Aqueous Solution Droplets Research Articles

Analysis of atmospheric pollutant metals by laser ablation inductively coupled plasma mass spectrometry with a radial line-scan dried-droplet approach

A novel method has been improved for analyzing atmospheric pollutant metals (Be, Mn, Fe, Co, Ni, Cu, Zn, Se, Sr, Cd, and Pb) by laser ablation inductively coupled plasma mass spectrometry. In this method, solid standards are prepared by depositing droplets of aqueous standard solutions on the surface of a membrane filter, which is the same type as used for collecting atmospheric pollutant metals. Laser parameters were optimized, and ablation behaviors of the filter discs were studied. The mode of radial line scans across the filter disc was a representative ablation strategy and can avoid error from the inhomogeneous filter standards and marginal effect of the filter disc. Pt, as the internal standard, greatly improved the correlation coefficient of the calibration curve. The developed method provides low detection limits, from 0.01ng m−3 for Be and Co to 1.92ngm−3 for Fe. It was successfully applied for the determination of atmospheric pollutant metals collected in Lhasa, China. The analytical results showed good agreement with those obtained by conventional liquid analysis. In contrast to the conventional acid digestion procedure, the novel method not only greatly reduces sample preparation and shortens the analysis time but also provides a possible means for studying the spatial distribution of atmospheric filter samples.

Water Evaporation from Acoustically Levitated Aqueous Solution Droplets.

We present a systematic study of the effect of solutes on the evaporation rate of acoustically levitated aqueous solution droplets by suspending individual droplets in a zero-relative humidity environment and measuring their size as a function of time. The ratios of the early time evaporation rates of six simple salts (NaCl, NaBr, NaNO3, KCl, MgCl2, CaCl2) and malonic acid to that of water are in excellent agreement with predictions made by modifying the Maxwell equation to include the time-dependent water activity of the evaporating aqueous salt solution droplets. However, the early time evaporation rates of three ammonium salt solutions (NH4Cl, NH4NO3, (NH4)2SO4) are not significantly different from the evaporation rate of pure water. This finding is in accord with a previous report that ammonium sulfate does not depress the evaporation rate of its solutions, despite reducing its water vapor pressure, perhaps due to specific surface effects. At longer evaporation times, as the droplets approach crystallization, all but one (MgCl2) of the solution evaporation rates are well described by the modified Maxwell equation.

Superhydrophobic behavior of lauryl methacrylate copolymers on the cotton fabric surface

The authors propose to use glycidyl methacrylate and lauryl methacrylate copolymers to provide the cotton fabric surface with superhydrophobic properties. The modification was carried out by immersion of cotton fabric in solutions of copolymers in methylethylketone and following heat treatment at 140°C. Stability of the superhydrophobic state of polymer coatings is studied during long-term contact with droplets of water and aqueous acidic, alkaline and salt solutions under saturated vapor conditions. Under the conditions of high humidity and absence of contact with the external environment, a low drop evaporation rate is ensured, which allows to trace the long-term change of the contact angle of the sessile drop. The mechanism of stability of superhydrophobic properties is discussed. Polymer coatings that are based on the glycidyl methacrylate and lauryl methacrylate block copolymer provide better stability of superhydrophobic properties compared with the random copolymer. It is shown that modification of the cotton fabric surface by glycidyl methacrylate and lauryl methacrylate copolymers does not affect its vapor permeability.

Publisher’s note

Macroscopic hollow polymeric particles are attractive materials for various applications such as surgery, food industry, agriculture, etc. However, protocols reporting their synthesis have hitherto made use of organic solvents and/or sacrificial templates, compromising the encapsulation of different bioactive compounds and the process yield. Here, millimeter-size, hollow polymeric particles were synthesized, for the first time, in a solvent- and template free manner onto superhydrophobic surfaces (SHS). The particles were produced upon assembly and double superficial crosslinking of liquid droplets of DNA and methacrylamide chitosan aqueous solutions (CH:MA), leading to liquid-core particles with a hardened hydrogel shell. The particles displayed appealing physical and biological properties. The millimeter-size hydrogel shell, resulting from the double ionic/covalent crosslinking of CH:MA, endowed the hollow particles with softness to the touch and an outstanding structural stability against manipulation by hand and with forceps. Meanwhile, the liquid DNA core guaranteed a biocompatible cell encapsulation followed by a superior release and proliferation of viable cells, as compared to solid CH:MA particles prepared as a blank. Particles with these characteristics show promise for surgical protocols practiced in Tissue Engineering and Regenerative Medicine, where manipulable and biocompatible synthetic implants are often needed to supply living cells and other sensitive bioactive compounds.

