Uneven Evaporation Research Articles

Effect of the Dispersion Process and Nanoparticle Quality on Chemical Sensing Performance.

On the surface of chemiresistive films, the scarce heterogeneity of a molecularly capped gold nanoparticle (MCGNP) colloidal dispersion and uneven evaporation of the MCGNP-contained drying drop applied to this surface are among the main factors that affect reproducibility, and repeatable fabrication of thin films of MCGNPs. This article shows that an increase in reproducibility and repeatability is possible using a dispersant and a surfactant during the deposition and annealing processes of the MCGNP. The results show higher sensitivity and accuracy of the sensors for the detection of volatile organic compounds in air and an increased limit of detection. These simple and practical additions might serve as a launching pad for fabrication of other types of thin-film-based sensors.

Role of pH and substrate on drying patterns of Laponite® droplet

Disc shaped Laponite® particles in aqueous solution have a net negative charge on the faces and a positive charge along the rim. The charge on the rim of these particles changes to negative around a pH of 12. The pH of the aqueous Laponite® solution thus determines whether the Laponite® particles arrange themselves in a repulsive glass phase or form a cage-like gel structure as they gel during drying. When droplets of Laponite® solution having different pH are allowed to evaporate, the arrangement of the particles during drying is further perturbed by currents of mass transfer due to uneven evaporation flux from the droplet surface. In this work we report the interesting patterns left by drying droplets of aqueous Laponite® with different pH, dried on substrates of different hydrophobicity. On hydrophobic substrate, in highly acidic pH, a prominent coffee-ring was observed , while in a strong basic pH, salt crystals on Laponite® particles gathered at the centre of the dried droplet. Similar studies were done on hydrophilic substrates and quite different dried patterns obtained. Further in the case of very acidic pH of the aqueous Laponite® droplet drying on a hydrophobic substrate, the dried pattern revealed collapsed cavities whose walls showed rings of salt crystallites. We offer an explanation of the different patterns on the basis of particle-substrate electrostatic interactions in tandem with the role of pH, in the determination of the final pattern.

Self-Assembly CNTs@PANi Coffee Rings on Poly(styrene-ethylene-butylene-styrene) Triblock Copolymer for Largely Stretchable Electronics.

In this paper, CNTs@PANi nanocomposites were prepared by in-situ oxidation polymerization of aniline, and their structure, morphology and conductivity were characterized. A mixed solvent of toluene and tetrahydrofuran was used to prepare dispersions of CNTs@PANi and poly(styrene-ethylene-butylene-styrene) (SEBS) triblock copolymer, and bilayer composite film was prepared. According to the solvent phase separation and uneven evaporation flux, CNTs@PANi self-assembled into the interconnected coffee ring structure on the SEBS matrix. The prepared bilayer composite film had excellent stretchability, and the conductivity of the functional layer was close to that of CNTs@PANi, which could light up an LED lamp under 100% strain and restore the topological structure. Electrochemical tests showed that the bilayer film had obvious heterogeneity. The impedance characteristics of the CNTs@PANi functional layer and the SEBS matrix were analyzed, and its heterogeneous corrosion resistance mechanism further discussed.

High‐Resolution Pixelated Light Emitting Diodes Based on Electrohydrodynamic Printing and Coffee‐Ring‐Free Quantum Dot Film

AbstractInkjet printing for preparing quantum dot light emitting diodes (QLEDs) has become increasingly attractive due to its large‐area, low‐cost fabrication features. However, the rather low resolution and coarse morphology induced by the printing and uneven evaporation of droplet restrict its further development. Herein, electrohydrodynamic (EHD) printing and mixed solvent method are employed to fabricate high‐resolution pixelated QLEDs. By optimizing the mixed solvent ratio to weaken capillary flow, coffee‐ring‐free quantum dot (QD) films can be obtained. With the help of EHD printing process optimization and pixel defining layers introduction, high‐resolution pixelated QLED with 306 pixels per in. is achieved, which can meet the requirements of manufacturing mobile phones. Finally, an inverted pixelated QLED with a low turn‐on voltage of 3 V and a maximum luminance of 8533 cd m−2 is fabricated, demonstrating that the presented strategy has huge potential in high‐resolution and high‐quality QD display manufacturing.

