Moment Of Nucleation Research Articles

The effect of bubble nucleation on the performance of a wickless heat pipe in microgravity

Bubble nucleation was investigated in a 20-mm-long, wickless heat pipe on the International Space Station. Over 20 h of running experiments using pentane as the working fluid, more than 100 nucleation events were observed. Bubble nucleation at the heater end temporarily boosted peak pressures and vapor temperatures in the device. At the moment of nucleation, the heater wall temperature significantly decreased due to increased evaporation and the original vapor bubble collapsed due to increased pressure. A thermal model was developed and using the measured temperatures and pressures, heat transfer coefficients near the heater end of the system were extracted. Peak heat transfer coefficients during the nucleation event were over a factor of three higher than at steady-state. The heat transfer coefficient data were all collapsed in the form of a single, linear correlation relating the Nusselt number to the Ohnesorge number.

Analysis of the initial stage of fatigue wear in heterostructure materials under contact cyclic loading

Introduction. The process of formation of fatigue defects in metal alloys with different structural morphology is considered. The work objective is to develop a computational tool for determining the moment of the defect nucleation under cyclic loading.Materials and Methods. A physical model is built, calculation expressions are presented. The physical model is based on the theory of dislocations. It is shown that a structure factor is particularly important in the process of fracture nucleus origination under dynamic cyclic loading. Depending on the structure and properties of the material, as well as on the nature of the loads, the critical fatigue defect develops in the form of cracks, pores or micro-crater wear.Research Results. A numerical experiment was performed to determine the moment of nucleation of the critical-size defect in iron-base alloys under the drop hypervelocity impacts. Comparative data of calculations and bench tests for droplet impingement erosion of steels and alloys with the structure of ferrite, austenite, sorbitol and martensite are presented. The efficiency of the nucleation stage during the incubation period of erosive wear of the materials studied was evaluated.Discussion and Conclusions. There are no strict instrumental methods for determining the duration of the nucleation stage; therefore, it is recommended to use the proposed analytical model. In addition, the work performed gave a significant application result, i.e. it showed that the focused design of the material structure can significantly increase the wear resistance.

Supercooled Liquid Serum Physiologic Solution Instantly Crystallized on the Nurse Table Used for Cooling of Periorbital Region During Rhinoplasty

Formation of less periorbital ecchymosis in post-operative period of rhinoplasty is a popular trend. We present the use of instantly crystallizing supercooled serum physiologic solution for periorbital cooling. There are circumstances in which water temperature drops below its freezing point, but no phase transition happens while water remains in the liquid phase. This is called supercooling. Pure water can be supercooled below the freezing temperature without transforming into ice. Tap water will not supercool because it contains impurities that serveas nucleation sitesfor crystallization. For freezer temperatures in the range of - 4°C, - 6°C, and - 8°C, nucleation was not observed and pure water remained in the supercooled condition for a long time. Sterile serum physiologic solution at + 5°C can be supercooled in the freezer at - 14°C only between the 257 and 277min time interval. But when it is supercooled in the freezer at - 8°C it is possible to save it in liquid form for at least 7days as we have observed in our trials. It is easily possible to transform this supercooled liquid sterile serum physiologic within a few seconds into moldable snow-like ice that can be used safely and more nicely rather than solid ice for periorbital cooling in rhinoplasty operations. Its sterile inner bag is held tight and struck over the sterile nurse table and it crystallizes within a few seconds. For frozen solutions, tearing of the inner plastic bag and extracting the ice and then crushing of big masses of ice to small pieces is exhaustive and a time-consuming process. The temperature of the supercooled fluid will be zero at the moment of nucleation with no risk of frostbite. The crystallized serum physiologic solution preserves its ice-gel form for nearly 25min. The instant crystallization of supercooled liquid serum physiologic solution can be applied as a tissue cooling method in rhinoplasty and in several other surgical procedures. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Nucleation and early stages of growth of lead onto copper electrodes from dilute electrolytes

The processes of nucleation and growth of lead from the dilute electrolytes on copper substrates were investigated by chronoamperometry and by scanning electron microscopic (SEM) analysis of the deposits obtained in the potentiostatic regime of electrolysis. In the dependence of electrodeposition conditions, the nucleation of Pb followed either progressive or instantaneous type. The type of nucleation changed from progressive to instantaneous one with increasing the concentration of Pb(II) ions and the overpotential of electrodeposition. Regardless of the type nucleation, a novel type of Pb particles like cobweb was formed by the potentiostatic electrodeposition in the moment of nucleation and at the early stages of growth. On the basis of the shape of cobweb-like particles and the electrodeposition conditions leading to their formation, these particles were situated in the group of spongy-like ones. Also, comparative morphological analysis of Pb deposits obtained in the conditions of progressive and instantaneous nucleation confirmed the existence of two limiting types of nucleation.

In Situ Raman Monitoring of the Formation and Growth of Carbon Nanotubes via Chemical Vapor Deposition

An in situ measurement technique based on Raman spectroscopy is established to simultaneously monitor the parameters of interest of the solid and gas phases during the formation and growth of carbon nanotubes (CNT) in a cold wall reactor for catalytic chemical vapor deposition (CCVD). Iron nanoparticles were used as a catalyst and acetylene as a carbon source. This new developed technique makes possible the simultaneous in situ measurement of the gas phase composition, the gas phase temperature and the micro structure of the deposited CNTs, such that finally the decomposition of the carbon source and its effect on the gas temperature and on the CNTs formation and growth can be followed as a function of the reaction time. Comprehensive qualitative and quantitative analyses have been performed to assess the catalyst performance with respect to the growing carbon nanostructures. On the basis of the real-time in situ Raman spectra of the emerging solid phase, the moment of nucleation or first signal detection, the time-range of the CNT growth and the end of the deposition is monitored via the characteristic D and G Raman bands for carbon materials. Information about catalytic activity is qualitatively provided and the estimated in situ D/G ratio reveals the defect and disorder content of the growing nanostructures. The Raman spectroscopy setup for the gas phase analysis exhibits high detection sensitivity as well as high accuracy and precision. Correlation could be found between the catalyst activity, acetylene conversion and micro structure of the resulting CNTs.

A note on infinities in eternal inflation

In some well-known scenarios of open-universe eternal inflation, developed by Vilenkin and co-workers, a large number of universes nucleate and thermalize within the eternally inflating mega-universe. According to the proposal, each universe nucleates at a point, and therefore the boundary of the nucleated universe is a space-like surface nearly coincident with the future light cone emanating from the point of nucleation, all points of which have the same proper-time. This leads the authors to conclude that at the proper-time t = t_{nuc} at which any such nucleation occurs, an infinite open universe comes into existence. We point out that this is due entirely to the supposition of the nucleation occurring at a single point, which in light of quantum cosmology seems difficult to support. Even an infinitesimal space-like length at the moment of nucleation gives a rather different result -- the boundary of the nucleating universe evolves in proper-time and becomes infinite only in an infinite time. The alleged infinity is never attained at any finite time.

Influence of magnetic field on the properties of freshly precipitated calcium phosphate: implication in adhesion to glass surface

Static magnetic field (MF) (S–S, B = 0.1 T) effects on spontaneous precipitation of calcium phosphates from equilimolar aqueous solutions of CaCl2 and Na2HPO4, or a mixture of Na2HPO4 and NaH2PO4 solutions ([Ca] = [P] = 10−2 M) at 25°C were studied. Besides the zeta potential and pH measurements as a function of time after the solution mixing, in situ deposition of calcium phosphates on glass surface was also investigated. The deposits were photographed at 400× magnification using an optical polarizing microscope equipped with a camera. It was found that MF effects on the properties of in situ precipitated calcium phosphates appeared in terms of changes in nucleation and precipitation rates, pH and zeta potential. The largest changes in the studied parameters occurred during 20 min from the moment of nucleation and precipitation. In the first system (CaCl2 and Na2HPO4 solutions) the magnetic field increases the precipitation rate, which results in a faster pH decrease relative to the reference system. However, in the second system (CaCl2 and a mixture of Na2HPO4 and NaH2PO4 solutions) the initial pH of the slurry is lower in comparison to the above system and in the absence of the MF it decreases at a slower rate. Based on the optical microscope photographs, it was found that the MF ordered the structure of calcium phosphates crystals precipitated in both systems, but to a higher degree when the sediment was precipitated from the mixture of phosphates. From the X-ray diffractograms of the samples it can be concluded that the MF does not change the chemical composition of the precipitated mixture of calcium phosphates, although in the second system it enhances the transformation of dicalcium phosphate into octacalcium phosphate.

Evaluation of the tendency toward cold-crack formation during the welding of steels

Using the AE method, the moment of nucleation of cold cracks in the studied material has been determined.

Ultrasonic processing of thermoplastic foam

AbstractUltrasonically induced bubble formation for the production of thermoplastic foam was investigated experimentally and theoretically as a basic study. A general purpose polystyrene and blends of low density polyethylene and polyethylene wax were saturated with nitrogen gas under various pressures and the ultrasonic excitation was applied to the polymer system upon release of gas pressure. The ultrasonic nucleation of bubbles in the polymer matrix was modeled by utilizing the classical nucleation theory. The negative pressure generated by the ultrasonic excitation was considered as the environmental pressure at the moment of nucleation. The experimental results showed that the heterogeneous nucleation must be used for ultrasonic foaming of the viscous fluid and the homogeneous nucleation for the low viscosity fluid. The theoretical analysis also indicated that the ultrasonic nucleation can be applied to the production of thermoplastic foam if the ultrasonic excitation generates large enough negative pressure.

The evolution of microcrystalline structures in supercooled metal powders

A numerical model has been developed to study the relative effects of nucleation and growth kinetics on the evolution of ultrafine grain structures observed in some supercooled metal powders. The thermal history during solidification is analyzed using a Newtonian heat transfer formulation coupled to classical models for homogeneous nucleation and continuous growth with a diffuse interface. The results indicate that decreasing particle size increases both the supercooling prior to solidification and the thermal excursion beyond the nucleation temperature. After the first nucleus appears, a compctition is established between the formation of new nuclei and the growth of the existing one(s). There is a range of particle sizes in which the achievable supercoolings are high enough to produce massive nucleation before any significant growth—and the ensuing recalescence—can take place. The probability of multiple nucleation may be evaluated from a dimensionless parameter combining the characteristic frequencies of the nucleation, growth, and heat transfer processes at the moment of nucleation. Calculations for Al and Ni model systems confirm the experimental observation that the latter has a stronger tendency to supercool and develop microcrystalline structures.

Crystal nucleation and growth in liquid rubidium

Molecular dynamics technique is used to study the homogeneous nucleation of an ordered stable phase from a liquidlike state of aggregation. The rubidium potential is used for a 500 particles system with reduced density 0.95. At the start of the calculation the system is quenched rapidly from a reduced temperature of 0.8 to 0.1. The moment of nucleation and the explosive growth of the nucleus to engulf the whole system have been monitored by studying the characteristics of Voronoi polyhedra in addition to quantities such as temperature, pressure, pair correlation functions, etc. After nucleation, the surface of the nucleus is found to advance at a rate of about 130 m/sec; the size of the critical nucleus is estimated to contain 40 to 70 particles. The ordered stable phase is found to have a bcc structure. In spite of the very rapid rate of cooling which was used to start the calculation, a metastable disordered structure was not obtained.