Drastic enhancement of crystal nucleation in a molecular liquid by its liquid–liquid transition

Crystallization is one of the most familiar and fundamental phase transition phenomena. There is a possibility that crystallization may be enhanced by critical-like fluctuations associated with another nearby phase transition if the order parameter of the former is coupled to that of the latter; however, the mechanism of such order parameter coupling and its generality remain elusive due to the lack of experimental studies. Here we report experimental evidence for a nontrivial coupling between crystallization and liquid–liquid transition (LLT) for a molecular liquid, triphenyl phosphite. We find that the crystal nucleation frequency is drastically enhanced by short-time preannealing near but above the spinodal temperature of LLT. By successfully separating the thermodynamic and kinetic factors governing crystal nucleation, we show that this enhancement is induced by the lowering of the crystal–liquid interfacial energy due to the presence of critical-like order parameter fluctuations. This finding may be regarded as a fingerprint of the presence of LLT below the melting point. Thus, it may allow us not only to control the crystal nucleation frequency by LLT but also to unveil LLT hidden behind crystallization. This enhancement of nucleation frequency by critical-like fluctuations of another ordering phenomenon may be general to a variety of combinations of phase transitions. It would provide a way to control a crystal grain structure, which is a crucial control factor of mechanical and thermal properties of crystalline materials.

Image processing for identification of vapor phase on a heating surface under the nonstationary boiling conditions

The paper presents a two-step approach to identify bubbles and their properties such as average sizes, and nucleation frequency in a two-phase liquid flow near the heating surface under the nonstationary boiling conditions. The first step of the approach, namely bubble identification, is based on the conventional cross-correlation algorithm applied to an experimental image acquired from the high-speed camera using the bubble image templates manually assigned in advance. Further post-processing step uses temporal identified bubble data to evaluate bubble dynamic properties and track the evolution of local vaporization sites on the heating surface.

Эволюция микролунок на широких террасах поверхности Si(111) в процессе высокотемпературного отжига

We have investigated in situ the morphological transformation of the Si(111) surfaces with micro-pits at large terraces during high-temperature annealing at T = 1200–1400°C. Experimental observation of the micro-pits kinetic decay have been performed by means of ultrahigh vacuum reflection electron microscopy. Focused ion beam system have been used for micro-pits creation at Si(111) terraces of large size. We have found that kinetic of micro-pit decay processes is affected by two dimensional vacancy islands nucleation at the micro-pit bottom when the micro-pit reaches the critical lateral size. The simple theoretical model has been proposed for describing the changes of the lateral size of micro-pit. The temperature dependence of two-dimensional vacancy islands nucleation frequency at micro-pit bottom is found to be described by the activation energy of 4.1 ± 0.1 eV.

Saturated Nucleate Pool Boiling of Ethanol-Water Binary Mixtures on Smooth and Enhanced Laser Processed Metal Surfaces

Pool boiling experiments of water and ethanol-water binary mixtures were conducted on smooth and laser textured stainless steel foils. High-speed IR thermography was used to measure transient temperature field during boiling in order to determine nucleation frequencies, nucleation site densities, bubble activation temperatures, wall-temperature distributions and average superheats as well as heat transfer coefficients. Saturated pool boiling experiments were conducted at atmospheric pressure over a heat flux range of 5–250 kW m−2 for pure water and ethanol-water mixtures (1% and 10% m/m). For both mixtures and both types of surfaces we measured significant decrease in average heat transfer coefficient and increase in bubble activation temperatures in comparison to pure water. However, laser textured surface in average provided around 60% higher nucleation frequency and more than 100% higher nucleation site density compared to smooth surface for both of the tested binary mixtures. Consequentially, heat transfer coefficient was enhanced for more than 30%. Our results show that laser textured surfaces can improve boiling performance for water and ethanol-water mixtures, but at the same time the addition of ethanol reduces heat transfer coefficient despite the enhancement of nucleation site density and nucleation frequency. This is also in agreement with available experimental data and existing theoretical models.

Crystallographic Investigation of the Initial Solidification Grain Structure in Al-Si Alloy

As-solidified structure of an ingot is composed of the chill, columnar and equiaxed zones. The whole solidified structure is strongly affected by the chill crystals. Some initial solidification grains have been observed on the ingot surface and thought to be traces of the nucleation point. The aim of this study is, therefore, to develop the experiment technique to make one ‘grain’ and to crystallographically investigate the initial solidification grain using EBSD analysis. In order to start solidification at a very specified position, a small metallic protrusion was installed on an insulating plate. Al-6 wt%Si alloy was melted at 800 °C and was poured on the metallic protrusion. In this study, the amount of protrusion was varied to investigate the growth mechanism of the initial solidification grain. The longitudinal cross section of the specimen was observed by an optical microscope, a scanning electron microscope. The starting position of solidification was the area that was on the metallic protrusion. In this initial solidification grain, it was difficult to observe the dendritic structure. The shape of this grain was about hemispherical. The grain area seemed to increase with increasing the amount of protrusion. The results of EBSD analysis showed that almost all initial solidification grains were composed by several crystals. The reason of this is that the nucleation frequency may increase with the amount of protrusion. The dendrite grew radially from the initial solidification grain continuously. The crystallographic structure was also continuous on the boundary of the initial solidification grain.

Investigation of the Dynamics of Nucleate Boiling in Subcooled Falling Liquid Films

This paper deals with the features of nucleation dynamics at boiling in falling water films at different subcooling, Reynolds number and heat fluxes. With the use of high-speed infrared and digital video the local parameters of nucleate boiling in falling liquid films such as: bubbles’ diameter before condensation, frequency of nucleation and temperature of onset of bubble appearance were received. Analysis of the experimental data showed that bubbles’ diameter before condensation has strong dependence on initial temperature and increases with the rise of heat flux. The main influence on nucleation frequency has the variation of heat flux density. At the same time the experimental data on nucleation frequency in falling water films are close to the frequency of nucleation at pool boiling. To identify the main features the comparison of received data on the local characteristics at boiling in subcooled falling liquid film with existing models for pool boiling was made

The Heater Size and Viscosity Effects on Thermal Bubble Growth and Dynamics after Departure

A single bubble growth after boiling in square vertical tube is studied numerically to better understand bubble formation factors and as well as its moving behavior after departure. The effects of heating surface area and viscosity of fluid are investigate with nucleation time, bubble departure time, bubble dimension as well as boiling curve. The study indicate that the larger heating surface area, the smaller bubble departure diameter and higher nucleation frequency. The larger of viscosity of fluid, the smaller moving deviation from central axis after bubble departure and smaller deformation of the bubble.

An Experimental Study of Bubble Nucleation Frequency in Subcooled Flow Boiling

In this paper, a simultaneous visualization and measurement study have been carried out to investigate bubble nucleation frequency of water in micro-channel at various heat fluxes and mass fluxes. A single micro-channel with an identical rectangular cross-section having a hydraulic of 137 μm and a heating length of 30 mm was used in this experiment. It is shown that the frequency of bubble nucleation increased drastically with the increase of heat flux and was also strongly dependent on the mass flux. A dimensionless frequency of bubble nucleation was correlated in terms of the Boiling number. The predictions of bubble nucleation frequency in the microchannel are found in good agreement with experimental data with a MAE of 10.4%.