Degassed micromolding lithography for rapid fabrication of anisotropic hydrogel microparticles with high-resolution and high uniformity.

Replica molding techniques, which are used to synthesize microparticles inside anisotropic micromolds, have been developed to enable the mass production of hydrogel particles. However, these techniques are limited in their ability to synthesize only a narrow range of particle compositions and shapes because of the difficulty in loading precursors into the micromolds as well as the low particle homogeneity due to the uneven evaporation of the precursors. Herein, we describe a simple yet powerful technique, called degassed micromolding lithography, which can load precursors within 1 min regardless of the wettability. This technique is based on the gas-solubility of a degassed micromold that acts as a suction pump to completely fill the mold by drawing precursor liquids in. The semi-closed system within the micromold prevents the uneven evaporation of the precursor, which is essential for the production of homogeneous particles. Furthermore, controlled uniformity of the hydrogel microparticles (C.V. < 2%) can be achieved by engineering the design of the micromold array.

Evaporation induced hollow cracks and the adhesion of silver nanoparticle film

Solution-based technology has potential in electrode patterning for low-cost, large-scale and flexible electronics. However, the formation of cracks often leads to unpredictable experimental results. This report explores the critical impact of solvent evaporation on the hollow crack formation and the adhesive property of silver nanoparticle film deposited using an ink-jet printable precursor ink. Uneven evaporation causes directional flows inside and along the surface of wet film, which contributes to nanoparticle film assembly initially at the periphery surface region. The area further away from the contact line tends to form thinner surface film, which can be easily deformed to form ripples under compressive stress. The suppressed evaporation of the liquid under the surface film makes the initially outward compensation flow reversed to inward, leaving a hollow inside morphology of the ripples, which can be transferred to cracks after complete solidification and will seriously degrade the adhesive property. This work has referential significance for the optimization of ink chemistry and film processing for high-quality film fabrication using solution-based method.

Capillary Coatings: Flow and Drying Dynamics in Open Microchannels.

Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10-3-10 s) to the formation of a dry thin film (a "capillary coating"; 102-103 s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings.

Effects of sub-base surface roughness on shrinkage cracking of industrial floors

Avoiding the formation of shrinkage cracks is one of the most important tasks in construction of concrete industrial floors. Cracks appear on the surface of floors when the nonlinear processes in the structure lead to internal stresses that exceed the actual tensile strength of the concrete. The tensile stresses developed depend on the constrained deformation of the floor during shrinkage and the elastic modulus of the material of the concrete slab. However, on the top surface of the floor, the tensile stresses can be increased, if the shrinkage deformations are constrained by the uneven evaporation and the generated friction between the sub-base and the floor. The value of friction coefficient is depended primarily on the surface roughness of the subbase and the type of polyethylene foil used between the sub-base and the concrete slab. The paper presents the results of experimental investigations on the friction coefficient and the effects of its value on the cracking process of industrial concrete floor...

中国西南部云贵高原一个湿地系统氧氢同位素的季节性变化:地下水流入通量定量评价

The Caohai Wetland serves as an important ecosystem on the Yunnan–Guizhou Plateau and as a nationally important nature reserve for migratory birds in China. In this study, surface water, groundwater and wetland water were collected for the measurement of environmental isotopes to reveal the seasonal variability of oxygen and hydrogen isotopes (δ18O, δD), sources of water, and groundwater inflow fluxes. Results showed that surface water and groundwater are of meteoric origin. The isotopes in samples of wetland water were well mixed vertically in seasons of both high-flow (September) and low-flow (April); however, marked seasonal and spatial variations were observed. During the high-flow season, the isotopic composition in surface wetland water varied from −97.13 to −41.73‰ for δD and from −13.17 to −4.70‰ for δ18O. The composition of stable isotopes in the eastern region of this wetland was lower than in the western region. These may have been influenced by uneven evaporation caused by the distribution of aquatic vegetation. During the low-flow season, δD and δ18O in the more open water with dead aquatic vegetation ranged from −37.11 to −11.77‰, and from −4.25 to −0.08‰, respectively. This may result from high evaporation rates in this season with the lowest atmospheric humidity. Groundwater fluxes were calculated by mass transfer and isotope mass balance approaches, suggesting that the water sources of the Caohai Wetland were mainly from groundwater in the high-flow season, while the groundwater has a smaller contribution to wetland water during the low-flow season.

