The growth of instabilities and the subsequent break-up of liquid sheets have been widely studied to understand nozzle atomisation. Two different theoretical approaches have been proposed to explain the spatial growth of instabilities. The first is based on the aerodynamic interactions between the liquid sheet and the surrounding air, in which the sheet becomes unstable to infinitesimal perturbations, similar to the Kelvin-Helmholtz instability. In the absence of aerodynamic interactions, the theory predicts zero spatial growth. An alternate theory ignores the surrounding air and instead relies on the thinning of the liquid sheet to predict spatial growth. Here, we rederive the governing equations for the thinning theory and show that when a small vertical displacement (or perturbation) with zero slope is introduced at the centre of the liquid sheet, the perturbation is convected downstream with finite amplification, but the perturbation does not diverge at the edge of the sheet. On the other hand, a perturbation with a finite slope diverges. We compare the model predictions with displacement measurements over a range of Weber numbers and geometries to assess the model's robustness. We demonstrate that low-frequency perturbations, in the absence of aerodynamic interactions, such as noise from the experimental set-up or impact waves generated at the point of impingement, can explain the spatial growth, particularly in the low-Weber-number regime. The theory predicts growth rates close to the measured values at low Weber numbers, where aerodynamic interactions are negligible.

Abstract The morphology of spray-dried granules critically influences their performance, with applications requiring either hollow shells or dense particles. We investigated shell formation and deformation in spray drying using suspensions of silica particles and polymers. In monodisperse silica systems, buckling was driven by capillary stresses, with the critical buckling size determined by particle radius and shell modulus. Adding smaller particles increased packing density and shell stiffness, but the accompanying rise in capillary pressure still promoted buckling. Replacing small particles with polymers fundamentally altered the deformation mechanism. Poly(vinylpyrrolidone) lowered the effective shell modulus, producing hollow granules with multiple buckling sites, whereas maltodextrin formed a dense surface layer, resulting in dented and blow-holed granules. The results demonstrate a transition from capillary-driven shell buckling to polymer shrinkage and viscoelastic deformation, providing design principles for tailoring granule porosity, mechanical integrity, and internal architecture.

Drying films of colloidal dispersion containing hard particles crack while fast drying drops of similar dispersion buckle, both caused by the capillary pressure exerted by the liquid menisci between particles. For drying films on substrates, there exists a maximum crack-free thickness while for fast drying drops, there is a maximum buckle-free shell size, with both quantities depending on particle size, elastic modulus of the particles and nature of particle packing. Here, we measure the critical cracking thickness for a drying colloidal film made of a mixture of hard and soft elastic particles to extract the effective modulus of the film for various ratios of hard and soft particles. Scanning electron images of the cross section of the film reveal the spatial distribution of the hard and soft particles. The measured effective modulus exhibits a trend that is identical to that obtained using the nanoindentation technique indicating that measurement of the critical cracking thickness gives a quantitative measure of the effective modulus of the packing. The values of effective modulus from the two techniques fall between the two limits obtained from the simple rule of mixtures for composite materials. The deviation from either limit is attributed to particle segregation caused by sedimentation of the heavier particles.

In early 2022, the European Union delisted titanium dioxide (TiO2) from the list of approved food additives, creating a need to explore potential alternatives to titanium dioxide. Given the concerns regarding TiO2 and the possibility of similar restrictions to their use in tablet coatings, there is a need to evaluate alternatives to TiO2 opacifiers in tablet coatings. Specifically, the alternative opacifiers are expected to influence the mechanical and adhesion properties of tablet coatings. The current study performs a comprehensive evaluation of six different commercially available titanium dioxide-free (TF) tablet coatings. The TF coatings contain calcium carbonate, rice starch and magnesium carbonate as opacifiers, with polyvinyl alcohol (PVA) or hydroxypropyl methylcellulose (HPMC) as the base polymers. Detailed measurements of drying stress, mechanical strength, and adhesion testing were conducted to evaluate their suitability as pharmaceutical coatings. The TF coatings resulted in drying stresses that were comparable to those observed for TiO2 coatings. However, the tensile strength and Young's modulus of all TF coatings, except one, were observed to be lower, indicating a relatively poor mechanical strength. Adhesive strength of TF coatings was similar to that of TiO2 coatings, with waxy tablets exhibiting lower adhesion strength due to higher hydrophobicity. In summary, the HPMC-HPC-based TF coating showed superior mechanical strength and comparable adhesive properties when compared to TiO2 coatings. This positions it as a viable alternative to TiO2 among the coatings tested. In contrast, all other TF coatings did not achieve the performance levels of the TiO2 coatings.