Evaporation induced natural convection inside a droplet of aqueous solution placed on a superhydrophobic surface

Numerical study has been conducted to study the fluid convection inside sessile and pendant evaporating droplets of aqueous NaCl solution placed on a superhydrophobic surface. Evaporation from the droplet surface leads to increase in the solute concentration at the evaporating liquid–air interface generating concentration gradient inside the droplet. Concentration gradient inside the droplet induces Rayleigh convection. Fluid with higher concentration in the liquid–air interface region moves downward along the interface and lighter fluid rises upward along the central region of the droplet like a plume. The average concentration, evaporation rate and the maximum velocity inside the sessile droplet on a superhydrophobic surface is higher than that on a hydrophobic surface. Both sessile and pendant droplet configurations have been investigated. Fluid near the interface moves towards the substrate in case of sessile droplet, where as the fluid moves away from the substrate along the interface for the pendant droplet configuration. Two counter rotating recirculation bubbles are observed in both sessile and pendant droplets. The magnitude of maximum convection velocity and circulation strength of the recirculation bubbles for pendant droplet is smaller than that of sessile droplet at the same evaporation conditions. This may be attributed to the stratification of higher density fluid for the pendant droplet configuration which suppresses the Rayleigh convection. The effect of initial solute concentration on the strength of convection is stronger for sessile droplet configuration compared to that of pendant droplet. The present study establishes strong influence of superhydrophobicity and orientation on the internal convection inside a droplet of aqueous solution.

Salt‐induced iron corrosion under evaporating sessile droplets of aqueous sodium chloride solutions

The corrosion process induced by evaporation of sessile droplets from aqueous sodium chloride solutions on planar iron surfaces was quantitatively investigated. The spatial distribution of the salt inside the evaporating sessile droplet, which is influenced by the initial salt concentration in the droplet bulk, is correlated to the localization of the anodic and cathodic reactions at the electrolyte–metal interface over the footprint droplet area. At low salt concentration, the inverse of the classical well‐accepted Evans model is observed due to the hydrodynamic flows linked to evaporation: the anode area is established near the three‐phase contact line region. Increasing salt concentration leads to a more uniform pitted sessile droplet: we observed locally anodic and cathodic areas over the droplet surface at the metal–electrolyte interface, where local variations in chloride concentrations occur. In addition, the presence of corrosion products and some pits demonstrate that pitting corrosion takes place if the salt concentration reaches a threshold value, experimentally determined by an optical set‐up.

Subvisible cirrus clouds – a dynamical system approach

Abstract. Ice clouds, so-called cirrus clouds, occur very frequently in the tropopause region. A special class are subvisible cirrus clouds with an optical depth lower than 0.03, associated with very low ice crystal number concentrations. The dominant pathway for the formation of these clouds is not known well. It is often assumed that heterogeneous nucleation on solid aerosol particles is the preferred mechanism although homogeneous freezing of aqueous solution droplets might be possible, since these clouds occur in the low-temperature regime T < 235 K. For investigating subvisible cirrus clouds as formed by homogeneous freezing we develop a reduced cloud model from first principles, which is close enough to complex models but is also simple enough for mathematical analysis. The model consists of a three-dimensional set of ordinary differential equations, and includes the relevant processes as ice nucleation, diffusional growth and sedimentation. We study the formation and evolution of subvisible cirrus clouds in the low-temperature regime as driven by slow vertical updraughts (0 < w ≤ 0. 05 m s−1). The model is integrated numerically and also investigated by means of theory of dynamical systems. We found two qualitatively different states for the long-term behaviour of subvisible cirrus clouds. The first state is a stable focus; i.e. the solution of the differential equations performs damped oscillations and asymptotically reaches a constant value as an equilibrium state. The second state is a limit cycle in phase space; i.e. the solution asymptotically approaches a one-dimensional attractor with purely oscillatory behaviour. The transition between the states is characterised by a Hopf bifurcation and is determined by two parameters – vertical updraught velocity and temperature. In both cases, the properties of the simulated clouds agree reasonably well with simulations from a more detailed model, with former analytical studies, and with observations of subvisible cirrus, respectively. The reduced model can also provide qualitative interpretations of simulations with a complex and more detailed model at states close to bifurcation qualitatively. The results indicate that homogeneous nucleation is a possible formation pathway for subvisible cirrus clouds. The results motivate a minimal model for subvisible cirrus clouds (SVCs), which might be used in future work for the development of parameterisations for coarse large-scale models, representing structures of clouds.

