On the spray drying of uniform functional microparticles

Spray drying is a typical method to produce particles in dry powder forms at industrial scale. Most spray-dried products often show a wide range of particle properties even within the same batch. At Monash University, we utilise a microfluidic spray drying approach to generate uniform microparticles with tightly controlled characteristics and sizes in a scalable, almost waste-free process. The technique is useful to correlate the effects of formulation and spray drying conditions on the properties of spray-dried particles, and can be used to test new formulations for targeted applications such as encapsulation and release of active ingredients. The synthesis route can be applied to other self-assembling systems, including mesoporous, crystalline, and hierarchically structured microparticles. As spray drying is commonly used in commercial scales, the understanding of how functional particles are formed in relation to formulations and process conditions could assist in developing a cost effective, energy and material-efficient route to produce powders with better properties and ease of handling for more advanced applications such as selective adsorption and bio-separation.     

Numerical investigation of droplet pre-dispersion in a monodisperse droplet spray dryer

Monodisperse droplet spray dryers have great advantages in particle formation through spray drying because of their ability to produce uniform sized particles. Experimental analyses of this system have shown that droplets atomized through the piezoceramic nozzle need to be sufficiently well dispersed before entering the drying chamber to achieve sufficiently dried particles. However, the dispersion dynamics cannot be readily observed because of experimental limitations, and key factors influencing the dispersion state currently remain unclear. This study carried out numerical simulations for droplet dispersions in the dispersion chamber, which allow this important process to be visualized. The systematic and quantitative analyses on the dispersion states provide valuable data for improving the design of the dispersion chamber, and optimizing the spray drying operation.     

Scalable gas-phase processes to create nanostructured particles

The properties of nanoparticles are often different from those of larger grains of the same solid material because of their very large specific surface area. This enables many novel applications, but properties such as agglomeration can also hinder their potential use. By creating nanostructured particles one can take optimum benefit from the desired properties while minimizing the adverse effects. We aim at developing high-precision routes for scalable production of nanostructured particles. Two gas-phase synthesis routes are explored. The first one - covering nanoparticles with a continuous layer - is carried out using atomic layer deposition in a fluidized bed. Through fluidization, the full surface area of the nanoparticles becomes available. With this process, particles can be coated with an ultra-thin film of constant and well-tunable thickness. For the second route - attaching nanoparticles to larger particles - a novel approach using electrostatic forces is demonstrated. The micron-sized particles are charged with one polarity using tribocharging. Using electrospraying, a spray of charged nanoparticles with opposite polarity is generated. Their charge prevents agglomeration, while it enhances efficient deposition at the surface of the host particle. While the proposed processes offer good potential for scale-up, further work is needed to realize large-scale processes.     

Surface structuring of case hardened chain pins by cold-sprayed microparticles to modify friction and wear properties

This paper presents the results of the application of a cold spray technique for structuring metallic surfaces with microparticles.The resulting changes in surface properties were characterized to observe their influences on the tribological behavior of the structured surface.The spray technique was applied to a technical component,a 16MnCr5 steel chain pin,designed to be mounted in a linear reciprocating tribometer.TiO_2 microparticles were used to structure the surface with a homogeneous distribution of singly dispersed particles,rather than a homogeneous closed coating on the surface.Tribometer tests were performed to directly compare structured and unstructured chain pins,and a significantly reduced sliding friction coefficient was observed for the structured pin.The pins were characterized in detail by surface analysis prior to and after application of the tribological load to set the surface parameters and surface chemistry,even on the microscale.It was confirmed that the particle structuring induced changes in the surface properties,and the durability of the changes after tribological loading was evaluated.     

Surface structuring of case hardened chain pins by cold-sprayed microparticles to modify friction and wear properties

This paper presents the results of the application of a cold spray technique for structuring metallic surfaces with microparticles. The resulting changes in surface properties were characterized to observe their influences on the tribological behavior of the structured surface. The spray technique was applied to a technical component, a 16MnCr5 steel chain pin, designed to be mounted in a linear reciprocating tribometer. TiO₂ microparticles were used to structure the surface with a homogeneous distribution of singly dispersed particles, rather than a homogeneous closed coating on the surface. Tribometer tests were performed to directly compare structured and unstructured chain pins, and a significantly reduced sliding friction coefficient was observed for the structured pin. The pins were characterized in detail by surface analysis prior to and after application of the tribological load to set the surface parameters and surface chemistry, even on the microscale. It was confirmed that the particle structuring induced changes in the surface properties, and the durability of the changes after tribological loading was evaluated.     

