Droplet Drying Research Articles

Evaporation and Electrowetting of Sessile Droplets on Slippery Liquid-Like Surfaces and Slippery Liquid-Infused Porous Surfaces (SLIPS).

Sessile droplet evaporation underpins a wide range of applications from inkjet printing to coating. However, drying times can be variable and contact-line pinning often leads to undesirable effects, such as ring stain formation. Here, we show voltage programmable control of contact angles during evaporation on two pinning-free surfaces. We use an electrowetting-on-dielectric approach and Slippery Liquid-Infused Porous (SLIP) and Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) surfaces to achieve a constant contact angle mode of evaporation. We report evaporation sequences and droplet lifetimes across a broad range of contact angles from 105°–67°. The values of the contact angles during evaporation are consistent with expectations from electrowetting and the Young-Lippman equation. The droplet contact areas reduce linearly in time, and this provides estimates of diffusion coefficients close to the expected literature value. We further find that the total time of evaporation over the broad contact angle range studied is only weakly dependent on the value of the contact angle. We conclude that on these types of slippery surfaces, droplet lifetimes can be predicted and controlled by the droplet’s volume and physical properties (density, diffusion coefficient, and vapor concentration difference to the vapor phase) largely independent of the precise value of contact angle. These results are relevant to applications, such as printing, spraying, coating, and other processes, where controlling droplet evaporation and drying is important.

Effect of Immiscible Secondary Fluid on Particle Dynamics and Coffee Ring Characteristics during Suspension Drying.

Particle motion and coffee ring patterns in water-borne suspensions of polystyrene (PS) particle added with small amounts of secondary hydrophobic decalin are investigated during the drying of the suspension droplets, mainly employing light scattering methods. Very tiny secondary fluid insertions via high-speed agitation effectively link the particles through hydrophobic dissolution leading to the formation of multimodal particulate clusters, with resistance to the outward capillary flow and suppression of coffee ring formation after drying. The impact of decalin on particles is corroborated by actual images acquired from an optical profiler and a scanning electron microscope (SEM). The average particle motion inside the suspension changed by decalin was expressed in terms of mean square displacement (MSD) based on diffusing wave spectroscopy (DWS). Employing multispeckle diffusing wave spectroscopy (MSDWS), the rapid motion or β-relaxation of particles in various suspensions with and without decalin is quantified in early lag time during the drying of droplets. The change in particle dynamics during suspension drop drying, when adding a small secondary fluid, plays a key role in tuning coffee ring patterns.

Design of water-soluble whole rice glutelin: The rendezvous of two rice subspecies, Japonica and Indica

The preparation route of the water-soluble rice glutelin-glutelin (extracted from two subspecies of Japonica and Indica rice, respectively) system is proposed and tested in laboratory-scale, which can potentially be scaled up for industrial production. The two-components in highly alkaline condition was carefully acidified to neutral to produce whole rice glutelin. The interfacial properties of the system can be adjusted by changing the ratio of rice glutelin origins (Japonica group (JRG) or Indica group (IRG)), so that the surface hydrophobicity can be reduced to expose the charged groups. The created protein structure is robust and could maintain desirable wettability after single droplet drying and freeze-drying. To date, the application of plant proteins has been largely limited by the poor solubility of hydrophobic aggregates. The proposed JRG-IRG system is a crucial step toward aqueous rice protein materials. The potential to prepare aqueous composites was also demonstrated by synergistic modification of plant proteins from different subspecies.

Unraveling Particle Formation: From Single Droplet Drying to Spray Drying and Electrospraying.

Spray drying and electrospraying are well-established drying processes that already have proven their value in the pharmaceutical field. However, there is currently still a lack of knowledge on the fundamentals of the particle formation process, thereby hampering fast and cost-effective particle engineering. To get a better understanding of how functional particles are formed with respect to process and formulation parameters, it is indispensable to offer a comprehensive overview of critical aspects of the droplet drying and particle formation process. This review therefore closely relates single droplet drying to pharmaceutical applications. Although excellent reviews exist of the different aspects, there is, to the best of our knowledge, no single review that describes all steps that one should consider when trying to engineer a certain type of particle morphology. The findings presented in this article have strengthened the predictive value of single droplet drying for pharmaceutical drying applications like spray drying and electrospraying. Continuous follow-up of the particle formation process in single droplet drying experiments hence allows optimization of manufacturing processes and particle engineering approaches and acceleration of process development.