Heterogeneous ice nucleation of α‐pinene SOA particles before and after ice cloud processing

AbstractThe ice nucleation ability of α‐pinene secondary organic aerosol (SOA) particles was investigated at temperatures between 253 and 205 K in the Aerosol Interaction and Dynamics in the Atmosphere cloud simulation chamber. Pristine SOA particles were nucleated and grown from pure gas precursors and then subjected to repeated expansion cooling cycles to compare their intrinsic ice nucleation ability during the first nucleation event with that observed after ice cloud processing. The unprocessed α‐pinene SOA particles were found to be inefficient ice‐nucleating particles at cirrus temperatures, with nucleation onsets (for an activated fraction of 0.1%) as high as for the homogeneous freezing of aqueous solution droplets. Ice cloud processing at temperatures below 235 K only marginally improved the particles' ice nucleation ability and did not significantly alter their morphology. In contrast, the particles' morphology and ice nucleation ability was substantially modified upon ice cloud processing in a simulated convective cloud system, where the α‐pinene SOA particles were first activated to supercooled cloud droplets and then froze homogeneously at about 235 K. As evidenced by electron microscopy, the α‐pinene SOA particles adopted a highly porous morphology during such a freeze‐drying cycle. When probing the freeze‐dried particles in succeeding expansion cooling runs in the mixed‐phase cloud regime up to 253 K, the increase in relative humidity led to a collapse of the porous structure. Heterogeneous ice formation was observed after the droplet activation of the collapsed, freeze‐dried SOA particles, presumably caused by ice remnants in the highly viscous material or the larger surface area of the particles.

Solution electrostatic levitator for measuring surface properties and bulk structures of an extremely supersaturated solution drop above metastable zone width limit.

We report on the first integrated apparatus for measuring surface and thermophysical properties and bulk structures of a highly supersaturated solution by combining electrostatic levitation with real-time laser/x-ray scattering. Even today, a proper characterization of supersaturated solutions far above their solubility limits is extremely challenging because heterogeneous nucleation sites such as container walls or impurities readily initiate crystallization before the measurements can be performed. In this work, we demonstrate simultaneous measurements of drying kinetics and surface tension of a potassium dihydrogen phosphate (KH2PO4) aqueous solution droplet and its bulk structural evolution beyond the metastable zone width limit. Our experimental finding shows that the noticeable changes of the surface properties are accompanied by polymerizations of hydrated monomer clusters. The novel electrostatic levitation apparatus presented here provides an effective means for studying a wide range of highly concentrated solutions and liquids in deep metastable states.

Formation of Highly Metastable β Glycine by Confinement in Inkjet Printed Droplets

Inkjet printing was used to deposit picoliter droplets of aqueous glycine solution onto glass or aluminum substrates and it was found that the volume constraint on crystal growth resulted in formation of the highly metastable β form. When the droplet volume increased to 0.1 μL or greater, a mixture of the α and β forms was seen in a coffee-ring formation. When the droplet volume increased to 10 μL, needle-like β crystals formed in the bulk of the ring and bipyramidal α crystals on the outer edge at the air–water interface. The β form was stable under ambient conditions for at least 6 weeks. The observations suggest that glycine nucleates to, and remains stable as, the β form when constrained in small volumes, because on the nanometer scale the β form is thermodynamically stable. As the droplet volume increases, and constraint on crystal growth is reduced, the α form becomes thermodynamically stable, and so phase transformation occurs, probably facilitated by a solvent-mediated mechanism. Inkjet printing p...

