On the predictions for diffusion-driven evaporation of sessile droplets with interface cooling

The diffusion-driven evaporation of sessile droplets from planar surfaces is influenced by cooling at the air-liquid interface. Here, corrections to the available models for predicting the evaporation process are presented. The mass conservation for diffusion-driven evaporation is resolved by considering the effect of interface cooling on the change in density of saturated vapour along the liquid-vapour interface of sessile droplets. Corrections to the predictions for the spatial distribution of vapour density around a sessile droplet and the evaporative flux of vapour at the interface are obtained. The classical models are recovered from the new predictions if interface cooling is negligible. Comparison between the new and classical predictions for the local surface evaporative flux is obtained using the literature data. Our analysis shows a significant effect of interface cooling which should be considered in predicting diffusion-driven evaporation of sessile droplets on planar surfaces

Effect of energy source, salt concentration and loading force on colloidal interactions between Acidithiobacillus ferrooxidans cells and mineral surfaces

The surface appendages and extracellular polymeric substances of cells play an important role in the bacterial adhesion process. In this work, colloidal forces and nanomechanical properties of Acidithiobacillus ferrooxidans (A. f) interacted with silicon wafer and pyrite (FeS2) surfaces in solutions of varying salt concentrations were quantitatively examined using the bacterial probe technique with atomic force microscopy. A. f cells were cultured with either ferrous sulfate or elemental sulfur as key energy sources. Our results show that A. f cells grown with ferrous ion and elemental sulfur exhibit distinctive retraction force vs separation distance curves with stair-step and saw tooth shapes, respectively. During the approach of bacterial probes to the substrate surfaces, surface appendages and biopolymers of cells are sequentially compressed. The conformations of surface appendages and biopolymers are significantly influenced by the salt concentrations. (c) 2015 Elsevier B.V. All rights reserved

A new way of assessing droplet evaporation independently of the substrate hydrophobicity and contact line mode: a case study of sessile droplets with surfactants

Evaluating and predicting evaporation kinetics of a sessile droplet present challenging exercises because of its strong dependence on the substrate hydrophobicity (contact angle) and the evaporation mode at the three-phase contact line. For sessile water droplets containing surfactants, the existence of surfactant would not only change droplet's geometry during the evaporation course, but could also affect the liquid cohesive energy density; thereby interactively affecting the evaporation kinetics. Here we present a new analytical model for the Hildebrand solubility parameter, which represents the cohesive energy density, as a function of time, thus a function of surfactant concentration inside a drying droplet. This distinct expression for the solubility parameter is independent of the droplet's geometry, therefore can be used to assess the variation of cohesive energy density during the evaporation course independently of both surface hydrophobicity and the evaporation mode. Our model resolve conflicting effects of water-soluble surfactants like sodium dodecyl sulfate (SDS) reported in the literature. It is shown that SDS can alter the evaporation rate differently (significantly or insignificantly) depending on the closeness of the studied system to the theoretical droplet lifetime extremum at the contact angle of 90°. The presence of SDS, up to the critical micellar concentration, does not significantly ease the vaporization of water through the air-liquid interface; nor SDS lead to faster evaporation rate by preferentially oscillating and vibrating of the adsorbed SDS molecules at the free water surface. Our theoretical analysis also provides a new approach to calculating the Hildebrand solubility parameter to estimate the cohesive energy density of different solvents

Interaction forces between goethite and polymeric flocculants and their effect on the flocculation of fine goethite particles