The contribution of dry indoor built environment on the spread of Coronavirus: Data from various Indian states

Coronavirus spread is more serious in urban metropolitan cities compared to rural areas. It is observed from the data on the infection rate available in the various sources that the cold and dry conditions accelerate the spread of coronavirus. In the present work, the existing theory of respiratory droplet drying is used to propose the mechanism of virus spread under various climates and the indoor environment conditions which plays a greater role in the virus spread. This concept is assessed using four major parameters such as population density, climate severity, the volume of indoor spaces, and air-conditioning usage which affect the infection spread and mortality using the data available for various states of India. Further, it is analysed using the data from various states in India along with the respective climatic conditions. It is found that under some indoor scenarios, the coronaviruses present in the respiratory droplets become active due to size reduction that occurs both in sessile and airborne droplet nuclei causing an increase in the spread. Understanding this mechanism will be very useful to take the necessary steps to reduce the rate of transmission by initiating corrective measures and maintaining the required conditions in the indoor built environment.

Investigation of Wettability, Drying and Water Condensation on Polyimide (Kapton) Films Treated by Atmospheric Pressure Air Dielectric Barrier Discharge

In this study, we report on the investigation of influence of air atmospheric pressure dielectric barrier discharge on polyimide (Kapton) films. It is shown that plasma treatment causes a significant increase of Kapton wettability that is connected with alterations of its chemical composition (oxidation) induced by dielectric barrier discharge. Observed variations in the wettability of Kapton were also found to be accompanied by changes in the dynamics of water droplets drying on plasma-treated Kapton, namely by the reduction of the constant contact angle phase of the droplet drying. This effect may be ascribed to the higher surface heterogeneity of plasma-treated Kapton that causes pinning of the edges of drying droplet on the Kapton surface. Finally, the differences in wettability induced by the plasma treatment led to a different way, how the water condensates on the Kapton surface: while the condensing water forms large amount of small droplets on untreated Kapton, much bigger water structures were found on the Kapton exposed to atmospheric plasma.

In situ Raman composition profiling in drying droplets

Spray drying has become a tool for engineering solid particles to meet the growing demand for better end-use particulate material properties. However, this industrial process still presents scientific challenges, such as a detailed description of the components distribution during drying of multi-component droplets. In order to help overcoming such challenge, in the present work, an in situ Raman technique is applied to assess the temporal and spatial distribution of the components in acoustically levitated drying droplets. Using a dextran-sucrose-water droplet with different solute proportions, different results were experimentally demonstrated: a dextran-rich surface and a sucrose-rich core in a particle; dense or bowl-like particles depending on the dextran-to-sucrose initial proportion.

Hydrophobicity of abiotic surfaces governs droplets deposition and evaporation patterns

Surface contamination with droplets containing bacteria is of concern in the food industry and other environments where hygiene control is essential. Deposition patterns after the drying of contaminated droplets is affected by numerous parameters. The present study evaluated the rate of evaporation and the shape of deposition patterns after the drying of water droplets on a panel of materials with different surface properties (topography, hydrophobicity). The influence of the particle properties (in this study 1 μm-microspheres and two bacterial spores) was also investigated. Polystyrene microspheres were hydrophobic, while Bacillus spores were hydrophilic or hydrophobic, and surrounded by different surface features. In contrast to material topography, hydrophobicity was shown to deeply affect droplet evaporation, with the formation of small, thick deposits with microspheres or hydrophilic spores. Among the particle properties, the spore morphology (size and round/ovoid shape) did not clearly affect the deposition pattern. Conversely, hydrophobic spores aggregated to form clusters, which quickly settled on the materials and either failed to migrate, or only migrated to a slight extent on the surface, resulting in a steady distribution of spores or spore clusters over the whole contaminated area.Adherent bacteria or spores are known to be highly resistant to many stressful environmental conditions. In view of all the quite different patterns obtained following drying of spore-containing droplets, it seems likely that some of these would entail enhanced resistance to hygienic processes.