PREPARATION AND CHARACTERIZATION OF SUPERPARAMAGNETIC FUNCTIONAL POLYMERIC MICROPARTICLES

Superparamagnetic poly(styrene-divinylbenzene-glycidyl methacrylate) (Pst-DVB-GMA) microparticles were prepared via a modified suspension polymerization process. A magnetic fluid was first prepared by a chemical co-precipitation method. Then magnetic microparticles were produced by mixing the monomers and the magnetic fluid with water in the presence of a stabilizer poly(vinyl pyrrolidone) (PVP) to form a suspension, and finally benzoyl peroxide was added to initiate the co-polymerization. The morphology and magnetic properties of the microparticles were examined by TEM and VSM. The spherically shaped microparticles, with a size range of 4 to 7 pm, showed distinct superparamagnetic characteristics. XRD was used to investigate the structure of the magnetite particles dispersed in the polymer matrix. The microparticles with epoxy groups on their surface can be applied directly to the seoaration of biomolecules.     

Droplet clustering in sprays

In this contribution, the spatial particle distribution in sprays of different atomizers is analyzed. Steady and unsteady particle structures are identified by evaluating the interparticle arrival time statistics at a certain position, which is the time increment between two succeeding particles. In addition to its characteristics of size and velocity, each particle exhibits an individual interparticle arrival time that is used to identify unsteady characteristics in the flow. Unsteadiness in sprays is thereby of interest for several reasons and in several applications, for example, in the combustion industry. A typical example of an unsteady spray behaviour is droplet clustering which can be caused, for example, by pulsating liquid disintegration procedures or particle interaction with large-scale eddy structures in the gas. The aim of the investigation is the analysis of such unsteady spray conditions. The evaluation of spray unsteadiness is done by means of point wise and time resolved PDA measurements in the spray of a pressure and twin-fluid atomizer, respectively.     

Amorphous particle deposition and product quality under different conditions in a spray dryer

Deposition of amorphous particles, as a prevalent problem particularly in the spray drying of fruit and vegetable juices, is due to low-molecular weight sugars and is strongly dependent on the condition of the particles upon collision with the dryer wall. This paper investigates the condition of the amorphous particles impacting the wall at different drying conditions with the aim of elucidating the deposition mechanism and physical phenomena in the drying chamber. A model sucrose-maltodextrin solution was used to represent the low-molecular-weight sugar. Particle deposits were collected on sampling plates placed inside the dryer for analyses of moisture content, particle rigidity (using SEM) and size distribution. Moisture content was adopted as a general indicator of stickiness. Product particles collected at the bottom of the experimental dryer were found to have higher moisture than particle deposits on samplers inside the dryer. Moisture content profile in the dryer shows that apart from the atomizer region, where particles are relatively wet, particle deposits at other regions exhibit similar lower moisture content. At the highest temperature adopted in the experiments, particles became rubbery suggesting liquid-bridge formation as the dominant deposition mechanism. Further analysis on particles size distribution reveals a particle segregation mechanism whereby smaller particles follow preferentially to the central air stream while larger particles tend to re-circulate in the chamber, as predicted in past CFD simulation. The findings from this work will form the basis and provide validating data for further modeling of wall deposition of amorphous particles in spray drying using CFD.     

A new framework for population balance modeling of spray fluidized bed agglomeration

Previous work (Hussain et al. (2013). Chemical Engineering Science, 101, 35) has pointed out that the conventional, one-dimensional population balance equation for aggregation can be expanded to accurately reproduce the results of discrete simulations of spray fluidized bed agglomeration. However, some parameters had to be imported from the discrete simulation (Monte-Carlo). The present paper shows how the expanded population balance can be run without importing parameters from the Monte-Carlo simulation. The expanded population balance still reproduces the results of Monte-Carlo simulations accurately, taking into account key micro-scale phenomena (sessile droplet drying, efficiency of collisions), but with much lower computational cost. Required input parameters are just the drying time of sessile droplets (calculated in advance), and the pre-factor of an equation that correlates particle collision frequency with fluidized bed expansion. In this way, the expanded population balance is, apart from autonomous, also (nearly) predictive. Its performance is demonstrated by comparisons with both Monte-Carlo results and experimental data for various operating conditions (binder mass flow rate, gas temperature). Despite formally being a one-dimensional expression, the expanded population balance captures additional properties, such as the number of wet particles and the number of droplets in the system, which are even difficult to measure in experiments.     

