Droplet Drying Research Articles

A Universal Cosolvent Evaporation Strategy Enables Direct Printing of Perovskite Single Crystals for Optoelectronic Device Applications.

Solution-processed metal halide perovskite (MHP) single crystals (SCs) are in high demand for a growing number of printed electronic applications due to their superior optoelectronic properties compared to polycrystalline thin films. There is an urgent need to make SC fabrication facile, scalable, and compatible with the printed electronic manufacturing infrastructure. Here, a universal cosolvent evaporation (CSE) strategy is presented by which perovskite SCs and arrays are produced directly on substrates via printing and coating methods within minutes at room temperature from drying droplets. The CSE strategy successfully guides the supersaturation via controlled drying of droplets to suppress all crystallization pathways but one, and is shown to produce SCs of a wide variety of 3D, 2D, and mixed-cation/halide perovskites with consistency. This approach works with commonly used precursors and solvents, making it universal. Importantly, the SC consumes the precursor in the droplet, which enables the large-scale fabrication of SC arrays with minimal residue. Direct on-chip fabrication of 3D and 2D perovskite photodetector devices with outstanding performance is demonstrated. The approach shows that any MHP SC can now be manufactured on substrates using precision printing and scalable, high-throughput coating methods.

Intrinsically Viscoelastic Supramolecular Conjugated Polymer toward Suppressing Coffee-Ring Effect

Intrinsically Viscoelastic Supramolecular Conjugated Polymer toward Suppressing Coffee-Ring Effect

Modeling of Drying of Droplets in Airflow Containing Passive Solid Particles

Modeling of Drying of Droplets in Airflow Containing Passive Solid Particles

Influence of surfactant‐humectant adjuvants on physical properties, droplet size, and efficacy of glufosinate formulations

AbstractGlufosinate efficacy is inconsistent among weed species and under environmental conditions that favor rapid droplet drying. Surfactant‐humectant adjuvants could maximize glufosinate efficacy by increasing wetting and penetration into the leaf surface while decreasing evaporation rate (ER). However, there is a lack of information in the literature about the interaction of surfactant‐humectants adjuvants with glufosinate. The objective of this study was to investigate the influence of surfactant‐humectant adjuvants on the physical properties, droplet size, and efficacy of two glufosinate formulations. Laboratory, greenhouse, and field studies were conducted at the Pesticide Application Technology Laboratory of the University of Nebraska‐Lincoln. Treatment design was a 2 × 5 factorial with two glufosinate formulations combined with five adjuvant treatments plus an untreated control. Density and viscosity of glufosinate solutions mostly increased with the addition of adjuvants. However, the influence of the adjuvants on dynamic surface tension (dST), static contact angle (sCA), and evaporation rate (ER) varied by glufosinate formulation, adjuvant, and relative humidity (RH). Under greenhouse conditions, an improvement in efficacy by adding adjuvants was mainly observed for Interline solutions. The addition of adjuvants to Interline solutions increased biomass reduction up to 19 and 35% for common lambsquarters (Chenopodium album L.) and kochia [Bassia scoparia (L.) A. J. Scott], respectively. Also, some of the adjuvants presented null or antagonistic influence on herbicide efficacy. No increase in control, biomass reduction, and mortality of horseweed (Erigeron canadensis L.) and Palmer amaranth (Amaranthus palmeri S. Watson) was observed with the use of adjuvants under field conditions. Herbicide‐adjuvant‐plant‐environment interaction is complex. Thus, the use of surfactant‐humectant adjuvants may not increase herbicide efficacy.

Rheological behaviour of concentrated maltodextrins describes skin formation and morphology development during droplet drying

We have studied the rheological behaviour of concentrated maltodextrins varying in dextrose equivalence value (DE) over a range of temperatures (≤70 °C) and concentrations (≥62% (w/w) solids) using small amplitude oscillatory experiments. Rheological data could be mapped onto master curves for storage and loss modulus as described with a model for transient or entangled polymeric networks. Maltodextrins DE5 and DE12 showed a rubber-to-glass transition, where the rubber-like behaviour is hypothesized to be the consequence of physical entanglements at sufficiently high concentrations of polysaccharides. Maltodextrins DE21 and DE38 mostly consisting of small sugars and oligomers exhibited viscous behaviour over a large range of frequencies and only at high frequencies the onset of a cross-over was visible which could signify the vicinity of the glass transition. An effective diffusion drying model validated with sessile single droplet drying experiments was coupled to rheological models to estimate rheological behaviour during droplet drying. The predicted rheological properties of the skin formed at the surface of the droplet could be related to the occurrence of surface instabilities later during drying, including cavitation for elastic skins and wrinkling for viscous skins. Insight in rheological behaviour at high concentrations may therefore offer help in predicting the morphology of droplets generated with spray drying.

