Droplet Drying Research Articles

Drying Kinetics of Salt Solution Droplets: Water Evaporation Rates and Crystallization.

Drying and crystallization of solution droplets is a problem of broad relevance, determining the microstructures of particles formed in spray-drying, the phase of particles delivered by, for example, aerosol formulations for inhalation therapies, and the impact of aerosols on radiative forcing and climate. The ephemeral nature of free droplets, particularly when considering the drying kinetics of droplets with highly volatile constituents, has often precluded the accurate measurement of transient properties such as droplet size and composition, preventing the robust assessment of predictive models of droplet-drying rates, nucleation, and crystallization. Here, we report novel measurements of the drying kinetics of individual aqueous sodium chloride solution droplets using an electrodynamic balance to isolate and trap single aerosol droplets (radius ≈ 25 μm). The initial solution droplet size and composition are shown to be highly reproducible in terms of drying rate and crystallization time when examined over hundreds of identical evaporating droplets. We introduce a numerical model that determines the concentration gradient across the radial profile of the droplet as it dries, considering both the surface recession because of evaporation and the diffusion of components within the droplet. Drying-induced crystallization is shown to be fully determined for this system, with nucleation and instantaneous crystallization occurring once a critical supersaturation level of 2.04 ± 0.02 is achieved at the surface of the evaporating droplet. This phenomenological model provides a consistent account of the timescale and surface concentration of free-droplet crystallization on drying for the different drying conditions studied, a necessary step in progress toward achieving control over rates of crystallization and the competitive formation of amorphous particles.

Dynamics of Salt Precipitation on Graphene Oxide Membranes

We investigated the drying dynamics and resulting salt deposition patterns from evaporating saline droplets on fabricated graphene oxide (GO) and reduced graphene oxide (rGO) membranes. As evaporation proceeds, the salt concentration increases until it exceeds substantially the solubility limit which results in the formation of salt crystals. Scanning electron microscopy imaging enabled us to investigate how the properties of the fabricated membranes influence the salt deposition patterns and crystal morphology, providing us with an opportunity to control salt deposition patterns on the surface. The observation from the drying of saline droplets on fabricated GO and rGO membranes leads us to conclude that combined effects of the membrane roughness, interlayer spacing, and nanochannel connectivity determines the drying dynamics and the final salt deposition patterns. Our results extend the understanding of the interaction between surface properties and salt deposition patterns, which is relevant informatio...

Co-encapsulation of coenzyme Q10 and vitamin E: A study of microcapsule formation and its relation to structure and functionalities using single droplet drying and micro-fluidic-jet spray drying

The objective of this research was to study the co-encapsulation of Coenzyme Q10 (CoQ10) and vitamin E (VE) using single droplet drying (SDD) and micro-fluidic-jet spray drying (MFJSD), to study the particle formation-structure-function relationships of the microcapsules. Single droplets of selected CoQ10 and VE-containing emulsions were studied for their particle formation under convective drying (at 70 and 90 °C for 10 min) and rehydration processes using the SDD device. The emulsions were also spray-dried (at 160 and 190 °C) by MFJSD to form monodisperse microcapsules, followed by analyses of powder properties and functionalities. Results from SDD showed emulsion droplets with 30% w/w solids content were associated with earlier crust formation (30 s faster) and less particle shrinkage during drying compared to those with 10% w/w solid content. Comparing the spray-dried microcapsules produced from the emulsions containing 30% and 10% w/w solids content, the former gave larger particle size (172–175 μm vs. 116 μm), higher microencapsulation efficiency (ME) (98–99% vs. 96%), lower tapped density (0.532–0.599 g/cm3 vs. 0.406–0.502 g/cm3) and better flow properties (Carr's index of 22.06–29.19% vs. 22.70–40.64%). Overall, the CoQ10 and VE retention, antioxidant capacities and colour of the microcapsules were relatively stable when spray-dried at 190 °C than at 160 °C.

