Evaporation Of Small Droplets Research Articles

Experiments on the breakup and evaporation of small droplets at high Weber number

Shock-driven multiphase mixing is present in numerous physical systems such as detonation-driven propulsion engines, liquid–vapor cloud explosions, and hypersonic flight droplet impacts. At the microscale, droplets experience deformation, breakup, and evaporation under extreme conditions (high Weber and Reynolds regimes). For small droplets, these phenomena are simultaneous and highly transient, making their interactions and interdependencies warrant further investigation. In this study, experiments are conducted in a shock tube facility to investigate these simultaneous droplet-scale phenomena. An interface consisting of small acetone droplets (⌀ 10–40 [μm]) is impulsively accelerated by a strong planar shock wave (Mach ∼2.09). The droplet size distribution is well-characterized in-situ utilizing a Phase Doppler Particle Analyzer (PDPA) and shadowgraphy. The development of child droplet clouds is captured through an ensemble of Mie scattering images. A simplified model is developed to interpret the experimental results, combining deformation, breakup, and evaporation models. The results indicate that the breakup of small droplets at high Weber numbers is likely dominated by the Rayleigh–Taylor (RT) mechanism, aligning with previous empirical models for low Weber numbers.

Modeling the dynamic behavior of a droplet evaporation device for the delivery of isotopically calibrated low-humidity water vapor

Abstract. A model is presented that gives a quantitative description of the dynamic behavior of a low-humidity water vapor generator in terms of water vapor concentration (humidity) and isotope ratios. The generator is based on the evaporation of a nanoliter-sized droplet produced at the end of a syringe needle by balancing the inlet water flow and the evaporation of water from the droplet surface into a dry-air stream. The humidity level is adjusted by changing the speed of the high-precision syringe pump and, if needed, the dry-air flow. The generator was developed specifically for use with laser-based water isotope analyzers in Antarctica, and it was recently described in Leroy-Dos Santos et al. (2021). Apart from operating parameters such as temperature, pressure, and water and dry-air flows, the model has as “free” input parameters: water isotope fractionation factors and the evaporation rate. We show that the experimental data constrain these parameters to physically realistic values that are in reasonable to good agreement with available literature values. With the advent of new ultraprecise isotope ratio spectrometers, the approach used here may permit the measurement of not only the evaporation rate but also the effective fractionation factors and isotopologue-dependent diffusivity ratios, in the evaporation of small droplets.

Modeling Evaporation of Water Droplets as Applied to Survival of Airborne Viruses

Many viruses, such as coronaviruses, tend to spread airborne inside water microdroplets. Evaporation of the microdroplets may result in a reduction of their contagiousness. However, the evaporation of small droplets is a complex process involving mass and heat transfer, diffusion, convection and solar radiation absorption. Virological studies indicate that airborne virus survival is very sensitive to air humidity and temperature. We employ a model of droplet evaporation with the account for the Knudsen layer. This model suggests that evaporation is sensitive to both temperature and the relative humidity (RH) of the ambient air. We also discuss various mechanisms such as the effect of solar irradiation, the dynamic relaxation of moving droplets in ambient air and the gravitational sedimentation of the droplets. The maximum estimate for the spectral radiative flux in the case of cloudless sky showed that the radiation contribution to evaporation of single water droplets is insignificant. We conclude that at small and even at moderately high levels of RH, microdroplets evaporate within dozens of seconds with the convective heat flux from the air being the dominant mechanism in every case. The numerical results obtained in the paper are in good qualitative agreement with both the published laboratory experiments and seasonal nature of many viral infections. Sophisticated experimental techniques may be needed for in situ observation of interaction of viruses with organic particles and living cells within microdroplets. The novel controlled droplet cluster technology is suggested as a promising candidate for such experimental methodology.

Microdrop Deposition Technique: Preparation and Characterization of Diluted Suspended Particulate Samples

The analysis of particulate matter (PM) in dilute solutions is an important target for environmental, geochemical, and biochemical research. Here, we show how microdrop technology may allow the control, through the evaporation of small droplets, of the deposition of insoluble materials dispersed in a solution on a well-defined area with a specific spatial pattern. Using this technology, the superficial density of the deposited solute can be accurately controlled. In particular, it becomes possible to deposit an extremely reduced amount of insoluble material, in the order of few μg on a confined area, thus allowing a relatively high superficial density to be reached within a limited time. In this work, we quantitatively compare the microdrop technique for the preparation of particulate matter samples with the classical filtering technique. After having been optimized, the microdrop technique allows obtaining a more homogeneous deposition and may limit the sample amount up to a factor 25. This method is potentially suitable for many novel applications in different scientific fields such as demanding spectroscopic studies looking at the mineral fraction contained in ice cores or to pollution investigations looking at the detection of heavy metals present in ultra-trace in water.

