Melting dynamics of ice in the mesoscopic regime

Abstract

How does a crystal melt? How long does it take for melt nuclei to grow? The melting mechanisms have been addressed by several theoretical and experimental works, covering a subnanosecond time window with sample sizes of tens of nanometers and thus suitable to determine the onset of the process but unable to unveil the following dynamics. On the other hand, macroscopic observations of phase transitions, with millisecond or longer time resolution, account for processes occurring at surfaces and time limited by thermal contact with the environment. Here, we fill the gap between these two extremes, investigating the melting of ice in the entire mesoscopic regime. A bulk ice I h or ice VI sample is homogeneously heated by a picosecond infrared pulse, which delivers all of the energy necessary for complete melting. The evolution of melt/ice interfaces thereafter is monitored by Mie scattering with nanosecond resolution, for all of the time needed for the sample to reequilibrate. The growth of the liquid domains, over distances of micrometers, takes hundreds of nanoseconds, a time orders of magnitude larger than expected from simple H-bond dynamics.Mie scattering | temperature jump | superheating | laser heating | anvil cell M elting of ice is one of the most common occurrences on our planet, from polar ices to glaciers, to the morning frost and the preparation of our drinks. Ices are present as well in extraterrestrial environments, planetary and intersellar, going through extremely different pressure and temperature conditions, where many phase boundaries are encountered. The molecular mechanisms of melting and the characteristic timescales of the process are not well elucidated, as for any other phase transition. In fact, great effort is presently being made with state of the art experimental techniques to understand the intrinsic kinetics of structural transformations, particularly under dynamic compression, by which phase-transition boundaries up to extreme pressures (P) and temperatures (T) can be accessed (1-6). Melting is the least hindered of phase transitions, requiring the disruption of the crystalline order and the achievement of the local structure of the liquid phase.The dynamics of melting have been studied up to the present in the picosecond timescale, observing structural changes on a length scale of few nanometers, in the attempt to explain the fundamental mechanisms of the transition onset. In both simulations and experiments, micrometric-or submicrometric-sized crystals can be rapidly heated above the melting temperature (Tm ), into a superheated state where melting occurs. Metals and water ice have been the test systems. Simulations have revealed a nucleation and growth mechanism (7-11), and nucleation has been especially addressed, in trying to determine the minimum number of atoms/molecules needed to form a critical nucleus for the transitions, the solid/melt interfacial energy, the transient local structures achieved during the transformation, and the role of defects and surfaces. Experimentally...

Superheating and Homogeneous Melting Dynamics of Bulk Ice

“…These results are consistent with those obtained in transient Mie scattering experiments evidencing how the superheating duration measured in this study nicely fits the time regime, about 100 ns, with the maximum growth rate of the molten domains which were estimated to reach diameters of the order of 400 nm. 2 These dimensions were found to further increase for longer pump−probe delay but at a much slower rate, as expected by the recovery of the initial ice temperature.…”

“…In the same time window the temperature of superheated ice drops down to about 295 K. Melting is therefore the relevant thermalization mechanism at this stage and not heat diffusion, which has much longer characteristic times and for a similar sample and initial temperature can be estimated to last a few milliseconds. 2 After this fast step, a much slower further increase of the liquid amount is revealed by the small negative slope (−3.1 × 10 −5 ns −1 ) of the ice absorbance data versus pump−probe delay in the range exceeding 50 ns with the fraction of molten ice increasing from 32.4% after about 25 ns to ∼37(1)% after 2.0 μs (see Figure S1). These results are consistent with those obtained in transient Mie scattering experiments evidencing how the superheating duration measured in this study nicely fits the time regime, about 100 ns, with the maximum growth rate of the molten domains which were estimated to reach diameters of the order of 400 nm.…”

“…A 50 μm thick ice I h , kept to a temperature ranging between 250 and 270 K in a cell equipped with CaF 2 or sapphire windows, is heated by a single 20 ps pulse at 1920 nm, resonant with the ν 1 + ν 2 ; ν 3 + ν 2 combination band of water, delivering an energy density per pulse up to 1 kJ/cm 3 , thus exceeding that calculated for complete melting. 2 The sample's low absorbance at this wavelength guarantees a uniform irradiation (heating) across the entire sample length. The repetition rate of this pump pulse was set to 2 Hz to ensure the recrystallization and the recovery of a good optical crystal quality before each measurement.…”

“…The melting of a crystalline material spans over a time scale extending, after the necessary energy has been received, from femtoseconds (fs) to seconds. − The time duration of the process is strictly related to the nature of the cohesive interactions characterizing the crystal (molecular, covalent, or metallic) and to the thermalization process (electronic or vibrational). During this interval the system explores metastable states whose nature and persistency are difficult to address even in the case of homogeneous melting occurring when the more common heterogeneous melting at crystal surfaces , or interfaces is suppressed.…”

Thermodynamic state of the interface during acoustic cavitation in lipid suspensions

“…Thus for every 5 water molecules entering the expanding boundary, 4 molecules will be deposited as ice at the interface (50) and 1 molecule will contribute to the volume of expanding cavity. Similar to relaxation of condensed lipids observed in this study, the ice should also disappear in millisecond time scales (51), as the pressure equilibrates to initial conditions and water returns to the equilibrium liquid state. However, if the initial state is that of supercooled water (metastable liquid), upon nucleation of microscopic ice during the negative pressure tail, the crystal will expand into the surrounding supercooled water as the system will relax to the new equilibrium state of a macroscopic mixture of ice and water, as has been observed during sonocrystalization experiments (52).…”