Impact of operating conditions on a single droplet and spray drying of hydroxypropylated pea starch: Process performance and final powder properties

AbstractFor an efficient production of spray‐dried fine powders having controllable properties (size, size distribution, and morphology), finding suitable operating conditions and an appropriate initial composition of the fluid material is important. For this purpose, a suspension device is employed to investigate the drying kinetics of a single droplet of hydroxypropylated pea starch (HPS). In the current work, the effect of the drying air temperature (80–160°C) and of the initial solid content of the agent (15–30%w/w) on the drying kinetics, shrinkage, and locking point of a single droplet was systematically investigated to acquire the optimal conditions required for producing HPS powder using a pilot‐scale spray dryer. In addition to the previously mentioned parameters, the atomization pressure effect (2–3 bars) on the spray‐drying process yield, thermal efficiency, and final powder properties were also considered. A laboratory‐scale X‐ray microtomograph and a camsizer‐XT were employed for the acquisition of three‐dimensional images and surface morphology of single dried particles and spray‐dried powder, respectively. Hollow particles were obtained in all single droplet experiments. The drying kinetic study results were supported by those obtained by the spray drying of HPS. An increased temperature of the drying air and the initial solid content of the solution results in higher powder recovery, lower residual moisture content, and larger particle size. Otherwise, the final particles are larger at a decreased atomization pressure. The optimized parameters for the high spray‐drying process yield (24.54%; 3 bars, 140°C, 25 wt%) and better powder homogeneity (span = 2.31; 3 bars, 140°C, 20 wt%) were defined. A mixture of particle morphologies was observed among the different final powders. A broken shell corresponding to rigid particles was obtained at a drying temperature of 140°C and an initial solid content of 25 wt%. A nonbroken particle refers to a pliable particle that corresponds to inflated and collapsed particles dried at high and low drying temperatures.

A study on the structure formation and properties of noni juice microencapsulated with maltodextrin and gum acacia using single droplet drying

Spray drying-based microencapsulation has been applied to mask unpleasant odours and to protect bioactives in liquid based products. However, little research has been undertaken to explore how particle formation occurs during the drying process and the relationships associated with particle properties. This study aimed to fill the gap by investigating the relation of structure formation and properties of microencapsulated fermented noni juice (FNJ) powder containing maltodextrin (M1, 10–13 DE; M3, 17–20 DE) and gum acacia (GA) using a single droplet drying (SDD) technique, mimicking the spray drying process. The impact of wall materials and solids content on the structure formation and particle properties including solubility, hygroscopicity, particle size, bulk density and microstructure were predicted. The SDD process was designed to observe the drying process and to investigate the morphological changes of semi-dried particles by attaching a water droplet or by suspending the semi-dried/dried particle in the drying chamber without air flow. Microscopic analysis show that microstructures of the particles were influenced by the wall materials. M3 particles showed the best solubility, and are predicted to have higher bulk density and hygroscopicity than M1 and GA particles. The M1 particles displayed good solubility and had a larger particle size than the M3 particles. Particles containing GA had the lowest dissolution rate, and are expected to have the lowest bulk density among the three wall materials. Overall, this research has provided insight into particle structure formation and the properties of microencapsulated FNJ powders by the above hydrocolloid materials.

Photoluminescence of Drying Droplets with Silicon Nanoparticles.

This study is dedicated to the formation of structures during drying of droplets of sols of silicon nanoparticles (SiNPs) in dimethylsulfoxide (DMSO) with a diameter of 1-5 mm on the horizontal glass and mica surfaces. Drying of such droplets with pinning (sticking) of the droplet contact line causes gradual gathering of the SiNPs on its edge with the formation of a thin ring. It has been found that the integral photoluminescence intensity IPL greatly varies during the drying process. At the initial stage, IPL monotonically decreases by several orders of magnitude and then abruptly increases several times at the final stage of ring formation. It has been shown that the rate of IPL decrease is maximal at a very early stage and depends both on the aggregative state (solid film SiNPs/sols of the SiNPs) and volume of the SiNPs sols. It is minimal for the solid film SiNPs and gradually increases as the volume of SiNPs sol in DMSO decreases (optical cell → big droplet → small droplet). The obtained experimental dependencies between the luminescence decrease rate and aggregative state and volumes of the SiNPs sol in DMSO are attributed to the combination of three mechanisms of luminescence quenching: photobleaching, quenching with atmospheric oxygen, and Förster resonance energy transfer quenching. The appearing of the luminescence leap at the final stage of ring formation is associated with the emergence of cracks in the ring.