Goethite is a major component in many mineral processing operations. Its presence can have adverse effects on solid/liquid separations. To understand the interactions between goethite surfaces and their influence on particle flocculation, the anionic polymer flocculants ammonium polyacrylate (NHPA), hydrolysed polyacrylamide (PHPA), and hydroxamic polyacrylamide (HXPA) were studied using a combination of atomic force microscopy (AFM), floc structure analyses, and settling tests. The floc settling velocity related to the different flocculants had the following order: NHPA > HXPA > PHPA. The different floc sizes indicated many small and large sized flocs formed with NHPA and PHPA, while monomodal medium sized flocs formed with HXPA. The mass fractal dimension values showed that more compact flocs were formed with HXPA than the other flocculants. The direct force measurements without flocculants confirmed that the goethite surfaces strongly repel each other in alkaline solutions, which agrees with DLVO theory for similarly charged surfaces. The various interactions measured with different flocculants can be related to their molecular structures and molecular weights. The incubation of NHPA at various pH values resulted in long-range adhesion after surface contact with multiple elastic minima, indicating strong adsorption and an expanded molecular conformation for the adsorbed flocculant. The strong elastic minima and long-range adhesion for HXPA indicated a strong adsorption of this hydroxamic flocculant, consistent with the flocculation performance. The force interactions support the results of the settling tests and floc structures well, and AFM could be a good method for studying the relationship between surface interactions and particle flocculation in polymer flocculants

Theoretical and experimental analysis of droplet evaporation on solid surfaces

The evaporation of sessile drops is central to a number of important processes, including printing, washing and coating. In this paper, the evaporation of water sessile droplets on hydrophobised silicon wafers and Teflon was analysed from theoretical and experimental perspectives. The contact angle, volume and base radius of the water droplets as a function of time were determined using tensiometry. The theoretical analysis showed different evaporative flux phenomena for acute and obtuse contact angles. The non-linear evolution of residual droplet volume, contact angle and base radius are solved and depend on the hydrophobicity of the solid surface and droplet dimension. Good agreement between the theoretical and experimental results was observed during pinning and depinning stages of evaporation. It was shown that the surface roughness, hydrophobicity and the contact angle hysteresis significantly influenced the evaporation of sessile drops and need to be considered when quantifying the evaporation process

Assessing the effect of aromatic residues placement on α-helical peptide structure and nanofibril formation of 21-mer peptides

Coiled-coils with defined assembly properties are attractive materials for the manufacture of peptide-based hybrid nanomaterials. In tailoring such peptide assemblies, the incorporation of aromatic residues is increasingly being investigated due to their potential to deliver controllable functionalities, such as interaction with aromatic porphyrins, carbon nanotubes, or graphene. Aromatic residues have the potential to either destabilise or stabilise the α-helical peptide structure, depending on the quantity, type, combination, and position of these residues in the peptide chain. In this work, we used a known synthetic three heptad repeat peptide containing no aromatic residues as an α-helical template. We then substituted the aliphatic residues with two different types of aromatic residues (phenylalanine and tryptophan), varying their number, position, and combination in the peptide chain as a preliminary assessment of the impact on peptide architecture. Circular dichroism (CD) spectroscopy combined with coarse-grained (CG) and all-atom (AA) molecular dynamics (MD) simulation were used to analyse the peptide structure and assembly. Aromatic residues designed to be within the hydrophobic core were had impact on self-assembly than those placed on the outer face of the coil. Tryptophan was seen to destabilise α-helical structure more than phenylalanine, potentially due to steric hindrance and hydrogen-bonding interactions. Using atomic force microscopy (AFM) and supported by CG-MD simulation, substituting all phenylalanine residues with tryptophan appeared to completely destabilise fibril-formation propensity. Subsituting tryptophan into the first heptad repeat was seen to have a greater impact on fibril formation compared to subtitution into the third heptad repeat, suggesting the importance of sequence design. These results add to the body of knowledge used to inform the design of α-helical peptides when incorporating aromatic residues

Microstructural characteristics of naturally formed hardpan capping sulfidic copper-lead-zinc tailings