Liquid flow rate effects during partial evaporation in a falling film micro contactor

The focus of this study is the investigation of the effect of liquid flow rate on partial evaporation, enhanced by convective nitrogen flow, in a falling film micro contactor. Experiments are performed at different flow rates and for a certain heating liquid temperature. The temperatures of the gas and liquid phases are measured at their exit points, and the evaporation rate is determined while the channels are monitored with a microscope.The channels do not appear fully wetted at low liquid flow rates, and the liquid temperature is relatively low. The bad wetting is attributed to liquid flow maldistribution that can cause temperature gradients and subsequently enhances the maldistribution by uneven evaporation. A heat transfer model is developed and solved for different numbers of wetted channels. According to the modelling results the low liquid temperatures, can be fully explained by liquid flow maldistribution and channel drying.These wetting effects are minimized by restricting the liquid flow at its exit point and increasing the amount of liquid maintained at the bottom of the contactor. This improves the temperature profile and prevents drying, as it is possible that capillary forces tend to drive liquid upwards when dry spots occur.

Fatigue properties of Fe–Al intermetallic coatings prepared by plasma spraying

In this research, FeAl based coatings were deposited on low carbon steel using two different plasma spraying technologies – gas stabilized plasma gun (GSP) and water stabilized plasma gun (WSP) – resulting in different deposition conditions. Significant changes in local chemical composition compared to feedstock material were observed due to uneven evaporation of Fe and Al from the molten material during the interaction of feedstock with plasma. Influence of coatings on substrate fatigue properties was characterized at room temperature using special testing device “SF-Test”. Fatigue lifetimes of specimens with different coatings were compared with those of uncoated specimens. Fractographic analysis of failed specimens revealed that substrate fatigue cracks initiated at coating–substrate interface and grew by striation mechanism. The coatings failed due to intrasplat cracking and intersplat decohesion.

Composition of β″ precipitates in Al–Mg–Si alloys by atom probe tomography and first principles calculations

The composition of β″ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at 175 °C for 36 h shows that the β″ phase contains ∼20 at. % Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published β″-Mg5Si6 structure. Ab initio calculations show that replacing the Si3-site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the β″-phase.

Comparative study of the β″‐phase in a 6xxx Al alloy by 3DAP and HRTEM

AbstractThe metastable β″‐hardening phase in aged 6xxx‐series Al alloys has been investigated by three‐dimensional atom‐probe in order to evaluate the composition (Mg5Si6) of the structure, previously determined by high‐resolution transmission electron microscopy (HRTEM) and electron diffraction. For β″, a significant number of Mg and Si atoms are found to be lost during detection in atom‐probe analysis. The reason of this is non‐resolved events, in which the correlation between the time‐of‐flight (TOF) and the detected position of the field‐evaporated ion is lost. The problem is attributed to uneven evaporation and can be avoided by the use of an advanced delay‐line detector (ADLD), which is capable of discriminating extremely closely spaced detection events. Preliminary analysis of a β″‐particle using this detector gave a Mg/Si ratio of 0.9 and a composition of 15 at.% Al. The Al level is significantly lower than those previously measured by other atom‐probe detectors, because of the higher efficiency of the ADLD in detecting Mg and Si in particles. Atom‐probe analysis with the ADLD detector is consistent with the β″ Mg/Si ratio derived from electron microscopy analysis. The measurements indicate that Al atoms may substitute both Si and Mg positions. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Calcium sulphate scaling in falling film evaporators

The formation of calcium sulphate scale in Vertical Tube Evaporators (VTE) of the falling film type has been investigated in a single tube experiment using fluted tubes. It was found that the scaling threshold is dependent not only on the brine concentration and temperature, but also on the heat flux to the evaporating film. With normal sea water concentrations and a high heat flux, scale was observed at 240°F (115°C), whereas with a low heat flux scale-free operation was possible at 266°F (130°C). The effect of heat flux is attributed to the presence of local areas of high concentration in the falling film which form as a result of uneven evaporation from the film. A simple model has been developed to explain the results.