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

Abstract

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.

A new electrodynamic balance (EDB) design for low-temperature studies: application to immersion freezing of pollen extract bioaerosols

“…Considering that the measured droplet radius (r) is always greater than 2.5 µm, the Knudsen number, Kn = λ N 2 /r, is always much smaller than 1; therefore the N 2 gas flow is always in the continuum regime (Seinfeld and Pandis, 1998). Hence both the mass transfer of water molecules from the SWD surface to the surrounding N 2 atmosphere and the heat transfer is significant in influencing the kinetic evaporation rate of the trapped SWDs (Miles et al, 2012;Holyst et al, 2013). Due to the influence of heat transfer, the surface temperature of SWDs will be lower than the surrounding gas flow, and this reduces the observed evaporation rate.…”

Evaporation of sessile droplets

“…(see, for example, Refs. [1,2] and references therein). There is a lot of publications on the evaporation of free suspended droplets (see Refs.…”

“…There is a lot of publications on the evaporation of free suspended droplets (see Refs. [2,3] for review), but the problem complicates considerably for the droplets placed on a solid substrate, because in this case the mass transfer between liquid and vapour is coupled with the heat transfer between three phases involved. Moreover, solid/liquid interaction comes into play and the wetting properties of liquid should be taken into account [1].…”

Optical diagnostics of a single evaporating droplet using fast parallel computing on graphics processing units

“…Analysis of light scattered by various particles are needed in numerous research endeavours, such as interstellar dust stu− dy [1], combustion systems [2], dusty plasmas [3], evaporat− ing liquids [4], colloidal suspensions [5,6], or different bio− medical applications [7,8]. It is also vital for nanophotonics research [9,10].…”