Controlled drug delivery using bilayer osmotic tablets has been used to treat many critical diseases like diabetes, hypertension, chronic pain, high cholesterol, chest pain, depression and anxiety disorders. Despite their commercial success, these systems can exhibit high residual (more than 5% unreleased drug) for drug loading beyond 20%- the reasons for which are not well understood. Historically, a drug overage may have been added to account for the residual. However, overages are discouraged and must be justified in terms of the drug product's safety and efficacy based on ICH Q8(R2) guidelines. Therefore, reducing residual through improved design is the preferred approach. Here, we report experiments using glass beads as the model drug particle to understand the release behavior at higher loadings. Experiments are performed for loadings ranging from 10%-50% and varying particle size, membrane thickness, and orifice size to understand the influence of these factors on the release mechanism. Apart from the release profile, the release of the osmotic agent from the tablets was determined by measuring the conductivity of the bath solution. A special setup was designed to determine extrudate composition at each time point during the release process. These measurements were complemented with rheological measurements to estimate the extrudate viscosity. The measurements show that there is a preferential release of polymer (over drug particles), indicating that the high molecular polymer in the sweller layer not only swells to extrude the drug layer but may also play an important role in entraining the drug particles while itself getting extruded out of the orifice. A complete release was observed at all glass bead loadings, indicating that the extrudate viscosity is large enough to prevent settling. These results suggest that one plausible reason for the complete release of glass beads may be their inert nature. The incomplete release observed for real drug particles may be then attributed to the possible agglomeration that prevents them from getting extruded out of the orifice. Further investigation would be needed with real drug particles to understand the mechanism completely.

An understanding of the morphological transformations of drying, particle-laden drops, is important for industrial processes such as spray drying, where drops of particulate suspensions dry rapidly to produce granules. The high drying rates during spray drying produce particle-packed shells saturated with liquid, which may buckle, transforming spherical shells into crumpled granules. The morphology of granules depends on the particle's mechanical properties and size, interparticle interaction, and drying rates. A recent theory has shown that the morphological transformations are controlled by a dimensionless parameter that measures the competition between the compressive stress generated by capillary forces and the elastic strength of the packing. In this work, we perform experiments on a spray dryer with suspensions containing particles of varying sizes and moduli to test the theoretical predictions. We show remarkable agreement with the theory over many orders of magnitude of the dimensionless parameter. The results provide a fundamental understanding of the morphological changes observed in fast-drying drops containing particles and enable the design of buckle-free granules with desired properties.

In the aftermath of the COVID-19 pandemic, airborne transmission has been identified as a significant factor in disease spread. However, there have been very few direct comparisons of virus viability in airborne droplets versus those deposited on surfaces or fomites. This study compares the viability of the enveloped Phi6 virus and two non-enveloped viruses (T4 and MS2) in droplets on hydrophobic and hydrophilic surfaces at 25°C and 45%–55% relative humidity. The former results in spherical droplets similar to airborne droplets, while the latter pertains to spreading droplets comparable to a fomite state. Our research highlights the influence of various physical factors of the carrier droplet—such as its shape and size—as well as the type and concentration of the virus on its viability during the drying process. We found that, at a fixed volume, virus viability decreased with droplet size in spherical droplets, while high initial viral concentrations (~ 10^5 pfu/μL) improved survival on fomites. This suggests that fomites from individuals with high viral loads pose a greater risk of infection. Overall, droplets of a specific volume are more viable in the air than on surfaces. Smaller airborne droplets may have decreased viability but can linger longer and penetrate deeper into the respiratory tract. When viral loads are high, its comparable persistence in both spherical and flat droplets increases the risk of fomite transmission.

Polymer films coated on soft substrates find applications in diverse areas. Understanding the mechanical behavior of the film-substrate composite is important for achieving films that can sustain large strains without failure. To this end, we perform tensile stress experiments for thin films of a model polymer, ethyl cellulose, coated on a soft substrate, polydimethylsiloxane. Drying of wet polymer films in high humidity environment results in porous films that are opaque in appearance while those dried in low humidity conditions lead to transparent films without pores. The porous films show lower tensile strength compared to the transparent films. While the tensile strength of the opaque films was invariant with film thickness, the strength of the transparent film increased with decreasing film thickness. We explain the observations in terms of the Griffith’s fracture criteria wherein flaw size, proportional to the thickness of the film, sets the tensile strength. The results connect the polymer film drying process and microstructure evolution with its mechanical properties and fracture behavior.

