Although the Hertz-Knudsen (HK) relation is often used to correlate evaporation data, the relation contains two empirical parameters (the evaporation and condensation coefficients) that have inexplicably been found to span 3 orders of magnitude. Explicit expressions for these coefficients have yet to be determined. This review will examine sources of error in the HK relation that have led to the coefficients’ scatter. Through an examination of theoretical, experimental, and molecular dynamics simulation studies of evaporation, this review will show that the HK relation is incomplete, since it is missing an important physical concept: the coupling between the vapor and liquid phases during evaporation. The review also examines a modified HK relation, obtained from the quantum-mechanically based statistical rate theory (SRT) expression for the evaporation flux and applying a limit to it in which the thermal energy is dominant. Explicit expressions for the evaporation and condensation coefficients are define...

Expressions for the Evaporation and Condensation Coefficients in the Hertz-Knudsen Relation.

During powder based additive manufacturing processes, a component is realized layer upon layer by the selective melting of powder layers with a laser or an electron beam. The density of the consolidated material, the minimal spatial resolution as well as the surface roughness of the resulting components are complex functions of the material and process parameters. So far, the interplay between these parameters is only partially understood.In this paper, the successive assembling in layers is investigated with a recently described 2D-lattice Boltzmann model, which considers individual powder particles. This numerical approach makes several physical phenomena accessible, which cannot be described in a standard continuum picture, e.g. the interplay between capillary effects, wetting conditions and the local stochastic powder configuration. In addition, the model takes into account the influence of the surface topology of the previous consolidated layer on the subsequent powder layer.The influence of the beam power, beam velocity and layer thickness on the formation and quality of simple walls is investigated. The simulation results are compared with experimental findings during selective electron beam melting. The comparison shows that our model, although 2D, is able to predict the main characteristics of the experimental observations. In addition, the numerical simulation elucidates the fundamental mechanisms responsible for the phenomena that are observed during selective beam melting.

https://iopscience.iop.org/article/10.1088/0965-0393/21/8/085011/pdf

Fundamental consolidation mechanisms during selective beam melting of powders

Artificial spin ice arrays of micromagnetic islands are a means of engineering additional energy scales and frustration into magnetic materials. Despite much progress in elucidating the properties of such arrays, the `spins' in the systems studied so far have no thermal dynamics as the kinetic constraints are too high. Here we address this problem by using a material with an ordering temperature near room temperature. By measuring the temperature dependent magnetization in different principal directions, and comparing with simulations of idealized statistical mechanical models, we confirm a dynamical `pre-melting' of the artificial spin ice structure at a temperature well below the intrinsic ordering temperature of the island material. We thus create a spin ice array that has real thermal dynamics of the artificial spins over an extended temperature range.

Melting artificial spin ice

Patterned deposition at moving contact lines

https://iopscience.iop.org/article/10.1088/1468-6996/14/5/055004/pdf

Thermal stability and phase transformations of martensitic Ti-Nb alloys.

Evaporation is ubiquitous in nature. This process influences the climate, the formation of clouds, transpiration in plants, the survival of arctic organisms, the efficiency of car engines, the structure of dried materials and many other phenomena. Recent experiments discovered two novel mechanisms accompanying evaporation: temperature discontinuity at the liquid-vapour interface during evaporation and equilibration of pressures in the whole system during evaporation. None of these effects has been predicted previously by existing theories despite the fact that after 130 years of investigation the theory of evaporation was believed to be mature. These two effects call for reanalysis of existing experimental data and such is the goal of this review. In this article we analyse the experimental and the computational simulation data on the droplet evaporation of several different systems: water into its own vapour, water into the air, diethylene glycol into nitrogen and argon into its own vapour. We show that the temperature discontinuity at the liquid-vapour interface discovered by Fang and Ward (1999 Phys. Rev. E 59 417-28) is a rule rather than an exception. We show in computer simulations for a single-component system (argon) that this discontinuity is due to the constraint of momentum/pressure equilibrium during evaporation. For high vapour pressure the temperature is continuous across the liquid-vapour interface, while for small vapour pressures the temperature is discontinuous. The temperature jump at the interface is inversely proportional to the vapour density close to the interface. We have also found that all analysed data are described by the following equation: da/dt = P(1)/(a + P(2)), where a is the radius of the evaporating droplet, t is time and P(1) and P(2) are two parameters. P(1) = -λΔT/(q(eff)ρ(L)), where λ is the thermal conductivity coefficient in the vapour at the interface, ΔT is the temperature difference between the liquid droplet and the vapour far from the interface, q(eff) is the enthalpy of evaporation per unit mass and ρ(L) is the liquid density. The P(2) parameter is the kinetic correction proportional to the evaporation coefficient. P(2) = 0 only in the absence of temperature discontinuity at the interface. We discuss various models and problems in the determination of the evaporation coefficient and discuss evaporation scenarios in the case of single- and multi-component systems.

https://iopscience.iop.org/article/10.1088/0034-4885/76/3/034601/pdf

K Kowalski

Papers

Evaporation of freely suspended single droplets: experimental, theoretical and computational simulations