A massive and dense textured layer (ca. 35–50 cm thick) of hardpan was uncovered at the top layer, which capped the unweathered sulfidic Cu-Pb-Zn tailings in depth and physically supported gravelly soil root zones sustaining native vegetation for more than a decade. For the purpose of understanding functional roles of the hardpan layer in the cover profile, the present study has characterized the microstructures of the hardpan profile at different depth compared with the tailings underneath the hardpans. A suit of microspectroscopic technologies was deployed to examine the hardpan samples, including field emission-scanning electron microscopy coupled with energy dispersive spectroscopy (FE-SEM-EDS), X-ray diffraction (XRD) and synchrotron-based X-ray absorption fine structure spectroscopy (XAFS). The XRD and Fe K-edge XAFS analysis revealed that pyrite in the tailings had been largely oxidised, while goethite and ferrihydrite had extensively accumulated in the hardpan. The percentage of Fe-phyllosilicates (e.g., biotite and illite) decreased within the hardpan profile compared to the unweathered tailings beneath the hardpan. The FE-SEM-EDS analysis showed that the fine-grained Ca-sulfate (possibly gypsum) evaporites appeared as platelet-shaped that deposited around pyrite, dolomite, and crystalline gypsum particles, while Fe-Si gels exhibited a needle-like texture that aggregated minerals together and produced contiguous coating on pyrite surfaces. These microstructural findings suggest that the weathering of pyrite and Fe-phyllosilicates coupled with dolomite dissolution may have contributed to the formation of Ca-sulfate/gypsum evaporites and Fe-Si gels. These findings have among the first to uncover the microstructure of hardpan formed at the top layer of sulfidic Cu-Pb-Zn tailings, which physically capped the unweathered tailings in depth and supported root zones and native vegetation under semi-arid climatic conditions

Rhizosphere modifications of iron-rich minerals and forms of heavy metals encapsulated in sulfidic tailings hardpan

Hardpan caps formed after extensive weathering of the top layer of sulfidic tailings have been advocated to serve as physical barriers separating reactive tailings in depth and root zones above. However, in a hardpan-based root zone reconstructed with the soil cover, roots growing into contact with hardpan surfaces may induce the transformation of Fe-rich minerals and release potentially toxic elements for plant uptake. For evaluating this potential risk, two representative native species, Turpentine bush (Acacia chisholmii, AC) and Red Flinders grass (Iseilema vaginiflorum, RF), of which pre-cultured root mats were interfaced with thin discs of crushed hardpan minerals in the rhizosphere (RHIZO) test. After 35 days, the surface dissolution of hardpan minerals occurred and Fe-rich cement minerals were transformed from ferrihydrite-like minerals to goethite-like and Fe(III)-carboxylic complexes, as revealed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) and synchrotron-based X-ray absorption fine structure spectroscopy (XAFS) analysis. This transformation may result from the functions of root exudates. The transformation of hardpan cement minerals caused the co-dissolution of Cu and Zn initially encapsulated in the cements and their uptake by plants. Nevertheless, only was the minority of the plant Cu and Zn transported into shoots

3D Bombax-structured carbon nanotube sponge coupling with Ag3PO4 for tetracycline degradation under ultrasound and visible light irradiation

A novel photocatalytic carbon nanotube sponge with three-dimensional Bombax-structure was fabricated by a facile chemical vapor deposition followed by in situ ion-exchange approach. The as-prepared sponge achieved both high-efficiency adsorption and photocatalysis towards antibiotics, which can remove up to 90% of tetracycline within an hour. The morphology and mechanism of the photocatalytic CNT sponge were explored by multiple measures. Results show the functional groups and high specific surface area of CNT sponge play vital roles in preparing this Bombax-structured AgPO/CNT sponge, the band gap of which can be tuned by varying the ration between AgPO and CNT. The photodegradation experiments of tetracycline with the assistance of ultrasound irradiation were performed, AgPO/CNT sponge exhibits preferable photocatalytic activity, which can be attributed to both the enhancement of specific surface area of AgPO and the cavitation effect on CNT surface. The efficiency contributed by ultrasound could account for more than half of the degradation efficiency when the ultrasound power was 100 W. The improvement in transfer efficiency and the delay in charge recombination of AgPO/CNT sponge were further verified by Electrochemical impedance spectra (EIS) and Photoluminescence tests (PL). Reactive free-radical species were detected by the Electron Spin Resonance (ESR). The intermediates and possible pathway were analyzed by gas chromatography-mass spectrometer (GC–MS) technique