Evaporation Characteristics of a Single Desulfurization Wastewater Droplet in High-temperature Gas

Understanding the high-temperature evaporation process of atomized wastewater droplets is a prerequisite for designing an efficient spray evaporation wastewater treatment facility. However, some dynamic characteristics during the evaporation of high-temperature wastewater droplets have not yet been fully understood. Here, the convective evaporation process of desulfurization wastewater droplet in high-temperature gas, which is commonly encountered in the spray drying technology to treat desulfurization wastewater in power plants, was investigated adopting the single droplet drying (SDD) device. In-situ droplet information including the morphology, radius, temperature, and mass was recorded. The influence of bulk temperature and the initial concentration of dissolved solids on evaporation kinetics was studied. To explore the development of resistances during the evaporation process as well as develop a reliable and flexible droplet evaporation model useful for CFD simulations, experimental results were correlated with a modified Reaction Engineering Approach (REA) framework, which revised the previous linear shrinkage model into a hybrid model to illustrate the droplet/particle deformation considering the possible inflation phenomenon at elevated temperatures. The results show that crust formation and internal bubble nucleation induced by wastewater components contributed to unique evaporation features including decreasing evaporation rate and inflation of semi-dried particles. Increasing bulk temperature enhanced the evaporation rate and the degree of inflation. Finally, the rapid and accurate prediction of the modified REA model elucidated its effectiveness to illustrate the whole evaporation and drying process.

Drying of virus-containing particles: modelling effects of droplet origin and composition

Background and purposeVirus-containing aerosol droplets emitted by breathing, speech or coughing dry rapidly to equilibrium with ambient relative humidity (RH), increasing in solute concentration with effects on virus survival and decreasing in diameter with effects on sedimentation and respiratory uptake. The aim of this paper is to model the effect of ionic and macromolecular solutes on droplet drying and solute concentration.MethodsDeliquescence-efflorescence concepts and Kohler theory were used to simulate the evolution of solute concentrations and water activity in respiratory droplets, starting from efflorescence data on mixed NaCl/KCl aerosols and osmotic pressure data on respiratory macromolecules.ResultsIn NaCl/KCl solutions total salt concentrations were shown to reach 10-13 M at the efflorescence RH of 40-55%, depending on the K:Na ratio. Dependence on K:Na ratio implies that the evaporation curves differ between aerosols derived from saliva and from airway surfaces. The direct effect of liquid droplet size through the Kelvin term was shown to be smaller and restricted to the evolution of breath emissions. Modelling the effect of proteins and glycoproteins showed that salts determine drying equilibria down to the efflorescence RH, and macromolecules at lower RH.ConclusionDifferences in solute composition between airway surfaces and saliva are predicted to lead to different drying behaviour of droplets emitted by breathing, speech and coughing. These differences may influence the inactivation of viruses.

Effect of interface-active proteins on the salt crystal size in waterborne hybrid materials

Aqueous processes yielding hybrid or composite materials are widespread in natural environments and their control is fundamental for a multiplicity of living organisms. Their design and in vitro engineering require knowledge about the spatiotemporal evolution of the interactions between the involved liquid and solid phases and, especially, the interphases governing the development of adhesion during solidification. The present study illustrates the effects of distinct proteins on the precipitation of sodium chloride encompassing the size, shape and distribution of halite crystals formed during the drying of droplets containing equally concentrated saline protein solutions. The precipitates obtained from aqueous sodium chloride formulations buffered with tris(hydroxymethyl)aminomethane (Tris) contained either bovine serum albumin (BSA), fibrinogen or collagen and were characterized with respect to their structure and composition using optical and electron microscopy as well as x-ray analysis. The acquired findings highlight that depending on the protein type present during droplet drying the halite deposits predominantly exhibit cubic or polycrystalline dendritic structures. Based on the phenomenological findings, it is suggested that the formation of the interphase between the growing salt phase and the highly viscous saline aqueous jelly phase containing protein governs not only the material transport in the liquid but also the material exchange between the solid and liquid phases.

Aggregation patterns of copper sulphate salt via droplet drying: mediation by surface hydrophobicity and salt concentration.