Measurement of interaction between water droplets and curved super-hydrophobic substrates in the air

The interaction force is very important in the study of the contact process of droplets and super-hydrophobic substrates. Accurate interaction force measurement in the air has far-reaching impact on industrial production and biomimetic field. However, limited by the evaporation of small droplets, interaction force can only be measured in the liquid by AFM and other devices. A millimetric cantilever was used to make it possible to measure the interaction between droplets and super-hydrophobic substrates in the air. The optical lever was calibrated with the electrostatic force. The super- hydrophobic substrates were fabricated using nano particles and copper grids. We finally acquired the interaction force and wetting time between the droplet and super- hydrophobic substrates with different grid fractions and similar contact angle. The results showed that the interaction force decreased with the increase of the grid fraction. These would open a new way of understanding the mechanism of hydrophobic.

Statistical steady state in turbulent droplet condensation

Motivated by systems in which droplets grow and shrink in a turbulence-driven supersaturation field, we investigate the problem of turbulent condensation in a general manner. Using direct numerical simulations, we show that the turbulent fluctuations of the supersaturation field offer different conditions for the growth of droplets which evolve in time due to turbulent transport and mixing. Based on this, we propose a Lagrangian stochastic model for condensation and evaporation of small droplets in turbulent flows. It consists of a set of stochastic integro-differential equations for the joint evolution of the squared radius and the supersaturation along the droplet trajectories. The model has two parameters fixed by the total amount of water and the thermodynamic properties, as well as the Lagrangian integral time scale of the turbulent supersaturation. The model reproduces very well the droplet size distributions obtained from direct numerical simulations and their time evolution. A noticeable result is that, after a stage where the squared radius simply diffuses, the system converges exponentially fast to a statistical steady state independent of the initial conditions. The main mechanism involved in this convergence is a loss of memory induced by a significant number of droplets undergoing a complete evaporation before growing again. The statistical steady state is characterized by an exponential tail in the droplet mass distribution. These results reconcile those of earlier numerical studies, once these various regimes are considered.

Modelling of heat and fluid flow during the evaporation of volatile drops on hot substrates

In this paper a review on the convective patterns observed during sessile droplets evaporation is performed. A model describing a pinned evaporating drop is introduced; the numerical resolution of the model allows the description of heat and fluid flow within the drop. The role of humidity in the gas phase is investigated, by taking into account the vapour diffusion process. It is shown that the change in substrate temperature has the most important effect on the evaporation rate. The results show that surface tension in comparison to the gravity convection is more accurate in describing evaporation of small droplets.

Dispersion and settling characteristics of evaporating droplets in ventilated room

Movement and evaporation of small droplets in the room air are investigated in this paper through CFD simulations. A modified drift-flux model is presented with the droplet evaporation rate and the drift velocity expressed as simple algebra functions of droplet diameter, which is integrated in the transport equations of droplet number density and droplet bulk density. Evaporating droplets are treated as a continuum phase with one way coupling with the carrier phase, i.e. air. Our numerical simulations reveal that the distribution of the large evaporating droplets in the ventilated room air is characterized by a combination of the settling feature when droplets are first generated and released and the dispersion feature after the droplets are evaporated to be either very fine or become droplet nuclei. For droplets less than 50 μm in diameter, the dispersion feature is dominant in the test room that we simulated, while for droplets larger than 100 μm in diameter, the settling feature dominates. For evaporating droplets between these two sizes, the spatial distribution of droplets tends to be located at the lower part of the test room than that of small neutral aerosol particles. Within this size range, a lower initial position of the droplets in the room results in a higher deposition rate of the droplets on the floor.

Droplet Spectra Broadening by Ripening Process. Part I: Roles of Curvature and Salinity of Cloud Droplets

Abstract The “ripening process” occurs due to thermodynamic instability of droplet size spectra in clouds. This instability results from the existence of droplets with different salinity and size in the droplet spectra. The ripening process is independent of turbulent fluctuations of supersaturation for a closed cloud parcel. Because of the ripening process, droplet number concentration continuously decreases with time after the initial peak supersaturation, which occurs during the initial updraft. Droplet spectra broaden to large sizes by evaporation of small droplets. Both mean droplet size and the standard deviation of droplet size spectra increase with time. This mechanism is suggested to be a potential physical mechanism for the formation of droplet size spectra in stratiform clouds.

