Freezing of few nanometers water droplets

Water-ice transformation of few nm nanodroplets plays a critical role in nature including climate change, microphysics of clouds, survival mechanism of animals in cold environments, and a broad spectrum of technologies. In most of these scenarios, water-ice transformation occurs in a heterogenous mode where nanodroplets are in contact with another medium. Despite computational efforts, experimental probing of this transformation at few nm scales remains unresolved. Here, we report direct probing of water-ice transformation down to 2 nm scale and the length-scale dependence of transformation temperature through two independent metrologies. The transformation temperature shows a sharp length dependence in nanodroplets smaller than 10 nm and for 2 nm droplet, this temperature falls below the homogenous bulk nucleation limit. Contrary to nucleation on curved rigid solid surfaces, ice formation on soft interfaces (omnipresent in nature) can deform the interface leading to suppression of ice nucleation. For soft interfaces, ice nucleation temperature depends on surface modulus. Considering the interfacial deformation, the findings are in good agreement with predictions of classical nucleation theory. This understanding contributes to a greater knowledge of natural phenomena and rational design of anti-icing systems for aviation, wind energy and infrastructures and even cryopreservation systems.

Toward a theory of the evolution of drop morphology and splintering by freezing

The drop freezing process is described by a phase-field model. Two cases are considered: when the freezing is triggered by central nucleation and when nucleation occurs on the drop surface. Depending on the environmental temperature and drop size, different morphological structures develop. Detailed dendritic growth was simulated at the first stage of drop freezing. Independent of the nucleation location, a decrease in temperature within the range from ~ −5 to −25°C led to an increase in the number of dendrites and a decrease in their width and the interdendritic space. At temperatures lower than about −25°C, a planar front developed following surface nucleation, while dendrites formed a granular-like structure with small interdendritic distances following bulk nucleation. An ice shell grew in at the same time (but slower) as dendrites following surface nucleation, while it started forming once the dendrites have reached the drop surface in the case of central nucleation. The formed ice morphology at the first freezing stage predefined the splintering probability. We assume that stresses needed to break the ice shell arose from freezing of the water in the interdendritic spaces. Under this assumption, the number of possible splinters/fragments was proportional to the number of dendrites, and the maximum rate of splintering/fragmentation occurred within a temperature range of about −10 °C to −20°C, in agreement with available laboratory and in situ measurements. At temperatures < −25°C, freezing did not lead to the formation of significant stresses, making splintering unlikely. The number of dendrites increased with drop size, causing a corresponding increase in the number of splinters. Examples of morphology that favors drop cracking are presented, and the duration of the freezing stages is evaluated. Sensitivity of the freezing process to the surface fluxes is discussed.

Oriented nucleation and crystal growth of Sr-fresnoite (Sr2TiSi2O8) in 2SrO·TiO2·2SiO2 glasses with additional SiO2

The oriented nucleation of Sr-fresnoite is barely affected by increasing the amount of SiO2 in glasses of the mol composition 2SrO·TiO2·2SiO2 + xSiO2 (x = 0 to 1.5) while keeping the crystallisation temperature constant. Bulk nucleation, growth selection and phase separation occur in the bulk.

Oriented nucleation and crystal growth of Ge-fresnoite (Ba2TiGe2O8) in 2BaO·TiO2·2GeO2 glasses with additional GeO2

The oriented nucleation of Ge-fresnoite is clearly affected by increasing the amount of GeO2 in glasses of the mol composition 2BaO·TiO2·2GeO2 + xGeO2 (x = 0.0–1.5) while keeping the crystallization temperature constant. Bulk nucleation and growth selection occur in the bulk.

Analytical Expressions for Formal Kinetics

In recent papers Rios and Villa resorted to developments in stochastic geometry to revisit theclassical KJMA theory and generalize it for situations in which nuclei were located in space accordingto both homogeneous and inhomogeneous Poisson point processes as well as according to Materncluster process and surface and bulk nucleation in small specimens. Rigorous mathematical methodswere employed to ensure the reliability of the new expressions. These results are briefly described.Analytical expression for inhomogeneous Poisson point process nucleation gives very good agreementwith Cellular Automata simulations. Cellular Automata simulations complement the analyticalsolutions by showing the corresponding microstructural evolution. These new results considerablyexpand the range of situations for which analytical solutions are available.

Reaction Sintering Kinetics of Mullite Ceramics Prepared from High Aluminum Fly Ash

This research presented the kinetics of mullite formation in fly ash-bauxite reactants couples. Experiment on isothermal conversion of fly ash-bauxite at 1100°C, 1300°C, 1500°C has been carried out. XRD was used to quantitatively measure the content of mullite specimens sintered at different temperatures and times. The kinetics curve of conversion ratio versus time has been drawn. The results obtained showed that the full transformation of fly ash-bauxite to mullite takes place between 1300 °C and 1500 °C. The activation energy of secondary mullite formation is deduced to be 151 kJ/mol in the range of 1100-1500°C. The growth morphology parameter, n, is about 1.14 and 0.45 at 1100°C and 1500°C, respectively, indicating that bulk nucleation is dominant in mullite crystallization followed by three-dimensional growth of mullite crystal controlled by diffusion, but at 1500 °C diffusion process dominates mullite formation process.