Patterns in drying droplets formed from colloidal solution of copper sulphate and gelatin are investigated with respect to variation of substrate hydrophobicity and salt concentration. Hydrophilic substrates as (i) glass, (ii) quartz and hydrophobic substrate as (iii) polypropylene (PP) have been used. It is observed that the dry residue pattern of salt crystals shows curved branches of crystalline aggregate growth about droplet centre for hydrophilic substrates, while thick, light and dark concentric bands of aggregates are observed for hydrophobic substrates. The geometry and topology of the patterns have been characterized through an analysis of fractal dimension and the topological measure, Euler characteristic. The fractal dimension of the deposit increases substantially with salt concentration for hydrophilic substrates, but decreases with concentration for hydrophobic substrate. Our analysis leads us to propose that an optimal viscosity contrast that facilitates prominent viscous fingers is a function of contact angle and salt concentration. We propose that substrate hydrophobicity and salt concentration together are responsible for DLA-like aggregation in evaporating droplets.

Large-Area Fabrication of Structurally Colored and Humidity Sensitive Composite Nanofilm via Ultrasonic Spray-Coating.

Intelligent structural colors have received extensive attention in recent years due to their diverse applications. However, the large-area, uniform, and cost-effective fabrication of ultra-thin structural color films is still challenging. Here, for the first time, we design and employ an ultrasonic spray-coating technique with non-toxic, green nano-silica and polyvinylpyrrolidone as raw materials, to prepare structural color films on silicon wafers. Due to the addition of polyvinylpyrrolidone, the coffee-ring effect during droplet drying is suppressed and uniform composite films are formed. We further perform a detailed study of the influence of various processing parameters including silica/polyvinylpyrrolidone concentration, substrate temperature, nozzle-to-substrate distance, and number of spray-passes on film roughness and thickness. By increasing the number of spray-passes from 10 to 30, the film thickness from 120 to 340 nm is modulated, resulting in different colors, and large-area and uniform colors on commercial round silicon wafers with 15 cm diameter are achieved. The silica/polyvinylpyrrolidone composite films show strong hydrophilicity and are sensitive to humidity changes, leading to quickly tunable and reversible structural colors. Quartz crystal microbalance with dissipation demonstrates water vapor adsorption and condensation on the nanofilm when increasing environmental humidity. Thereby, ultrasonic spray-coating as a novel film fabrication technique provides a feasible scheme for large-area preparation of intelligent structural colors.

Numerical study of single droplet drying in an acoustic levitator before the critical point of time

When using an acoustic levitator to perform single droplet drying studies, one has to be aware of its significant impact on the heat and mass transfer. In this paper, we propose a model for the first drying stage of an acoustically levitated water droplet containing suspended solids. Both the realistic geometry of the acoustic levitator and the deformation of the levitated droplet are taken into account by our numerical simulations. Empirical data from the literature are used to validate our single droplet levitation and drying model in case of pure water and binary water-maltodextrin droplets. We found that the acoustic streaming inside the levitated droplet has a retarding effect on its critical point of time when the solute forms a solid crust at the free phase boundary. Moreover, the temperature dependence of the sound wave length turned out to have a crucial impact on the performance of the acoustic levitator.

Transition from saliva droplets to solid aerosols in the context of COVID-19 spreading.

To control the evolution of a pandemic such as COVID-19, knowing the conditions under which the pathogen is being transmitted represents a critical issue, especially when implementing protection strategies such as social distancing and wearing face masks. For viruses and bacteria that spread via airborne and/or droplet pathways, this requires understanding how saliva droplets evolve over time after their expulsion by speaking or coughing. Within this context, the transition from saliva droplets to solid residues, due to water evaporation, is studied here both experimentally, considering the saliva from 5 men and 5 women, and via numerical modeling to accurately predict the dynamics of this process. The model assumes saliva to be a binary water/salt mixture and is validated against experimental results using saliva droplets that are suspended in an ultrasound levitator. We demonstrate that droplets with an initial diameter smaller than 21 μm will produce a solid residue that would be considered an aerosol of <5 μm diameter in less than 2 s (for any relative humidity less than 80% and/or any temperature greater than 20°C). Finally, the model developed here accounts for the influence of the saliva composition, relative humidity and ambient temperature on droplet drying. Thus, the travel distance prior to becoming a solid residue can be deduced. We found that saliva droplets of initial size below 80 μm, which corresponds to the vast majority of speech and cough droplets, will become solid residues prior to touching the ground when expelled from a height of 160 cm.