Deposition and distribution of gold nanoparticles in a coffee-stain ring on the HOPG terraces

Cetyltrimethylammonium bromide (CTAB)-stabilized low-aspect ratio gold nanorods have been synthesized using seed-mediated wet chemical approach. Freshly cleaved highly oriented pyrolytic graphite (HOPG) surface is used as a substrate for the deposition of nanoparticles via the droplet drying technique. Owing to the presence of CTAB molecules in suspension, nanoparticles on the non-wetting HOPG surface get deposited in the form of a coffee-stain ring. Such deposits at various positions of the coffee-stain ring have been characterized by scanning helium ion microscopy (HIM) and atomic force microscopy (AFM). Besides the assembly of nanoparticles, AFM is specially employed to investigate the deposition of CTAB molecules. The deposition and distribution of nanoparticles are studied and quantified at various positions of the coffee-stain ring. On flat HOPG terraces, HIM shows merely the deposits of the gold nanostructures whereas AFM reveals both nanoparticles and CTAB deposits over the surface. This research study primarily focused on the nanoparticle deposits within the coffee-stain ring formed over the non-wetting HOPG terraces.

The drying of liquid droplets**Project supported by the National Natural Science Foundation of China (Grant No. 21822302), the joint NSFC-ISF Research Program, China (Grant No. 21961142020), the Fundamental Research Funds for the Central Universities, China, and the National College Students’ Innovative and Entrepreneurial Training Plan Program, China (Grant No. 201910006142).

The drying of liquid droplets is a common phenomenon in daily life, and has long attracted special interest in scientific research. We propose a simple model to quantify the shape evolution of drying droplets. The model takes into account the friction constant between the contact line (CL) and the substrate, the capillary forces, and the evaporation rate. Two typical evaporation processes observed in experiments, i.e., the constant contact radius (CCR) and the constant contact angle (CCA), are demonstrated by the model. Moreover, the simple model shows complicated evaporation dynamics, for example, the CL first spreads and then recedes during evaporation. Analytical models of no evaporation, CCR, and CCA cases are given, respectively. The scaling law of the CL or the contact angle as a function of time obtained by analytical model is consistent with the full numerical model, and they are all subjected to experimental tests. The general model facilitates a quantitative understanding of the physical mechanism underlying the drying of liquid droplets.

A mechanistic model to predict droplet drying history and particle shell formation in multicomponent systems

Spray drying allows tuning the physical properties of the resulting powders widely. However, targeted process design is complicated by the interplay between process characteristics and the non-ideal physical properties exhibited by multicomponent mixtures, such as pharmaceutical formulations. This work presents a mechanistic model describing the drying of single droplets. The model includes heat and mass balances, non-ideal vapour-liquid equilibria, and population balances describing the evolution of particulates within the drying droplet. The model is applied to (up to) ternary mixtures of solvents, polymers and solutes, and predicts properties of the drying droplet such as the time of shell formation and the size of the particle obtained after drying. A comparison with experimental data from single droplet drying experiments carried out at defined relative vapour saturation and temperature shows that the model can be used to predict shell formation (as well as other properties) for the systems studied here.

Interfacial energy driven distinctive pattern formation during the drying of blood droplets

HypothesisDried blood droplet morphology may potentially serve as an alternative biomarker for several patho-physiological conditions. The deviant properties of the red blood cells and the abnormal composition of diseased samples are hypothesized to manifest through unique cell-cell and cell-substrate interactions leading to different morphological patterns. Identifying distinctive morphological trait from a large sample size and proposing confirmatory explanations are necessary to establish the signatory pattern as a potential biomarker to differentiate healthy and diseased samples. ExperimentsComprehensive experimental investigation was undertaken to identify the signatory dried blood droplet patterns. The corresponding image based analysis was in turn used to differentiate the blood samples with a specific haematological disorder “Thalassaemia” from healthy ones. Relevant theoretical analysis explored the role of cell-surface and cell-cell interactions pertinent to the formation of the distinct dried patterns. FindingsThe differences observed in the dried blood patterns, specifically the radial crack lengths, were found to eventuate from the differences in the overall interaction energies of the system. A first-generation theoretical analysis, with the mean field approximation, also confirmed similar outcome and justified the role of the different physico-chemical properties of red blood cells in diseased samples resulting in shorter radial cracks.

Design and Characterizations of Inhalable Poly(lactic-co-glycolic acid) Microspheres Prepared by the Fine Droplet Drying Process for a Sustained Effect of Salmon Calcitonin.