Analysis of evaporating droplets using ellipsoidal cap geometry

The evaporation of small droplets of volatile liquids from solid surfaces depends on whether the initial contact angle is larger or less than 90°. In the latter case, for much of the evaporation time the contact radius remains constant and the contact angle decreases. At equilibrium, the smaller the drop, the more it is possible to neglect gravity and the more the profile is expected to conform to a spherical cap shape. Recently published work suggests that a singular flow progressively develops within the drop during evaporation. This flow might create a pressure gradient and so result in more flattening of the profile as the drop size reduces, in contradiction to expectations based on equilibrium ideas. In either case, it is important to develop methods to quantify confidence in a deduction of elliptical deviations from optically recorded droplet profiles. This paper discusses such methods and illustrates the difficulties that can arise when the drop size changes, but the absolute resolution of the system is fixed. In particular, the difference between local variables, such as contact angle, cap height, and contact diameter, which depend on the precise location of the supporting surface, and global variables such as radii of curvature and eccentricity, is emphasized. The applicability of the ideas developed is not limited to evaporation experiments, but is also relevant to experiments on contact angle variation with drop volume.

Vertical gradients of dissolved chemical constituents in evaporating clouds

Vertical gradients in cloud-water composition were investigated during the Ground-based Cloud Experiment at Great Dun Fell (GDF) 1993. The cloud-water measurements were performed at two heights above the cloud base. The observed changes in cloud-water concentration were not only induced by dilution or concentration due to an increasing or decreasing liquid water content (LWC), but also by loss or uptake of chemical compounds, and, under appropriate meteorological conditions (downslope flow), by evaporation of small droplets between the two heights. The observed vertical gradients were found to be ion-specific. Higher amounts of total dissolved material were measured at greater distances above the cloud base, e.g. SO 4 2− during most of the time of the monitored cloud events. Thus, vertical gradients may be important for deposition calculations of trace substances onto vegetation via cloud-water interception. In any case, the cloud base is a very important parameter relevant for the cloud chemical studies, because it is of importance for data interpretation.

Computer Simulation of Photophoretic Force on Rapidly Vaporizing Heterogeneous Droplet

The theoretical/computational study is focused on the photophoretic (radiometric) effect caused by nonuniform absorption of anisotropic, high-temperature thermal radiation within a layered droplet. Transient values of the photophoretic force acting on the shrinking droplet are estimated by numerical integration of the momentum flux carried by vapor molecules interacting with the droplet's surface of nonuniform temperature. The corresponding stress tensor of the surrounding nonequilibrium gas is calculated by application of the results of the kinetic theory of evaporation of small droplet suspended in its own vapor. The radiant heat dissipation within the microheterogeneous, layered droplet is simulated by an application of the Monte Carlo method. Specific computations are performed for small droplets of ultrafine coal-water slurry (CWS) suspended in superheated steam and irradiated from one side to a high-temperature blackbody radiation. The conditions are simulating the situation of primary atomized CWS ...

Comparison of droplet-size distribution and matrix effects in flame atomic absorption spectrometry applying hydraulic high-pressure/perfomance nebulization and pneumatic nebulization

The solution to be analysed is forced through a very fine nozzle (20 μm) under a pressure of 100–400 bar (10–40 MPa) using a liquid chromatographic pump, resulting in an aerosol for sample introduction in atomic absorption spectrometry (AAS). Compared with pneumatic nebulization, the hydraulic high-pressure/performance nebulization (HHPN) causes an essentially smaller proportion of large droplets (> 16 μm): HHPN, 1%; standard flame AAS nebulization, 7%; inductively coupled plasma nebulization, 34%. The smaller droplets of the HHPN aerosol lead to higher sensitivity (2-fold in signal height, 5–6-fold in signal area) and to lower matrix interferences (better evaporation of smaller droplets in the spectroscopic source).

Note on the Temperature and Evaporation of Small Droplets

Abstract : Calculations of the temperature of a stationary drop using ordinary heat- and mass-transfer equations predict that the wet-bulb depression of a nonventilated drop is greater than that of a ventilated drop. An effort to resolve this question is given. (Author)

VII.–The Effect of the Transport of Heat on the Rate of Evaporation of Small Droplets I. Evaporation into a Large Excess of a Gas

SynopsisBeginning with fundamental results obtained by Mason for the effect of self-cooling on the evaporation of drops, and by Fuchs for the diffusional retardation of evaporation for small droplets of any radius, explicit expressions for the effect of the transport of heat on the rate of quasi-stationary growth or evaporation, are discussed.The simplest algebraic formulation of the results lends itself to interpretation as expressing a resistance to evaporation, the total resistance being the sum of four resistances in series. Two of these resistances, one to diffusion and one to the conduction of heat, are offered by the gaseous phase in bulk; and there are two corresponding resistances at the interface. Corrections are formulated for the effect of the heating of the droplet by radiation. These corrections may be expressed as a (finite) resistance in parallel with the other two resistances to the transfer of heat. Simplified equations are obtained for the evaporation of a liquid whose latent heat of vaporization is very large.Some remarks are made on the formation of a monodisperse aerosol by the growth of smaller droplets. Integrated expressions are obtained for particular cases of the evaporation of a droplet over a finite period of time.