Efflorescence of Ammonium Sulfate and Coated Ammonium Sulfate Particles: Evidence for Surface Nucleation

Using optical microscopy, we investigated the efflorescence of ammonium sulfate (AS) in aqueous AS and in aqueous 1:1 and 8:1 (by dry weight) poly(ethylene glycol)-400 (PEG-400)/AS particles deposited on a hydrophobically coated slide. Aqueous PEG-400/AS particles exposed to decreasing relative humidity (RH) exhibit a liquid−liquid phase separation below 90% RH with the PEG-400-rich phase surrounding the aqueous AS inner phase. Pure aqueous AS particles effloresced in the RH range from 36.3% to 43.7%, in agreement with literature data (31−48% RH). In contrast, aqueous 1:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 7.2 to 19.2 μm effloresced between 26.8% and 33.9% RH and aqueous 8:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 1.8 to 7.3 μm between 24.3% and 29.3% RH. Such low efflorescence relative humidity (ERH) values have never been reached before for AS particles of this size range. We show that these unprecedented low ERHs of AS in PEG-400/AS particles could not possibly be explained by the presence of low amounts of PEG-400 in the aqueous AS phase, by a potential inhibition of water evaporation via anomalously slow diffusion through the PEG coating, or by different time scales between various experimental techniques. High-speed photography of the efflorescence process allowed the development of the AS crystallization fronts within the particles to be monitored with millisecond time resolution. The nucleation sites were inferred from the initial crystal growth sites. Analysis of the probability distribution of initial sites of 31 and 19 efflorescence events for pure AS and 1:1 (by dry weight) PEG-400/AS particles, respectively, showed that the particle volume can be excluded as the preferred nucleation site in the case of pure AS particles. For aqueous 1:1 (by dry weight) PEG-400/AS particles preferential AS nucleation in the PEG phase and at the PEG/AS/substrate contact line can be excluded. On the basis of this probability analysis of efflorescence events together with the AS ERH values of pure aqueous AS and aqueous PEG-400/AS particles aforementioned, we suggest that in pure aqueous AS particles nucleation starts at the surface of the particles and attribute the lower ERH values observed for aqueous PEG-400/AS particles to the suppression of the surface-induced nucleation process. Our results suggest that surface-induced nucleation is likely to also occur during the efflorescence of atmospheric AS aerosol particles, possibly constituting the dominating nucleation pathway

Scalable Freeze-Tape-Casting Fabrication and Pore Structure Analysis of 3D LLZO Solid-State Electrolytes.

Nonflammable solid-state electrolytes can potentially address the reliability and energy density limitations of lithium-ion batteries. Garnet-structured oxides such as Li7La3Zr2O12 (LLZO) are some of the most promising candidates for solid-state devices. Here, three-dimensional (3D) solid-state LLZO frameworks with low tortuosity pore channels are proposed as scaffolds, into which active materials and other components can be infiltrated to make composite electrodes for solid-state batteries. To make the scaffolds, we employed aqueous freeze tape casting (FTC), a scalable and environmentally friendly method to produce porous LLZO structures. Using synchrotron radiation hard X-ray microcomputed tomography, we confirmed that LLZO films with porosities of up to 75% were successfully fabricated from slurries with a relatively wide concentration range. The acicular pore size and shape at different depths of scaffolds were quantified by fitting the pore shapes with ellipses, determining the long and short axes and their ratios, and investigating the equivalent diameter distribution. The results show that relatively homogeneous pore sizes and shapes were sustained over a long range along the thickness of the scaffold. Additionally, these pores had low tortuosity and the wall thickness distributions were found to be highly homogeneous. These are desirable characteristics for 3D solid electrolytes for composite electrodes, in terms of both the ease of active material infiltration and also minimization of Li diffusion distances in electrodes. The advantages of the FTC scaffolds are demonstrated by the improved conductivity of LLZO scaffolds infiltrated with poly(ethylene oxide)/lithium bis(trifluoromethanesulfonyl)imide (PEO/LITFSI) compared to those of PEO/LiTFSI films alone or composites containing LLZO particles

Heat and mass transfer effects of ice growth mechanisms in water and aqueous solutions

Includes bibliographical references.Research into ice crystallization processes is an important area of study. The desire to improve product quality and efficiency of processes involving ice crystallization in industries such as desalination by freezing, freeze drying, freeze concentration and freeze crystallization for food processing, requires insight into the ice growth mechanisms. More so, a novel technology called Eutectic Freeze Crystallization, where water is recovered in the form of ice, requires that ice crystals are of high purity as this directly determines the quality of the water obtained. During ice crystallization, ice growth mechanisms play an important role in determining the structure, size and morphology of ice which have an effect on separation processes and product purity. Heat and mass transfer play a fundamental role in ice growth processes as they affect the thermodynamics and kinetics of the crystallization process. Ice growth experiments were carried out in pure water, in 8.4 wt% and 16.8 wt% magnesium sulphate and in 8.4 wt% sodium nitrate using a 10x5x31 mm test cell made of Plexi-glass®. The Colour Schlieren optical technique was used to conduct the experiments. This is because of its capability to map refractive index gradients related to either temperature or/and concentration gradients of the solution during crystal growth

Crystal nuclei templated nanostructured membranes prepared by solvent crystallization and polymer migration