Nonequilibrium Self-Assembly of π-Conjugated Oligopeptides in Solution.

Supramolecular assembly is a powerful method that can be used to generate materials with well-defined structures across multiple length scales. Supramolecular assemblies consisting of biopolymer-synthetic polymer subunits are specifically known to exhibit exceptional structural and functional diversity as well as programmable control of noncovalent interactions through hydrogen bonding in biopolymer subunits. Despite recent progress, there is a need to control and quantitatively understand assembly under nonequilibrium conditions. In this work, we study the nonequilibrium self-assembly of π-conjugated synthetic oligopeptides using a combination of experiments and analytical modeling. By isolating an aqueous peptide solution droplet within an immiscible organic layer, the rate of peptide assembly in the aqueous solution can be controlled by tuning the transport rate of acid that is used to trigger assembly. Using this approach, peptides are guided to assemble under reaction-dominated and diffusion-dominated conditions, with results showing a transition from a diffusion-limited reaction front to spatially homogeneous assembly as the transport rate of acid decreases. Interestingly, our results show that the morphology of self-assembled peptide fibers is controlled by the assembly kinetics such that increasingly homogeneous structures of self-assembled synthetic oligopeptides were generally obtained using slower rates of assembly. We further developed an analytical reaction-diffusion model to describe oligopeptide assembly, and experimental results are compared to the reaction-diffusion model across a range of parameters. Overall, this work highlights the importance of molecular self-assembly under nonequilibrium conditions, specifically showing that oligopeptide assembly is governed by a delicate balance between reaction kinetics and transport processes.

Experimental study of geometrical characteristics of distilled water and aqueous ethanol solution droplets evaporating on aluminum surface

Change of geometrical characteristics of distilled water and aqueous ethanol solution droplets was studied under their evaporation on aluminum surface. According to change in the contact diameter three evaporation modes of distilled water droplet on polished aluminum surface were detected: increase in the contact area, pinning of a droplet (constant contact area), and droplet depinning (decrease in the contact diameter). During evaporation of aqueous ethanol solution droplets, two evaporation modes were detected: increase in the contact area, and droplet depinning.

Preparation of Microcapsules Containing Dye Aqueous Solution and Investigation of Release Feature According to Preparation Method

Microcapsules containing the dye aqueous solution were prepared with three kinds of preparation methods and the release feature of microcapsules with each preparation method was mainly investigated. As a dye tried to microencapsulate, methylene blue was adopted, because methylene blue aqueous solution was changed in color with light irradiation and utilized in order to check the degree of river pollution. Microencapsulation using multiple emulsion was performed with the suspension polymerization method, the inverse interfacial polycondensation method and the suspension polymerization in parallel with the interfacial polymerization method, respectively. The release feature of microcapsules prepared with each preparation method was estimated with the solute permeability coefficient. It was found that the release feature of dye aqueous solution was different according to the preparation method and could be delicately controlled by microencapsulating with the suspension polymerization in parallel with interfacial polycondensation reaction and forming the polyurethane shell on the surface of the dye aqueous solution droplets.

Temperature-dependent formation of NaCl dihydrate in levitated NaCl and sea salt aerosol particles.

Recent laboratory studies indicate that the hydrated form of crystalline NaCl is potentially important for atmospheric processes involving depositional ice nucleation on NaCl dihydrate particles under cirrus cloud conditions. However, recent experimental studies reported a strong discrepancy between the temperature intervals where the efflorescence of NaCl dihydrate has been observed. Here we report the measurements of the volume specific nucleation rate of crystalline NaCl in the aqueous solution droplets of pure NaCl suspended in an electrodynamic balance at constant temperature and humidity in the range from 250 K to 241 K. Based on these measurements, we derive the interfacial energy of crystalline NaCl dihydrate in a supersaturated NaCl solution and determined its temperature dependence. Taking into account both temperature and concentration dependence of nucleation rate coefficients, we explain the difference in the observed fractions of NaCl dihydrate reported in the previous studies. Applying the heterogeneous classical nucleation theory model, we have been able to reproduce the 5 K shift of the NaCl dihydrate efflorescence curve observed for the sea salt aerosol particles, assuming the presence of super-micron solid inclusions (hypothetically gypsum or hemihydrate of CaSO4). These results support the notion that the phase transitions in microscopic droplets of supersaturated solution should be interpreted by accounting for the stochastic nature of homogeneous and heterogeneous nucleation and cannot be understood on the ground of bulk phase diagrams alone.