Spray drying of desulfurization wastewater: Drying characteristics, product analysis and potential risk assessment

The Wastewater Evaporation System (WES) was a reliable and economical way to treat desulfurization wastewater. However, the evaporation mechanism was not clear, and the evaporation product properties were lack of awareness. In this study, the evaporation and product properties were revealed by the single droplet drying (SDD) method and spray drying system. It was found that the evaporation process of wastewater involved the constant rate and falling rate period. The component of the evaporation product from the SDD and spray drying method were similar, mainly consisted of MgCl2, CaCl2, NaCl and CaSO4. Besides, the potential risks of WES were assessed. After wastewater evaporation, the Electrostatic Precipitator (ESP) removal efficiency was enhanced due to the aggregation of fine particles and the enhanced specific resistance. For the influence of WES on comprehensive fly ash utilization, the chloride mass fraction in the final product was lower than 0.06% and could meet strict regulations.

“Coffee Ring” Effect of Ag Colloidal Nanoparticles Dried on Glass: Impact to Surface-Enhanced Raman Scattering (SERS)

It is well known that spontaneous drying of some fluid droplets on certain solid surfaces forms a “coffee ring” pattern. In this paper, we studied “coffee ring” formation for two kinds of Ag colloidal nanoparticles (borohydride-reduced (b.-r.) and hydroxylamine-reduced (h.-r.)) and its impact on surface-enhanced Raman scattering (SERS). Optical and scanning electron microscopies were used to observe the morphology of the dried rings as well. We used 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (TMPyP) as a testing SERS molecular probe. The results showed that the structure of the edge rings of dried drops of Ag colloid/TMPyP systems was different for b.-r. and h.-r. nanoparticles. The inherent limitation of our approach is inhomogeneity in particle and “hot spots” distribution, SERS signal fluctuation, and consequently low spectral reproducibility. However, in the case of h.-r. nanoparticles, it formed a structure with highly enhancing sites (“hot spots”) providing enormous SERS signal of TMPyP. Higher sensitivity and the possibility of spectral mapping over the dried pattern are advantages in comparison with the measurements from colloidal suspension. Although our approach is not reliable for quantitative analytical SERS applications, it can serve as a simple, cheap, and fast prescan method, which can be easily implemented for preliminary SERS analysis.

Droplet evaporation on inclined substrates

Drying of droplets on inclined surfaces is relevant to applications such as ink-jet printing, spray coating, and crime-scene reconstruction. We develop a lubrication-theory based model to investigate the effect of substrate inclination on the evaporation of pure-solvent and particle-laden droplets on smooth and rough inclined substrates. We find that the effect of inclination on evaporation mainly depends on substrate roughness and the Bond number. Our predictions qualitatively agree with previous experimental work and our model can be easily extended to more complex situations such as multiple droplets evaporating on inclined substrates.

Chloride release characteristics of desulfurization wastewater droplet during evaporation process using the single droplet drying method

The desulfurization wastewater evaporation technology is an effective way to realize zero-liquid discharge (ZLD). However, the release of chloride during the evaporation process is still a great challenge, which may cause severe chloride enrichment in the wastewater. In this work, the single droplet drying (SDD) method was utilized to investigate the chloride release characteristics during the evaporation process. The three periods of chloride release were first proposed based on the chloride release ratio and wastewater droplet's features (mass, temperature, and diameter). With the removal of the free moisture, the first period (Free water evaporation) was initiated, and almost no released chloride was detected; In the unstable shell period, 19.8% chloride would release there, mainly due to the increased droplet's temperature; Approximately 10% chloride would release during the shell pyrolysis period, which was directly related to the hydrolysis of the chloride-based salt hydrates in the evaporation particle. Besides, the chloride release characteristics at different cations species and ambient temperature were investigated. The desulfurization wastewater with a higher Mg2+ concentration would be more likely to release chloride during the evaporation process. With the temperature increased from 150 to 300℃, the chloride release ratio would improve from 14.56 to 29.8% due to the disruption of the shell and the hydrolysis degree at a higher temperature. In practical engineering practices, a balance point between chloride release and evaporation efficiency should be found, and the Mg2+ in the wastewater should be pre-treated before entering into the evaporation system.

