is very complicated and has been well studied. The temperature dependence of water’s Raman spectrum and implied structure has been studied at both ambient 25 and high pressure. 26 It is apparent from these studies that the dynamic changes within a group of molecules during boiling can be investigated using Raman spectroscopic methods. The O–H vibrational bands are very sensitive to the temperature of the bulk and the resulting Raman spectral changes have already been well under- stood. 25–29 Therefore the technique of Raman scattering is use- ful for us to evaluate the bulk temperature. 30 Further, cavity effects within droplets generated by mechanical means can amplify a Raman signal from the laser pulse (MDR) 31–34 and as we will describe droplet formation is eventually observed in this way in the overall process of boiling and re-condensation. In the discussion of our results, we examine the mechanism of the rapid change from liquid to gas (‘‘ phase explosion ’’) 3 that in our case occurs even though the temperature is below that of the spinodal. Moreover, we evaluate the velocity of vaporisation and discuss the pump power dependence of Raman spectral changes. We also discuss volume expansion 888 Phys. Chem. Chem. Phys., 2003, 5, 888–895 DOI: 10.1039/b210609d This journal is # The Owner Societies 2003 PCCP