Effects of porosity and contaminant on evaporation from nanopores

Evaporation from nanopores, owing to its high mass/heat fluxes and high heat transfer coefficients, have found widespread applications in various industrial process, including electronics cooling, solar steam generation, membrane distillation and power generation. To further improve the performance of these nanopore-evaporation-associated processes, it is necessary to experimentally quantify the ultimate transport limit of evaporation from nanopores and understand its dependence on nanoscale confinement and operating conditions. This ultimate transport limit has now been widely accepted to be dictated by evaporation kinetics at the liquid-vapor interface, which is very difficult to quantify experimentally due to the ultra-small evaporation rates from single nanopores. To overcome this challenge, a new measurement approach based on a hybrid nanochannel-nanopore device design has been developed recently. This measurement approach can accurately measure evaporation rates/fluxes from single nanopore and has been used to investigate the effect of nanopore diameter on kinetic-limited evaporation flux. Although this study provides us new fundamental understanding about how nanoscale confinements change evaporation from nanopore, the effects of contaminant and pore porosity, which to some extent determines the practical performance of evaporation from nanopores, have remained elusive. Such lacking understanding has prevented us from developing optimized evaporative nanoporous structures for practical applications. This works aims to investigate the effects of porosity and contaminant on kinetic-limited evaporation flux by experimentally measuring kinetic-limited evaporation rates from nanopore arrays. A modified hybrid nanochannel-nanopore device design is used to achieve this goal. In this modified device design, a nanopore array is directly connected to a 2-D nanochannel and the total evaporation rate from the nanopore array is measured by tracking meniscus receding in the nanochannel during a drying/evaporation process. Using this modified device design, we measured the kinetic-limited evaporation rates from 3x3 nanopore arrays with different interval distances ranging from 200 nm to 1 μm. To facilitate comparison between different devices, the total evaporation rates were converted to evaporation fluxes based on the nanopore projected area. Our results showed that that porosity or nanopore interval distance has negligible effect on the kinetic-limited evaporation flux. We also performed evaporation experiment using water with impurity and studied the effect of contaminant on kinetic-limit evaporation flux. It was observed that the contaminants in water can significantly reduce the kinetic-limited evaporation flux in nanopores and the contaminant effect becomes much more obvious in smaller nanopore due to contaminant-accumulation-induced pore blockage

Nanoparticle enhanced evaporation of liquids: A case study of silicone oil and water

Evaporation is a fundamental physical phenomenon, of which many challenging questions remain unanswered. Enhanced evaporation of liquids in some occasions is of enormous practical significance. Here we report the enhanced evaporation of the nearly permanently stable silicone oil by dispersing with nanopariticles including CaTiO3, anatase and rutile TiO2. The results can inspire the research of atomistic mechanism for nanoparticle enhanced evaporation and exploration of evaporation control techniques for treatment of oil pollution and restoration of dirty water

Deposit Growth in the Wetting of an Angular Region with Uniform Evaporation

Solvent loss due to evaporation in a drying drop can drive capillary flows and solute migration. The flow is controlled by the evaporation profile and the geometry of the drop. We predict the flow and solute migration near a sharp corner of the perimeter under the conditions of uniform evaporation. This extends the study of Ref. 6, which considered a singular evaporation profile, characteristic of a dry surrounding surface. We find the rate of the deposit growth along contact lines in early and intermediate time regimes. Compared to the dry-surface evaporation profile of Ref. 6, uniform evaporation yields more singular deposition in the early time regime, and nearly uniform deposition profile is obtained for a wide range of opening angles in the intermediate time regime. Uniform evaporation also shows a more pronounced contrast between acute opening angles and obtuse opening angles.Comment: 12 figures, submitted to Physical Review

Predicting evaporation rates from pools

A number of simplified approaches, based on semi-empirical models and algebraic formulas, often derived from the correlation of experimental data, are available to predict the evaporation rate of chemicals from pools. In this work they are compared, based on their performances in estimating the evaporation rates determined in some sets of experiments reported in the literature, involving different chemicals, pool geometries, wind velocities, etc. Generally, the models predict correctly the data used in the fitting to determine their own parameters, but most of them fail when applied to other data sets. No model appear capable of predicting all the available sets of literature data with errors within  30%: nevertheless, some models application guidelines can be derived. In particular, the model of Heymes et al. (2013), provides reasonable estimates of the evaporation rates of a variety of compounds in rather different operating conditions, for wind speed exceeding 1 m/s, while the simple model of Mackay and Wesembeeck (2014) should be preferred in the absence of ventilation. Other models, such as those of Hummel et al. (1996), Stiver and Mackay (1996) and EPA (1999) can be applied within proper intervals of wind speed, molecular weight and vapour pressure of the chemical

Controlling evaporation loss from water storages

[Executive Summary]: Evaporation losses from on-farm storage can potentially be large, particularly in irrigation areas in northern New South Wales and Queensland where up to 40% of storage volume can be lost each year to evaporation. Reducing evaporation from a water storage would allow additional crop production, water trading or water for the environment. While theoretical research into evaporation from storages has previously been undertaken there has been little evaluation of current evaporation mitigation technologies (EMTs) on commercial sized water storages. This project was initiated by the Queensland Government Department of Natural Resources and Mines (NRM) with the express aim of addressing this gap in our knowledge. The report addressed i) assessment of the effectiveness of different EMT’s in reducing evaporation from commercial storages across a range of climate regions, ii) assessment of the practical and technical limitations of different evaporation control products, and iii) comparison of the economics of different EMT’s on water storages used for irrigation

A macroscopic model for sessile droplet evaporation on a flat surface

The evaporation of sessile droplets on a flat surface involves a complex interplay between phase change, diffusion, advection and surface forces. In an attempt to significantly reduce the complexity of the problem and to make it manageable, we propose a simple model hinged on a surface free energy-based relaxation dynamics of the droplet shape, a diffusive evaporation model and a contact line pinning mechanism governed by a yield stress. Our model reproduces the known dynamics of droplet shape relaxation and of droplet evaporation, both in the absence and in the presence of contact line pinning. We show that shape relaxation during evaporation significantly affects the lifetime of a drop. We find that the dependence of the evaporation time on the initial contact angle is a function of the competition between the shape relaxation and evaporation, and is strongly affected by any contact line pinning.Comment: 13 pages, 8 figure

Droplet evaporation losses during sprinkler irrigation: an overview

A detailed understanding regarding the evaporation losses in sprinkler irrigation is important for developing as well as adopting appropriate water conservation strategies. To explain this phenomenon many theoretical and experimental studies have been conducted since the 1950‟s. Notwithstanding all these efforts, the contribution of droplet evaporation to the total evaporation losses during sprinkler irrigation is still a controversial issue in the irrigation community. There is a substantial difference among researchers regarding the magnitudes of the different components of the total evaporation in sprinkler irrigation especially droplet evaporation losses. Field studies reported that the droplet evaporation losses ranged from 2 – 45%, whereas theoretical studies indicated that it is less than 1%. This is due largely to the limitations of the traditional measurement methods. However, it is likely that these limitations can be overcome and accurate measurements obtained using the eddy covariance (ECV) technique