The present study aimed to develop inhalable poly (lactic-co-glycolic acid) (PLGA)-based microparticles of salmon calcitonin (sCT) for sustained pharmacological action by the fine droplet drying (FDD) process, a novel powderization technique employing printing technologies. PLGA was selected as a biodegradable carrier polymer for sustained-release particles of sCT (sCT/SR), and physicochemical characterizations of sCT/SR were conducted. To estimate the in vivo efficacy of the sCT/SR respirable powder (sCT/SR-RP), plasma calcium levels were measured after intratracheal administration in rats. The particle size of sCT/SR was 3.6 µm, and the SPAN factor, one of the parameters to present the uniformity of particle size distribution, was calculated to be 0.65. In the evaluation of the conformational structure of sCT, no significant changes were observed in sCT/SR even after the FDD process. The drug release from sCT/SR showed a biphasic pattern with an initial burst and slow diffusion in simulated lung fluid. sCT/SR-RP showed fine inhalation performance, as evidenced by a fine particle fraction value of 28% in the cascade impactor analysis. After the insufflation of sCT samples (40 µg-sCT/kg) in rats, sCT/SR-RP could enhance and prolong the hypocalcemic action of sCT possibly due to the sustained release and pulmonary absorption of sCT. From these observations, the strategic application of the FDD process could be efficacious to provide PLGA-based inhalable formulations of sCT, as well as other therapeutic peptides, to enhance their biopharmaceutical potentials.

Vaporization and particle formation during drying of multisolvent droplet without and with antisolvent-vapor infusion

The understanding of the evaporation-drying of multi-solvent-solute containing droplets was advanced with a unique single droplet drying (SDD) approach. Analyzing the drying of dual-solvent droplets revealed two stages of temperature increase and the alteration of solute precipitation behavior, from at droplet surface to at the interior of droplet. When a single-solvent droplet was dried with antisolvent vapor, droplet temperature remained constant along the precipitated solids forming microparticles, despite the simultaneous evaporation of original solvent and antisolvent. Toluene and water droplets during ethanol-vapor-infused drying at 31 °C reached the same constant temperature of 27.5 °C, namely the free-evaporating temperature of ethanol. Mathematical and chromatography analyses showed that the solvent composition kept changing. The changing rate, governed by antisolvent vapor density, could profoundly impact final particle morphology, producing lactose microparticles with five distinct morphologies. The proposed SDD technique enables coupled analyses of drying kinetics and microstructure formation for elucidating complex in-drying mechanisms.

Modeling and simulation of solid-containing droplet drying and different-structure particle formation

The formation of solid and hollow particles from solute precipitation of a liquid droplet was investigated using a simulative approach. The simulation model describes the evolution of the solute concentration, temperature gradient, and size change of the droplet and includes the vapor concentration and temperature gradient in the air surrounding the droplet. The volume of fluid (VOF) method was adopted to capture the gas–liquid–solid interface. The entire drying process was analyzed using a sodium chloride aqueous droplet as an example. The influence of ambient temperature on the final structure of the precipitate particle was studied. Two distinct particle structures were discussed. The droplet solidifies into a solid particle when the ambient temperature is low, and into a hollow shape when the ambient temperature is high. The mechanisms underlying these two particle structures were discussed.

Dextrose equivalence of maltodextrins determines particle morphology development during single sessile droplet drying.

Particle morphology development during spray drying is critical to powder properties. The aim of this study was to investigate whether the dextrose equivalence (DE) of maltodextrins can be used as an indicator for the final particle morphology. Maltodextrins were characterized on glass transition temperature (Tg) and viscosity, where low DE-value maltodextrins exhibited higher Tg and viscosity than high DE maltodextrins (≥21). A new custom-built sessile single droplet dryer was used to analyse morphology development of minute maltodextrin droplets (R0~100μm) at 60°C and 90°C. Droplets with low DE showed early skin formation (2-5s) and developed smoothly shaped particles with large cavities. Rheology on low DE maltodextrin films at dry matter of 82% (w/w) suggested that drying droplets acquired elasticity after locking providing resistance against surface compression. After locking morphology development is probably halted as the glassy state is approached. On the contrary, rheology on high DE maltodextrin (≥21) films at dry matter of 93% (w/w) suggested that drying droplets with high DE developed viscous skins, which are susceptible to surface deformations, leading to wrinkling, folding or creasing particle morphologies. The results demonstrated that DE-value may be used as an indicator for particle morphology development when interpreted in view of the process conditions.