The COVID-19 pandemic that afflicted the world recently has renewed the focus on transmission of respiratory diseases via aerosol route. An important question in this regard pertains to the size range of the droplets carrying virus that is most relevant to the transfer of pathogens released from an infected person in the course of respiratory activities, such as coughing, sneezing, speaking and breathing. The emitted droplets undergo rapid drying to form residues of nonvolatile solutes contained in the respiratory fluids. The residues may remain airborne or may settle on ground. The shrinkage ratio of the droplets have been investigated extensively to arrive at the airborne particle size range assuming typical solute concentrations in the saliva. In order to obtain a clearer understanding of the shrinkage process, it is important to examine the problem across a range of solute concentrations and test the measured shrinkage ratios against those predicted by mathematical models. From the perspective of residence times of a drying droplet in an enclosed space as well as for lung deposition, the aerodynamic diameters of the particles, rather than their physical diameters are a matter of significance, and the aerodynamic diameter is a sensitive function of the effective densities. The present work investigates both shrinkage ratios and effective densities of residue particles formed from saline solutions across a range of solute concentrations and compares the results with mathematical model of droplet drying and residue formation. The model is primarily tested for NaCl, which is most relevant to respiratory droplets, and as a part of its wider applicability, both experiments and models are compared for glucose as well. The experiments consist of subjecting the droplets placed on super-hydrophobic surfaces to evaporative drying under controlled ambient conditions at various solute concentrations. The mathematical model is based on solving the heat and mass transfer equations for drying of droplets, solute diffusion for build-up of concentration profiles and a critical supersaturationbased-nucleation model for crust formation leading to residues. The measured shrinkage ratios varied from 0.16 to 0.56 for NaCl across concentration of 2-80 kg/m3 and were in the range of 0.13-0.43 for glucose across the same range of concentrations. The model predictions agreed remarkably well with the experimental results leading closely to a concise formula: shrinkage ratio, SR = (C/rho eff)1/3, where, the effective density of the residue particle rho eff= 459 m3kg regardless of the concentration C kg for NaCl in the fluid. A significant aspect of the study has been the close agreement between model predictions and measured shrinkage ratios. Interestingly, both measurements and model predictions suggest the formation of hollow particles with effective densities (independent of solute concentrations) well below the crystalline solid densities of the solutes. The implications of these results are discussed.

We describe a unique method to measure the viscosity of liquids based on the fluid mechanics of thin films. The technique requires only a few drops of the sample and can measure accurately low viscosities.

Waterborne coatings with intrinsic antibacterial attributes have attracted significant attention due to their potential in mitigating microbial contamination while simultaneously addressing the environmental drawbacks of their solvent-based counterparts. Typically, antimicrobial coatings are designed to resist and eliminate microbial threats, encompassing challenges such as biofilm formation, fungal contamination, and proliferation of black mold. Iodine, when solubilized using ethylene glycol and incorporated as a complex into waterborne latex dispersions, has shown remarkable antimicrobial activity. Here, we demonstrate the effect of the film formation process of these iodinated latex dispersions on their antimicrobial properties. The effect of iodine on the surface morphology and mechanical, adhesion, and antimicrobial properties of the generated films was investigated. Complete integration and uniform distribution of iodine in the films were confirmed through UV-vis spectrophotometry and a laser Raman imaging system (LRIS). In terms of properties, iodinated films showed improved mechanical strength and adhesion compared with blank films. Further, the presence of iodine rendered the films rougher, making them susceptible to bacterial adhesion, but interestingly provided enhanced antibiofilm activity. Moreover, thicker films had a lower surface roughness and reduced biofilm growth. These observations are elucidated through the complex interplay among film thickness, surface morphology, and iodine properties. The insights into the interlink between the film formation process and antimicrobial properties of iodinated latex dispersions will facilitate their enhanced application as sustainable alternatives to solvent-based coatings.

A single-layer osmotic controlled-release tablet, also referred to as an extrudable core system (ECS) tablet, has been modelled to predict the drug release rate as function of various parameters related to the excipients and the active pharmaceutical ingredient, the geometry of the tablet and the coating thickness. The analysis leads to a better understanding of the drug release process resulting in a mathematical tool for design of new formulations. The model accounts for all the main events occurring during the drug release process from a tablet coated with a semi-permeable membrane, namely, the solvent influx driven by the difference in osmotic pressure across the coating, dispersion of core components (drug, polymer and osmogen), swelling of the tablet due to solvent accumulation, build-up of hydrostatic pressure inside the tablet, tensile stress acting on the coating, the extrusion of the dispersed core components and dissolution of drug particles outside the tablet in the bulk. We also derive the condition for successful entrainment of drug particles based on the viscosity of the hydrated phase and other parameters of the system. The model was validated by comparing the predictions with drug release data for two drugs of varying solubility. The agreement between the predicted release and the measurements confirmed the suitability of the model in describing the drug release process in the osmotic controlled-release tablet.