Drying kinetics and particle formation of potato powder during spray drying probed by microrheology and single droplet drying

Drying kinetics and particle formation of potato powder during spray drying probed by microrheology and single droplet drying

In situ crystallization kinetics and behavior of mannitol during droplet drying

In this paper, we explain the crystallization process of mannitol during convective droplet drying based on the crystallization kinetics calculated from two mathematical models coupled with experimental investigations. A novel differential-reaction engineering approach was developed, correlating the mannitol crystallization behavior to the deviation of droplet drying kinetics at different drying temperatures. The model was compared with a conventional glass transition-based model and the evolutions of droplet saturation state, density and morphology during drying were experimentally determined, to provide a comprehensive analysis on the crystallization of mannitol as droplet drying progressed. Two crystallization stages were identified. Mannitol solids firstly nucleated and precipitated at droplet surface, and the crystallization kinetics were similar at different drying temperatures (70, 90 and 110 °C). The removal of residual moisture at the second stage was dependent on the drying temperature, with lower temperatures of 70 and 90 °C exhibiting an extended crystallization period associated with slow release of moisture. The morphological and polymorphism analyses on dried mannitol particles suggested a possible growth of stable or metastable polymorphs during the prolonged crystallization event. The results for the first time unveil the unique in-drying crystallization behavior of mannitol droplets, providing fundamental information needed in industrial spray drying for controlling the crystal form of mannitol-based drugs. The proposed mathematical and experimental approaches can be utilized in analyzing other crystallization-prone materials.

(Invited) Drying Patterns from Nanofluid Droplet Evaporation

Droplet evaporation has received more attention for the applications including concentrating chemicals, coating, printing, spraying, drying, painting, biomedical sampling, etc. Deposition pattern form the evaporative colloidal droplet has many applications in the industrial applications. The process to form the configuration in the drying droplet has been investigated with simulation and experiments. Experimentally, the suspensions of colloidal droplet with the particle size at nano- and micro-scale nanoparticles in the deionized water were prepared. The suspensions are carried out by well stirring first and then putting in an ultrasonication bath to ensure the even initial dispersion of the particles in the water. The clean silicon wafer was used as a substrate for the colloidal sessile droplet drying. Right after the suspensions were prepared, we placed a tiny sessile droplet to dry in the open conditions. The drying patterns were directly recorded with the optical microscope. The formation of branched particle aggregation structures was observed during the droplet evaporation. The experiments demonstrated the dried patterns such as finger, coffee ring, uniform coverage and the combined structures. We have applied the kinetic Monte Carlo model and the diffusion-limited aggregation simulation to explain the self-assemble processes. A lattice-gas Kinetic Monte Carlo (KMC) is developed to explain the of the finger structure while the diffusion-limited aggregation simulation is development to explain the transition from the coffee ring to uniform coverage. The mathematical models control the variables which indicate the presence of liquid, particle or substrate in cells of the cubical lattice. The simulation domain mimics the shape of a sessile nanofluid droplet. The particle, liquid, vapor interaction, and the other factors are considered. The experiments can be reflected by the simulations.

A differential shrinkage approach for evaluating particle formation behavior during drying of sucrose, lactose, mannitol, skim milk, and other solid-containing droplets

Particle formation process during drying of solid-containing droplets exerts profound influence on the property and quality of dried particles. A differential shrinkage approach is proposed to describe the particle formation behavior of different materials, by evaluating how the droplet shrinkage kinetics of the given material(s) deviates from the ideal shrinkage. Sucrose was selected as the reference material to establish ideal shrinkage kinetics, as its shrinkage at 10, 30, and 50 wt.% initial concentrations well followed the ideal shrinkage at all temperatures tested (70, 90, and 110 °C). Comparing the differential shrinkage kinetics of sucrose, lactose, and mannitol showed that mannitol has a strong crust-forming tendency, which could not be explained by the difference in the solubility of the three materials. By establishing a differential shrinkage curve for each material, this approach offers a straightforward and powerful method to evaluate the particle formation property of the material at various initial concentrations.

Effect of Gelation on the Colloidal Deposition of Cellulose Nanocrystal Films.

One of the most important aspects in controlling colloidal deposition is manipulating the homogeneity of the deposit by avoiding the coffee-ring effect caused by capillary flow inside the droplet during drying. After our previous work where we achieved homogeneous deposition of cellulose nanocrystals (CNCs) from a colloidal suspension by reinforcing Marangoni flow over the internal capillary flow (Gençer et al. Langmuir 2017, 33 (1), 228-234), we now set out to reduce the importance of capillary flow inside a drying droplet by inducing gelation. In this paper, we discuss the effect of gelation on the deposition pattern and on the self-assembly of CNCs during droplet drying. CNC films were obtained by drop casting CNC suspensions containing NaCl and CaCl2 salts. A mixed methodology using rheological and depolarized dynamic light scattering was applied to understand the colloidal behavior of the CNCs. In addition, analysis of the mixture's surface tension, viscosity, and yield stress of the suspensions were used to gain deeper insights into the deposition process. Finally, the understanding of the gelation behavior in the drying droplet was used to exert control over the deposit where the coffee-ring deposit can be converted to a dome-shaped deposit.

Morphological transformations during drying of surfactant-nanofluid droplets

The effect of surfactants with different chain length on the drying dynamics of nanosized dispersion droplets and on the final morphology of the grains formed after water evaporation is investigated experimentally. An acoustic levitator was used to examine the drying dynamics of single droplets and SEM imaging was used to characterise the morphology of the final dried grains. Results show that the drying of drops with high molecular weight surfactants leads to more irregular grains and that the grain morphology is related to surface tension driven instability of the evaporating droplets which may lead to formation of hollow dried grains.

Gaining insight on spray drying behavior of foods via single droplet drying analyses

A continuous challenge for spray drying operations is the optimal control of product quality despite the complex process removal of water and particle formation. In general, high product functionality (e.g. in terms of reconstitution behavior, high enzyme activity or appropriate living probiotic bacteria) is key to the success of spray-dried powders. In this article, we review scientific studies that employ single droplet drying approaches to unravel underlying phenomena of spray drying process. Moreover, we identify scientific challenges to advance single droplet drying studies and thus contribute to development of mechanism-based guidelines for spray drying of functional food powders.

Heat stability of Lactobacillus rhamnosus GG and its cellular membrane during droplet drying and heat treatment

Dehydration and thermal stresses are generally considered as two mains factors deactivating probiotic cells during droplet drying, as typically in industrial spray drying for producing active dry probiotics. However, little is known about how cells respond to these interplaying stresses in the short period of drying. This study showed that dehydration process could alleviate the detrimental effect of thermal stress to a certain extent, evidenced by that probiotic cells could withstand higher temperature in a single droplet drying (SDD) process compared to sole heat treatment. During SDD at 90 °C, droplet temperature increased with time, and the inactivation of Lactobacillus rhamnosus GG (LGG) was initially observed at droplet temperature of 61–65 °C. By contrast, the transition from the maintenance of LGG viability to rapid deactivation occurred at around 54 °C in heat treatment without dynamic dehydration. Possible mechanisms for the enhanced thermotolerance were investigated from drying kinetics level and cellular level. The favorable temperature profile and the decrease in droplet water activity during drying may benefit cell survival. The cytoplasmic membrane of LGG was more stable at elevated temperatures of 60–65 °C during drying, which might be related to the high viscosity of semi-dried particles mitigating the leakage of intracellular substances. Trehalose demonstrated a strong thermoprotective effect over lactose in heat treatment, but the protection was less effective at the later stage of drying. These results dissected the influence of the interplaying stresses on probiotic cells for the first time during droplet drying and also suggested possible approaches for improving cell survival in dried particles. Components capable of protecting cellular membrane are recommended for developing protectant formulation in spray drying of probiotics.