Evaporation Rate of Aqueous Salt Solutions Droplets

The evaporation of aqueous salt solution droplets from metal surfaces has been studied experimentally. The volumetric evaporation rate is found to decrease in time for any initial droplet volume due to an increase in salt concentration and the efforts of system to take a state of thermodynamic equilibrium. Crystalline hydrate film was being formed during desorption of CaCl2 10%, LiCl 10%, LiBr 30% salts. When NaCl 10% salt solution evaporated, there was no film. The average evaporation rate of NaCl salt is higher than for other salts. The lowest values of the average evaporation rate were found for LiBr 30% salt solution.

Dissolution of Sessile Microdroplets of Electrolyte and Graphene Oxide Solutions in an Ouzo System.

Manipulating the way a droplet shrinks by evaporation or dissolution is an effective approach for assembling dissolved nanomaterials. In this work, we investigate the dissolution dynamics of a submicroliter sessile droplet of electrolyte aqueous solution and of graphene oxide suspension immersed in a binary mixture of solvents, among which one is miscible and the other is immiscible with water (i.e., an Ouzo system). Our measurements reveal an interesting two-stage dissolution of the droplet: a fast initial stage and a slow second stage. The duration of the first stage is longer at a lower temperature, leading to a counterintuitive result that the dissolution completes faster at reduced temperature. The presence of graphene oxide in the droplet dramatically alters the dissolution dynamics, possibly due to its enrichment at the droplet surface. The finding from this work provides useful guideline for designing conditions to pack nanomaterials by dissolving droplets, especially for those temperature sensitive components.

Microwave-assisted synthesis of trisiloxane superspreader and its superspreading behavior on plant leaves surfaces

This paper describes a facile microwave-assisted synthesis of a series of novel trisiloxane surfactants, their surface and aggregation properties, and superspreading behaviors on plant leaves surfaces. Kinetic study showed that hydrosilylation reaction followed a second-order rate law and that the activation energy was greatly reduced in case of microwave irradiation (84.395kJ/mol) as compared to the conventional heating method (109.272kJ/mol). The molecular structures of trisiloxane surfactants were characterized by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance spectroscopy (1HNMR). Their surface and aggregation properties were investigated by surface tensiometry, transmission electron microscopy (TEM), and dynamic laser particle size analysis (DLS). Dynamic spreading behavior of droplets of aqueous trisiloxane surfactant solutions on the surfaces of the rice and mango leaves were investigated by contact angle measurement, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Effects of various factors on the advancing contact angle (CA), radius (r), wetted area (S), velocity of spreading (dr/dt), spreading exponent (n), and critical wetting concentration (CWC) were analyzed. These surfactants were found to have low critical aggregation concentration (CMC) and surface tension (γCMC). With increase in number of ethoxy units, the values of CMC, γCMC, occupied area per surfactant molecule (Amin), and standard free energy of aggregation (△Gθmic) increased, whereas Гmax and standard free energy of adsorption (△Gθads) decreased. The surfactant, with an optimal HLB value of about 10 (10.37 for S-2), showed the best superspreading behavior. The superspreading behavior was related to molecular structure, the concentration of surfactant, and substrate wettability. A superspreading model was presented to illustrate the superspreading process. Synergistic effects of precursor water film and Marangoni effect were mainly responsible for the superspreading behavior.