Currently, production of porous polymeric membranes for filtration is predominated by the phase-separation process. However, this method has reached its technological limit, and there have been no significant breakthrough over the last decade. Here we show, using polyvinylidene fluoride as a sample polymer, a new concept of membrane manufacturing by combining oriented green solvent crystallization and polymer migration is able to obtain high performance membranes with pure water permeation flux substantially higher than those with similar pore size prepared by conventional phase-separation processes. The new manufacturing procedure is governed by fewer operating parameters and is, thus, easier to control with reproducible results. Apart from the high water permeation flux, the prepared membranes also show excellent stable flux after fouling and superior mechanical properties of high pressure load and better abrasion resistance. These findings demonstrate the promise of a new concept for green manufacturing nanostructured polymeric membranes with high performances

Nuclear magnetic resonance cryoporometry

Nuclear Magnetic Resonance (NMR) cryoporometry is a technique for non-destructively determining pore size distributions in porous media through the observation of the depressed melting point of a confined liquid. It is suitable for measuring pore diameters in the range 2 nm-1 mu m, depending on the absorbate. Whilst NMR cryoporometry is a perturbative measurement, the results are independent of spin interactions at the pore surface and so can offer direct measurements of pore volume as a function of pore diameter. Pore size distributions obtained with NMR cryoporometry have been shown to compare favourably with those from other methods such as gas adsorption, DSC thermoporosimetry, and SANS. The applications of NMR cryoporometry include studies of silica gels, bones, cements, rocks and many other porous materials. It is also possible to adapt the basic experiment to provide structural resolution in spatially-dependent pore size distributions, or behavioural information about the confined liquid

Effect of curing conditions and harvesting stage of maturity on Ethiopian onion bulb drying properties

The study was conducted to investigate the impact of curing conditions and harvesting stageson the drying quality of onion bulbs. The onion bulbs (Bombay Red cultivar) were harvested at three harvesting stages (early, optimum, and late maturity) and cured at three different temperatures (30, 40 and 50 oC) and relative humidity (30, 50 and 70%). The results revealed that curing temperature, RH, and maturity stage had significant effects on all measuredattributesexcept total soluble solids

THE EFFECT OF WATER-SOLUBLE POLYMERS ON THE MICROSTRUCTURE AND PROPERTIES OF FREEZE-CAST ALUMINA CERAMICS

Porous ceramics can be divided into three separate classes based on their pore size: microporous ceramics with pores less than 2 nm, mesoporous ceramics with pores in the range of 2-50 nm and macroporous ceramics with pores that are greater than 50 nm. In particular, macroporous ceramics are used in a variety of applications such as refractories, molten metal filtration, diesel particulate filters, heterogeneous catalyst supports and biomedical scaffolds.Freeze casting is a novel method used to create macroporous ceramics. In this method growing ice crystals act as a template for the pores and are solidified, often directionally, through a ceramic dispersion and removed from the green body through a freeze drying procedure. This method has attracted some attention over the past few years due to its relative simplicity, flexibility and environmental friendliness. On top of this freeze casting is capable of producing materials with high pore volume fractions, which is an advantage over processing by packing and necking of particles, where the pore volume fraction is typically less than 50%. Many of the basic processing variables that affect the freeze cast microstructure, such as the temperature gradient, interfacial velocity and solid loading of the dispersion have been well established in the literature. On the other hand, areas such as the effect of additives on the microstructure and mechanical properties have not been covered in great detail.In this study the concept of constitutional supercooling from basic solidification theory is used to explain the effects of two water-soluble polymers, polyethylene glycol and polyvinyl alcohol, on the microstructure of freeze cast alumina ceramics. In addition, changes in the observed microstructure will be related to experimentally determined values of permeability and compressive strength

Impact Dynamics of Surfactant-Laden Droplets on Non-Wettable Coatings

Owing to their excellent water repellency, non-wettable (superhydrophobic) coatings have gained tremendous attention in the past couple of decades. Alkyl ketene dimer (AKD), an inexpensive polymer frequently used in paper industry as a sizing agent, has shown potentials to become superhydrophobic. The formation of a porous structure after curing the solidified AKD for an extra-long time (4–6 days) results in superhydrophobicity, i.e., a static contact angle with water of \u3e150° and a roll-off angle of \u3c10°. In this work, a facile and low-cost method was used to turn the surface of AKD superhydrophobic in a very short period of time by briefly treating the coatings, obtained from isothermally heated molten AKD at 40 °C for 3 min, with ethanol. The resulting superhydrophobicity is due to the formation of porous, entangled irregular micro/nano textures that create air cushions on the surface leading to droplet state transition from Wenzel to Cassie. As a proof of concept, the same material was applied to the co-sputtered nickel-tungsten thin films, commonly used in micro/nano-electro-mechanical systems, to improve their hydrophobicity. According to the results, at least 20% increase was observed in the dynamic contact angles of the treated substrates. In addition, this work presents a detailed high-speed imaging analysis of the influence of the molecular weight, concentration and ionic nature of surfactants on droplet impact of such solutions on hydrophilic, hydrophobic and superhydrophobic substrates. Among all these surfaces, the concentration and ionic nature of the solutions were found to be more dominant parameters in determining the energy dissipation in the retraction phase of the droplet impact on the superhydrophobic (AKD) surfaces at room temperature. As the concentration decreases or when positive charges are present in the solution, it is more likely to observe a similar retraction dynamic to pure water when the droplet hits the superhydrophobic AKD having negatively charged surface sites. Finally, the impact dynamics and freezing behavior of these solutions were studied at very low temperatures of –10 to –30 °C. The results show that the dynamic behavior of the solutions is also a function of their temperature-dependent viscosity. The surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote heterogeneous ice nucleation both from within the liquid as well as a larger solid-liquid interfacial area, resulted from filling the air pockets of the surface by surfactants, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed