Melting dynamics of ice in the mesoscopic regime Margherita Citronia,b,1, Samuele Fanettia,c, Naomi Falsinia, Paolo Foggia,d,e, and Roberto Binia,b aLENS–European Laboratory for Non-Linear Spectroscopy, 50019 Sesto, Florence, Italy; bDipartimento di Chimica Ugo Schiff, University of Florence, 50019 Sesto, Florence, Italy; cIstituto di Chimica dei Composti Organo Metallici, Consiglio Nazionale delle Ricerche, 50019 Sesto, Florence, Italy; dIstituto Nazionale di Ottica, Consiglio Nazionale delle Ricerche, 50125 Florence, Italy; and eDipartimento di Chimica, University of Perugia, I-06123 Perugia, Italy Edited by Russell J. Hemley, The George Washington University, Washington, DC, and approved May 2, 2017 (received for review December 6, 2016) How does a crystal melt? How long does it take for melt nuclei The temperature increases by the redistribution of the absorbed to grow? The melting mechanisms have been addressed by sev- energy via the vibrational relaxation over the lattice modes. This eral theoretical and experimental works, covering a subnanosec- thermalization process takes ∼20 ps in water ice (13) and can ond time window with sample sizes of tens of nanometers and be slower in non–H-bonded molecular crystals. The structural thus suitable to determine the onset of the process but unable to changes following the heating pulse have been monitored in alu- unveil the following dynamics. On the other hand, macroscopic minum (14) and gold (15) by electron diffraction with a sample observations of phase transitions, with millisecond or longer time thickness of 20 nm, where the main signatures of melting com- resolution, account for processes occurring at surfaces and time plete in a few picoseconds. Water ice melting after an ultrafast limited by thermal contact with the environment. Here, we fill the T jump has been studied by time-resolved infrared spectra (13, gap between these two extremes, investigating the melting of 16), with a sample thickness of 1.6 µm and over a time window ice in the entire mesoscopic regime. A bulk ice Ih or ice VI sample of 250 ps. Here, the spectral changes attributed by the authors to is homogeneously heated by a picosecond infrared pulse, which melting mostly occur in the first 150 ps after the pulse (τ = 37 ps) delivers all of the energy necessary for complete melting. The evo- (16). At 250 ps an incomplete melting was still observed, consis- lution of melt/ice interfaces thereafter is monitored by Mie scat- tent with the amount of energy delivered. tering with nanosecond resolution, for all of the time needed for The knowledge at this picosecond–nanometer scale barely the sample to reequilibrate. The growth of the liquid domains, connects to the experiences in the macroscopic world. On the over distances of micrometers, takes hundreds of nanoseconds, macroscopic side, photographic investigations have been per- a time orders of magnitude larger than expected from simple formed with millisecond resolution to investigate premelting H-bond dynamics. structures in colloidal crystals (17), ice nucleation events upon cooling (18), or the rate and topology of ice and hydrate forma- Mie scattering j temperature jump j superheating j laser heating j tion under dynamic compression (19, 20). anvil cell Here, we use a picosecond T -jump technique and time- resolved Mie scattering to explore the melting and the successive elting of ice is one of the most common occurrences on our refreezing on bulk samples of ice Ih and ice VI. The main pecu- Mplanet, from polar ices to glaciers, to the morning frost and liarities of this experiment are the bulk sample thickness (50 µm) the preparation of our drinks. Ices are present as well in extrater- and the time sampling: We access a time window of tens of mil- restrial environments, planetary and intersellar, going through liseconds, which is the time needed to completely restore the extremely different pressure and temperature conditions, where initial conditions, with nanosecond resolution. We observe the many phase boundaries are encountered. The molecular mecha- scattering at ice/melt interfaces and the propagation of melting nisms of melting and the characteristic timescales of the process over distances of micrometers, accessing the characteristic time are not well elucidated, as for any other phase transition. In fact, of the process. As will be shown, the P, T, and V conditions in this great effort is presently being made with state of the art exper- experiment are quasi-static over the timescale of melting evolu- tion. The kinetics of melting propagation in a bulk crystal are imental techniques to understand the intrinsic kinetics of struc- CHEMISTRY tural transformations, particularly under dynamic compression, thus explored over the entire mesoscopic regime, connecting the by which phase-transition boundaries up to extreme pressures microscopic and the macroscopic realms. (P) and temperatures (T) can be accessed (1–6). Melting is the least hindered of phase transitions, requiring the disruption of Significance the crystalline order and the achievement of the local structure of the liquid phase. The complete unfolding of the melting process, from the The dynamics of melting have been studied up to the present nucleation onset to the achievement of a bulk liquid structure, in the picosecond timescale, observing structural changes on a is not known to date for the timescales and the sizes of the length scale of few nanometers, in the attempt to explain the heterogeneities involved. Here, we observe in ice the archety- fundamental mechanisms of the transition onset. In both simu- pal H-bonded molecular crystal, the evolution of solid/liquid lations and experiments, micrometric- or submicrometric-sized interfaces generated by the absorption of a picosecond infra- crystals can be rapidly heated above the melting temperature red pulse. The pulse delivers all of the energy needed for com- (Tm ), into a superheated state where melting occurs. Metals and plete melting; therefore, the lattice is homogenously heated water ice have been the test systems. Simulations have revealed from within, above its melting temperature. The growth of a nucleation and growth mechanism (7–11), and nucleation has liquid domains over distances of micrometers takes hundreds been especially addressed, in trying to determine the minimum of nanoseconds and is orders of magnitude slower than vibra- number of atoms/molecules needed to form a critical nucleus tional dynamics. for the transitions, the solid/melt interfacial energy, the tran- sient local structures achieved during the transformation, and the Author contributions: M.C., S.F., P.F., and R.B. designed research; M.C., S.F., and N.F. per- role of defects and surfaces. Experimentally, the sample can be formed research; M.C., S.F., and N.F. analyzed data; and M.C. and R.B. wrote the paper. rapidly heated by a pulse of infrared light at a wavelength that The authors declare no conflict of interest. the sample can absorb (T jump). With this “direct laser heat- This article is a PNAS Direct Submission. ing” the sample is heated from within, and transition nuclei can 1To whom correspondence should be addressed. Email: [email protected]fi.it. be formed in the bulk, overcoming the dominance of heteroge- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. neous nucleation, induced by surfaces or preexisting defects (12). 1073/pnas.1620039114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1620039114 PNAS j June 6, 2017 j vol. 114 j no. 23 j 5935–5940 Downloaded by guest on September 25, 2021 The experimental insight into the melting dynamics of ices with which we measure the transmission through the sample with has paramount relevance in understanding natural phenomena a fast Si photodiode (Fig. 1C). The focus of the probe (∼150 µm and is a fundamental benchmark for theories at any level of in diameter) lies inside the area irradiated by the pump. When a accuracy. Ices, ubiquitous in terrestrial and extraterrestrial envi- pump pulse is shot, the probe transmission drops to a minimum ronments, are characterized by hydrogen-bond networks that in tens or hundreds of nanoseconds, depending on the amount determine the great complexity of their physical and chemical of deposited energy, and then returns to a static value in a time behavior. The melting line of water borders at least five crys- (micro- to milliseconds) strongly dependent on T0. The trans- talline phases (Ih , III, V, VI, and VII) with different structures mission change is due to scattering at ice/melt interfaces, form- and H-bond arrangements. In this work we investigate the melt- ing with the T jump and evolving in time during melting (due ing dynamics in ice Ih and VI, accessed respectively by the use to superheating) and refreezing (due to the thermal contact with of low temperature and high static pressure, providing an unique the environment that decreases T to the thermal bath tempera- experimental study in the mesoscopic domain and opening the ture T0). When the transmitted beam is blocked (Fig. 1D), a pos- way to investigations in different P and T conditions. itive signal due to off-axis scattered light is consistently observed, with a similar time dependence to that of the transmission change. Results In both configurations the signal amplitude increases as λprobe Temperature Jump and Transient Transmission Loss. Ice Ih and ice decreases, as expected for scattering. As also expected, the trans- VI samples 50 µm thick, at fixed temperature (T0) and pres- mission change is not observed when liquid samples are heated sure (P0), are “instantaneously” heated by a 15-ps pulse of by the T -jump pulse, except at the highest E values (E ≥ 4.5 mJ) light at 1,930 nm (FWHM = 100 nm) (21) with a focus diame- where a strong oscillation of the transmission is observed, likely ter of 400 µm.