Preparation and optimization of a dry powder for inhalation of second-line anti-tuberculosis drugs

A spray drying process was standardized to prepare an inhalable powder comprising d-cycloserine and ethionamide, two “second line” drugs employed for treating multi-drug resistant (MDR) tuberculosis (TB). The aim of the process development effort was to maximize product yield. Contour plots were generated using a small central composite design (CCD) with face centered (α = 1) to maximize the process yield as the response criterion. The design space was experimentally validated. Powder was prepared and characterized for drug content (HPLC), geometric size (laser scattering), surface morphology (scanning electron microscopy) aerosol behaviour (cascade impaction) and powder flow properties. The optimized process yielded a powder with a median mass aerodynamic diameter (MMAD) of 1.76 µ ± 3.1 geometric standard deviation (GSD). Mass balance indicated that the major proportion of the particles produced by spray drying are lost to the outlet filter. The process represents a best-case compromise of spray-drying conditions to minimize loss during droplet drying, collection and process air discharge.

Polymer solution electrospraying: A tool for engineering particles and films with controlled morphology

Polymer solution electrospray is a relatively recent research area which has become increasingly active due to its potential for engineering particle morphologies at the micro- and nanoscales, which can be useful for pharmaceutical applications, energy devices, and other fields. Despite significant efforts in this research area, and the fact that electrospray (ES) theory for pure solvents is well-established, improved understanding is still needed on how polymeric structures form during electrospray, to be able to predict the diversity of particle morphologies encountered. This situation is due partly to the complexity of the mechanisms involved, which combine the rich behavior of polymer solutions with the unique physics of ES (with disparate temporal and spatial scales). In this article, we start by reviewing which application fields have driven research in polymer solution electrospraying, and which morphologies matter. We then develop a theoretical framework on the mechanisms responsible for the transformation of a polymer solution droplet made by ES into a solid morphology. We discuss the applicability of a droplet drying model used in Spray Drying to the case of electrospray droplets. We further highlight the importance of Coulombic instabilities in developing polymeric structures. These structures include particles with pointed ends, sometimes with long nanofilaments, as well as other elongated shapes, and globular shapes with a broad size distribution. We identify the conditions necessary to prepare monodisperse globular particles (e.g. spheres). Finally, we analyze the roles played by the presence of nonsolvent vapors during droplet drying. This includes ambient humidity when electrospraying non water-soluble polymers, whose effects on particle porosity have often been overlooked. We close with recommendations and clarifications of practical importance for practitioners.

Morphology development during single droplet drying of mixed component formulations and milk

We report on the influence of selected components and their mixtures on the development of the morphology during drying of single droplets and extend the results to the morphology of whole milk powder particles. Sessile single droplet drying and acoustic levitation methods were employed to study single droplet drying. The influence of carbohydrates (lactose and maltodextrin DE12) and proteins (micellar casein or whey protein) on morphology development is very different, since upon concentration protein systems will jam and undergo a colloidal glass transition, whereas carbohydrate systems will gradually increase in viscosity as a consequence of the concentration. Whey protein gives relatively rigid shells due to jamming of the “hard sphere” proteins, while casein micelles behave as “soft spheres” that can deform after jamming, which gives flexibility to the shell during drying. The influence of the carbohydrates on the final morphology was found much larger than the influence of the proteins. Caseins influenced morphology only in mixtures with lactose at higher concentrations due to its high voluminosity. Similar observations were done for whole milk, where fat appeared to have no influence. With maltodextrin the influence of the casein was again observed in the shape and smoothness of wrinkles. Both sessile and levitated droplet drying methods provide a similar and consistent view on morphology development.