Lattice Boltzmann Modeling of Drying of Porous Media Considering Contact Angle Hysteresis

Drying of porous media is governed by a combination of evaporation and movement of the liquid phase within the porous structure. Contact angle hysteresis induced by surface roughness is shown to influence multi-phase flows, such as contact line motion of droplet, phase distribution during drainage and coffee ring formed after droplet drying in constant contact radius mode. However, the influence of contact angle hysteresis on liquid drying in porous media is still an unanswered question. Lattice Boltzmann model (LBM) is an advanced numerical approach increasingly used to study phase change problems including drying. In this paper, based on a geometric formulation scheme to prescribe contact angle, we implement a contact angle hysteresis model within the framework of a two-phase pseudopotential LBM. The capability and accuracy of prescribing and automatically measuring contact angles over a large range are tested and validated by simulating droplets sitting on flat and curved surfaces. Afterward, the proposed contact angle hysteresis model is validated by modeling droplet drying on flat and curved surfaces. Then, drying of two connected capillary tubes is studied, considering the influence of different contact angle hysteresis ranges on drying dynamics. Finally, the model is applied to study drying of a dual-porosity porous medium, where phase distribution and drying rate are compared with and without contact angle hysteresis. The proposed model is shown to be capable of dealing with different contact angle hysteresis ranges accurately and of capturing the physical mechanisms during drying in different porous media including flat and curved geometries.

Tranilast-loaded amorphous solid dispersion prepared with fine droplet drying process for improvement of oral absorption and anti-inflammatory effects on chemically-induced colitis

The aim of the present study was to evaluate the applicability of a tranilast (TL)-loaded amorphous solid dispersion prepared with a fine droplet drying (FDD) process for the treatment of inflammatory bowel diseases. The FDD process, a novel powderization technology using an inkjet head, was employed to produce an amorphous solid dispersion of tranilast (ASD/TL). The physicochemical properties of ASD/TL were evaluated in terms of the morphology, particle size, crystallinity, drug-polymer interaction, and dissolution behavior. The oral absorption behavior of TL and anti-inflammatory effect on a chemically-induced colitis model were evaluated after oral administration of TL samples. The particle size of ASD/TL was 4 μm, and the span factor, one of the parameters to present the uniformity of particle size distribution, was calculated to be 0.28. In each ASD/TL particle, TL was dispersed in an amorphous state in the carrier polymer, and ASD/TL showed improved dissolution behavior of TL, as evidenced by a 2-times higher dissolution than the equilibrium solubility of crystalline TL even under acidic conditions. Orally-dosed ASD/TL exhibited rapid and improved oral absorption with 16-fold higher bioavailability than crystalline TL. ASD/TL could significantly attenuate the inflammatory symptoms in the colon tissues of colitis model rats based on the results of histological analyses and measurement of biomarkers. ASD/TL might contribute to the development of effective treatment for inflammatory bowel diseases due to its enhanced biopharmaceutical properties.

Wetting and Drying of Aqueous Droplets Containing Nonionic Surfactants CnEm.

This paper presents a systematic study of the wetting and drying of aqueous pico-liter droplets containing nonionic surfactants polyoxyethylene alkyl ethers (CnEm; n = 10, 12, 14, m = 6 or 8) in comparison with the anionic surfactant sodium dodecyl sulfate (SDS). The spreading and drying of droplets on hydrophilic substrates were studied by tracking the three-phase contact line (TCL) and by interferometry. CnEm droplets undergo phase separation during drying: a water-rich droplet retracts and leaves behind a thin film that is postulated to be a surfactant mesophase. This thin film either retracts or breaks up into small droplets on a longer time scale. The receding contact angle of the water-rich droplet on the thin film in the late stage of drying of CnEm droplets is independent of hydrophobicity of substrates, supporting the inference that a mesophase is present on the surface. Both CnEm and SDS solutions inhibit spreading on hydrophilic surfaces, which is attributed to Marangoni contraction as a result of a surface tension gradient across the gas-liquid interface. More pronounced suppression of spreading is observed in the case of CnEm solutions, possibly due to the phase transition of surfactant solution in the vicinity of the initial TCL leading to a viscous phase at the TCL that pins the droplet. Tracer particle measurements reveal that mild Marangoni flows exist for droplets with surfactant concentrations well above the critical micelle concentration (CMC). Origins of the surfactant gradients that result in Marangoni flows are discussed.

Single droplet drying of detergents: Experimentation and modelling

This study evaluates the single droplet drying (SDD) of detergents. Experimental data are used to validate a theoretical multistage model. The experiments are carried out in an acoustic levitator analysing six different detergent formulations at 393K. The work is completed by developing a model based on conservation equations. The model is composed of the three stages observed in the drying curve. In the first stage, an external surface drying stage occurs until the surface is saturated and the crust is formed. In the next stage, the drying rate is governed by diffusion through the pores. At the same time, the particle heats up until boiling conditions are achieved in the core of the particle. At this point, the third stage begins. In the final stage, the droplet core is in boiling conditions which are governed by the pressure drop through the crust and the concentration of salts. This model has been evaluated for the different formulations being flexible to predict the drying of different mixtures with good accuracy.