(Invited) EIS for the Early Detection of Unexpected Corrosion Phenomena in Highly Resistive Organic Solutions: Multiple Electrical Equivalent Circuit Fitting Validated By Parametric Continuity Conditions

Unexpected corrosion phenomena caused by a priori non-corrosive organic highly resistive solutions have been identified in some industrial petrochemical plants. Due to the extremely low conductivity of this kind of non-polar solution, performing reliable and reproducible electrochemical tests remains a difficult task. In which concerns EIS, Nyquist diagrams are prone to be crippled by a strong bulk-related capacitive behavior imposed by the dielectric nature of the electrolyte. The low frequency processes at the metal interface are then often hidden or at least badly resolved. In spite of these inherent difficulties, some of these phenomena have been successfully reproduced and studied in laboratory with a two-electrode cell in which two identical large surface plates of carbon or stainless steel were assembled facing each other and separated by a gap of 1 mm. EIS was then used to detect the onset and investigate the possible causes of the corrosion phenomena. Results appeared to be related either to water contamination or to the presence of solution additives. In the case of water contamination results showed that, even below the solubility coefficient, droplets of aqueous solution are ejected from the organic matrix at the metal interface creating clusters of aggressive electrolyte yielding localized corrosion. The corresponding Nyquist diagrams after water contamination have been fitted with a two time-constants electrical equivalent circuit in which the low frequency domain was straightforwardly associated to the aqueous droplets formation and was used to monitor the onset of corrosion. In the case of additive-containing organic matrix, however, the Nyquist diagrams exhibited a time-evolving profile that required a dual electrical circuit fitting strategy to be applied. Indeed, the process time evolution was so that diagrams were alternatively fitted with two different circuits for the same system depending on the immersion duration: a 2 time-constants EEC for the first ca 36 hours and a single bulk-related time-constant after that. This fitting methodology, although mandatory from an experimental point of view due to the time evolution profile of the diagrams, brings to the forefront of the problem the question of the validity of shifting between two or more different EEC for the survey of a monotonically evolving single interface. In other words, the shift between the two different circuits must match the physical continuity of characteristic parameters. This means that the fitting results for the different elements introduced in the EEC must not show leaps or discontinuities as the EEC are changed, since they are supposed to represent the same system. This parametric continuity condition has been used throughout the study and is illustrated in Figure 1 for the time evolution of the dielectric constant of the bulk organic phase and the solution resistance as obtained from the numeric values of the fitted EEC. It can be seen that in the initial stages the modified solution showed a higher dielectric constant that was associated with corrosion processes taking place till the reactive species were depleted at the interface or till the surface was blocked by adsorbed species. The bulk phase consistently retrieved his usual dielectric constant values after ca 36 hours in the conditions of Figure 1. Figure 1

Molecular Combing of λ-DNA using Self-Propelled Water Droplets on Wettability Gradient Surfaces.

Surface wettability gradients were used to elongate and align double stranded λ-DNA. Gradients were prepared by vapor phase deposition of octyltrichlorosilane (C8-silane) and fluorinated octyltrichlorosilane (F-silane) precursors. Gradient formation was confirmed by water contact angle and ellipsometric film thickness measurements. Placement of a droplet of aqueous DNA solution on the hydrophobic end of each gradient led to spontaneous motion of the droplet toward the hydrophilic end and deposition of the DNA. Fluorescence imaging of surface-adsorbed YOYO-1 labeled DNA molecules revealed that they are elongated and aligned perpendicular to the droplet-surface contact line at all positions along the gradient, consistent with a dominant role played by surface tension forces in elongating the DNA. The density of adsorbed DNA was found to be greatest on the C8-silane gradient at its hydrophobic end. DNA density decreased toward the hydrophilic end, while the length of the elongated DNA was less dependent on position. The elongation of DNA molecules by spontaneous droplet motion on chemical gradient surfaces has possible applications in DNA barcoding and studies of DNA-protein interactions.

Picoliter Drop-On-Demand Dispensing for Multiplex Liquid Cell Transmission Electron Microscopy.

Liquid cell transmission electron microscopy (LCTEM) provides a unique insight into the dynamics of nanomaterials in solution. Controlling the addition of multiple solutions to the liquid cell remains a key hurdle in our ability to increase throughput and to study processes dependent on solution mixing including chemical reactions. Here, we report that a piezo dispensing technique allows for mixing of multiple solutions directly within the viewing area. This technique permits deposition of 50 pL droplets of various aqueous solutions onto the liquid cell window, before assembly of the cell in a fully controlled manner. This proof-of-concept study highlights the great potential of picoliter dispensing in combination with LCTEM for observing nanoparticle mixing in the solution phase and the creation of chemical gradients.