Correlation of the kinetics of aggregation and inactivation of L-glutamate dehyrogenase during drying and particle formation of a levitated microdroplet

ABSTRACTMeasurement of single droplet drying in the acoustic levitator has been used to detect a correlation between the kinetics of aggregation and inactivation of the model protein l-glutamate dehydrogenase (GDH) during drying and particle formation. The droplet drying was performed at one set of conditions: drying gas temperature = 60°C; drying gas relative humidity = 1%, and drying gas velocity = 0.5 L/min. A centrifugation technique was used to remove insoluble aggregates from the rehydrated microparticles removed from the levitator chamber. Size exclusion chromatography could then be used to quantify the contents of nonaggregated native protein. During the constant-rate drying period up to the critical point of drying, there was retention of nonaggregated native structure. Only after the critical point of drying has been reached, a rapid decline in nonaggregated native content was observed owing to formation of insoluble aggregates. No sign of a soluble trimer of GDH was found. The kinetics of this behavior correlates closely with the kinetics of inactivation of the same enzyme in levitated microdroplets from the literature, i.e., retention of activity up to the critical point and rapid loss of activity after the critical point. The novelty of this work is that such a correlation between aggregation and inactivation can be detected during drying and particle formation on both sides of the critical point of drying.

Engineering Interfacial Processes at Mini-Micro-Nano Scales Using Sessile Droplet Architecture.

Evaporating sessile functional droplets act as the fundamental building block that controls the cumulative outcome of many industrial and biological applications such as surface patterning, 3D printing, photonic crystals, and DNA sequencing, to name a few. Additionally, a drying single sessile droplet forms a high-throughput processing technique using low material volume which is especially suitable for medical diagnosis. A sessile droplet also provides an elementary platform to study and analyze fundamental interfacial processes at various length scales ranging from macroscopically observable wetting and evaporation to microfluidic transport to interparticle forces operating at a nanometric length scale. As an example, to ascertain the quality of 3D printing we must understand the fundamental interfacial processes at the droplet scale. In this article, we review the coupled physics of evaporation flow-contact-line-driven particle transport in sessile colloidal droplets and provide methodologies to control the same. Through natural alterations in droplet vaporization, one can change the evaporative pattern and contact line dynamics leading to internal flow which will modulate the final particle assembly in a nontrivial fashion. We further show that control over particle transport can also be exerted by external stimuli which can be thermal, mechanical oscillations, vapor confinement (walled or a fellow droplet), or chemical (surfactant-induced) in nature. For example, significant augmentation of an otherwise evaporation-driven particle transport in sessile droplets can be brought about simply through controlled interfacial oscillations. The ability to control the final morphologies by manipulating the governing interfacial mechanisms in the precursor stages of droplet drying makes it perfectly suitable for fabrication-, mixing-, and diagnostic-based applications.

On the particle formation in spray drying process for bio-pharmaceutical applications: Interrogating a new model via computational fluid dynamics

Although spray drying is widely used to produce active pharmaceutical ingredients, its operation and improvement is often difficult to assess because of complex interactions between the chemical and physical properties of the solutions, and the ensuing fluid dynamic, and heat and mass transfer phenomena occurring during spray drying. Hence, computational fluid dynamics (CFD) modeling is used to provide insight into spray drying to produce pharmaceutical solid amorphous dispersions, the results of which are compared to experimental data from the modelled spray dryer during its operation. It is shown that droplets with different sizes, trajectories and breakup behaviors occur and cause droplet drying and solid powder formation differences that affect product characteristics. Based on the combined CFD and experimental information, a new drying approach is proposed and tested that is based on the precipitation of dissolved solid species and the introduction of swirl motion of the droplets during ejection from a spray nozzle. This new approach is shown to improve drying performance of difficult-to-handle pharmaceutical solutions.