Polymer coatings are used for a number of applications such as for decorative purposes, to protect surfaces and as functional parts of devices. The mechanical integrity of the coatings is critical to their function and hence it is important that the coatings do not fail during their lifetime. Here, we present a simple model to determine conditions under which drying films of polymer solutions can crack. The model accounts for the properties of the polymer film and substrate and predicts the tensile stress developed in the drying film. As the tensile stress increases and exceeds a critical value, the film relaxes by nucleating a crack. The model predicts a critical thickness below which the film does not crack. The predicted critical cracking thickness is compared with experiments performed on drying films of silicone resin on six different substrates with the value of Young's modulus spanning over six decades. The predicted trend matches the measurements.

In a recent comment, Mehring and Gyurkovich show that in our derivation of the time-dependent equations, we have neglected the perturbed term in the interface location while applying the kinematic boundary condition. When the neglected term is incorporated in the analysis, the time-dependent equation for the sinuous mode is different from our original expression. They conclude that the instability predicted by the original model does not exist for their new model. We show that, on the contrary, the new model predicts spatial growth of sinuous disturbance that depends on forcing frequency and the slope of the forcing disturbance.

With advancements in architectural coatings, the demand for antimicrobial coatings in this sector has increased significantly. The COVID-19 pandemic has also been a primary force behind the increased demand for and production of antimicrobial coatings. Typically, antimicrobial coatings are used to resist and decolonize microbial attacks such as biofilm formation, fungal contamination, and black mold formation. In this study, we synthesized a water-based antimicrobial polymer nanocomposite, I-P(MMA/BA), by blending an iodine complex with a poly (methyl methacrylate-butyl acrylate) latex with the aid of polyol and polyvinylpyrrolidone. Antimicrobial efficacy was evaluated in two habitats, namely planktonic and biofilm. Biomass studies indicated that iodinated latex nanocomposites and films show excellent antibacterial and antibiofilm activities. About 85–97

Fast evaporation of particle-suspension drops results in complex morphologies of the final dried granules. Understanding the morphological transformations is important to industrial processes such as spray drying where droplets of particulate suspensions are dried at a fast rate to produce granules of thermally sensitive materials. The transformation of an initial spherical shell to complex morphologies of the final dried granule has been attributed to the buckling of particle-packed shells. Here, we demonstrate a universal scaling law for buckling that depends on the particle size, hardness, particle packing and size of drying drop. The critical transition for buckling is set by a dimensionless number that measures the competition between the compressive stress generated by capillary forces and the elastic strength of the packing. The same dimensionless number is also responsible for cracking of drying colloidal films, suggesting a universality in the mechanical behaviour of particle packings saturated with a solvent. These results should enable design of hierarchically structured, buckle-free granules with varying porosity, surface composition and internal structure.

Paints and Coatings are ubiquitous with wide ranging applications in architectural and construction, aerospace, automotive, electronic, food, and the pharmaceutical industries. The manufacture and storage of paints, their application on a substrate, and the film formation process all involve fluid flow whose understanding and control is important for achieving the desired finish. Within this context, this special issue presents developments in advanced computational models, experiments, and analysis related to the various stages of paint formulation and their applications.

Polymer coatings find use in a wide range of industrial applications, from conventional paints and coatings in building and construction to the pharmaceutical industry, organic solar cell production, and lithium battery technology. Despite their importance, there are gaps in our understanding of the drying process, the stress development during drying, and their influence on the final mechanical properties of the dried film. This perspective focuses on the fundamental aspects of the drying and film formation process, highlights the gaps, and suggests directions for future work.

Immediate-release film coatings, also known as "non-functional" film coating, are applied to core tablets to improve product appearance and swallowability, impart taste-masking properties, improve handling and stability of the dosage form, and reduce exposure to active drug substance for caregivers. The coatings have no measurable impact on bio-performance of the drug product but they protect tablets from negative effects of environment such as humidity, oxidation, and light. The mechanical stability and integrity of tablet coatings are therefore important to maintain drug product quality attributes such as appearance and stability. Therefore, environmental conditions under which these coatings may crack are important to understand so as to prevent their occurrence. In this work, we present a novel computational framework to assess the mechanical integrity of tablet coatings exposed to rapid variations in environmental conditions. We perform detailed stress and strain analysis of tablet coatings on tablet surfaces with debossed regions and identify conditions for cracking. Coatings with both elastic and viscoelastic properties are considered. Rapid changes in environmental temperature and humidity can cause differential expansion/contraction of coating and tablet core resulting in stresses that are higher than those experienced during the drying process in a coater. Debossed regions on the tablet surface with sharp surface curvatures act as stress concentrators that nucleate cracks. Small changes in the design of the debossed regions lead to modest reductions in the peak stress. Stress calculations show that coatings that are well bonded to tablet surface can crack only under very extreme conditions.