Particle morphology and powder properties during spray drying of maltodextrin and whey protein mixtures

Application behavior of spray dried powders such as reconstitution behavior and flowability is indirectly influenced by powder particle morphology. We here investigated the influence of the drying conditions and composition for whey protein (WP), maltodextrin (MD) and their mixtures on particle morphology during pilot-scale spray drying. Even though all powders showed a variety of morphologies, MD powders contained more wrinkled particles and had high bulk density, whereas pure WP powders contained more hollow particles. Mixture powders (75:25 WP:MD) show more hollow particles with increasing inlet temperature, with a lower bulk density. The observed morphologies for different formulations corresponded roughly to previous observations during sessile single droplet drying, but the results indicate that the morphology is also influenced by the faster pilot-scale drying, which was not evident in the slower single droplet drying.

Size-Separation and Self-Assembly of Anisotropic Nanoparticles in a Coffee-Stain Ring

Nanoparticles deposition via droplet drying technique generally leads to a coffee-stain-like ring. In the case of different shapes and sizes of nanoparticles, such ring-like deposits also demonstrate various phases within the self-assembly superstructures. For broad spectrum sizes in suspension, size-separation effect has not been considered before. Herein, polydisperse gold anisotropic nanoparticles have been prepared via wet chemical synthesis technique. High-resolution scanning electron microscopy (HRSEM) at various positions of a coffee-stain ring displayed both size-separation and self-assembly of nanoparticles. Especially at the outer edges of a ring-like deposit, side-by-side organized arrays of anisotropic nanoparticles have been observed. Likewise, such ordered arrays generally begin with thin nanorods and the width of nanoparticles increases as one moves inward within such arrays. Also, assembly of deposited arrays is more orderly arranged at the periphery of the ring, whereas such order significantly decreases at the interior region of the ring. Furthermore, it has been observed for a mixture of thick–thin nanorods and spheres that only thick nanorods self-assemble into ordered arrays whereas thin rod-sphere was found disordered everywhere within the ring. Nanoscale interactions are quantified for these systems to understand self-assembly behavior of such anisotropic nanoparticles.

Role of trapped water on electroresponsive characteristic of silica-graphene oxide composite microspheres

Electroresponsive microspheres containing nanosilica and graphene oxide (GO) sheets have been achieved by the one-step evaporation induced assembly of mixed colloids using spray drying. The interlocking of the nanostructured constituents by the attractive capillary force during drying of dispersion droplets results in the formation of powder grains with correlated nanoparticles and nanosheets. The presence of GO sheets arrests the buckling of the drying droplets during evaporative assembly and results in well-defined spherical shaped grains. Interestingly, this approach of random jamming in colloidal droplets is capable of preventing the spontaneous stacking of the GO sheets and, therefore, facilitates a way to confine the atomic-thick GO sheet in the solid matrix. A plausible mechanism for the formation of silica-GO binary composite microspheres is elucidated. Microspheres dispersed in insulating oil show interesting electroresponsive characteristics where the microspheres form a chainlike percolation path under the applied electric field. It has been established that the adsorbed water in the microspheres plays a profound role as far its electroresponsive behavior is concerned. GO sheets in the microspheres allow for tuning of electroresponse strength owing to the increased interaction of water molecules to the oxygen-containing functional groups anchored on the GO sheets.

Inkjet Printing of Magnetic Particles Toward Anisotropic Magnetic Properties

Unique properties of one-dimensional assemblies of particles have attracted great attention during the past decades, particularly with respect to the potential for anisotropic magnetism. Patterned films can be created using inkjet printing; however, drying of particle-laden colloidal droplets on solid surfaces is usually accompanied by the well-known coffee-ring effect, deteriorating both the uniformity and resolution of the printed configurations. This study examines the effect of externally applied magnetic field on particle deposition patterns. Ferromagnetic Gd5Si4 particles were formulated in terpineol oil and directly deposited via magnetic field-assisted inkjet printing on a photopaper to generate patterned films with suppressed coffee-ring effect. The particle deposition morphology is determined by both solvent imbibition and particle-magnetic field interactions. Three characteristic times are considered, namely, the critical time for solvent imbibition into the substrate (tim), the time it takes for particles to form chains in the presence of the magnetic field (tch), and the time in which the particles reach the substrate in the direction normal to the substrate (tpz). The characteristic time ratios (tpz/tim) and (tpz/tch) determine the final deposition morphology in the presence of magnetic field. The ability to control particle deposition and assembly, thus tuning the magnetic anisotropic properties of nanostructured materials is a promising approach for many engineering applications.