Shock wave driven microparticles for pharmaceutical applications

Ablation created by a Q-switched Nd:Yttrium Aluminum Garnet (Nd:YAG) laser beam focusing on a thin aluminum foil surface spontaneously generates a shock wave that propagates through the foil and deforms it at a high speed. This high-speed foil deformation can project dry micro- particles deposited on the anterior surface of the foil at high speeds such that the particles have sufficient momentum to penetrate soft targets. We used this method of particle acceleration to develop a drug delivery device to deliver DNA/drug coated microparticles into soft human-body targets for pharmaceutical applications. The device physics has been studied by observing the process of particle acceleration using a high-speed video camera in a shadowgraph system. Though the initial rate of foil deformation is over 5 km/s, the observed particle velocities are in the range of 900–400 m/s over a distance of 1.5–10 mm from the launch pad. The device has been tested by delivering microparticles into liver tissues of experimental rats and artificial soft human-body targets, modeled using gelatin. The penetration depths observed in the experimental targets are quite encouraging to develop a future clinical therapeutic device for treatments such as gene therapy, treatment of cancer and tumor cells, epidermal and mucosal immunizations etc.      

Relationship between single and bulk mechanical properties for zeolite ZSM5 spray-dried particles

In this work typical mechanical properties for a catalyst support material, ZSM5 （a spray-dried granular zeolite）, have been measured in order to relate the bulk behaviour of the powder material to the single particle mechanical properties. Particle shape and size distribution of the powders, determined by laser diffraction and scanning electron microscopy （SEM）, confirmed the spherical shape of the spray-dried particles. The excellent flowability of the material was assessed by typical methods such as the Hausner ratio and the Cart index, This was confirmed by bulk measurements of the particle-particle internal friction parameter and flow function using a Schulze shear cell, which also illustrated the low compressibility of the material. Single particle compression was used to characterize single particle mechanical properties such as reduced elastic modulus and strength from Hertz contact mechanics theory. Comparison with surface properties obtained from nanoindentation suggests heterogeneity, the surface being harder than the core. In order to evaluate the relationship between single particle mechanical properties and bulk compression behaviour, uniaxial confined compression was carried out. It was determined that the Adams model was suitable for describing the bulk compression and furthermore that the Adams model parameter, apparent strength of single particles, was in good agreement with the single particle strength determined from single particle compression test.     

Multiphysics numerical investigation of photothermal self-driving characteristics of SiO2@Au Janus microparticles for drug delivery

The photothermal self-driving process of Janus microparticles has wide application prospects in the fields of biomedicine. Since silica and gold have good biocompatibility and high photothermal conversion efficiency, the SiO₂@Au Janus microparticles are widely used as drug carriers. Based on the multiphysics coupling method, the photothermal self-driving process of SiO₂@Au Janus microparticles was investigated, wherein the substrate was SiO₂ particles and one side of the particles was coated with gold film. Under a continuous wave laser with irradiation of 20 W/cm², the distance covered by the Janus particles was increased by increasing the thickness of the gold film and reducing the size of the SiO₂ particles; the self-driving characteristics of the Janus particles were controlled substantially by increasing the intensity of the incident laser. Based on the simulation results, the thermophoretic motion and Brownian motion of particles can be measured by comparing the absolute values of the thermophoretic force impulse, Brownian force impulse, and drag force impulse. The Brownian force acting on Janus microparticles under low laser power cannot be ignored. Furthermore, the minimum laser power required for Janus particles to overcome Brownian motion was calculated. The results can effectively guide the design of Janus particles in biomedicine and systematically analyze the mechanism of particle thermophoretic motion during drug delivery.     

Experiment and numerical simulation of heat and mass transfer during a spray freeze-drying process of ovalbumin in a tray

Spray freeze-drying is a promising technology for producing high-quality porous particles primarily for pharmaceutical uses. The advantages of freeze-drying in the production of pharmaceuticals and biomedical products, such as minimization of thermal and chemical degradation, retention of volatile components, preservation of high porosity, and a very low content of residual water after drying, are mostly retained in spray freeze-drying. In this study, we performed spray freeze-drying of a 3% (w/w) chicken egg ovalbumin solution in a tray with a batch-type spray freeze-dryer that we developed. The physical characteristics of the spray freeze-dried particles were qualitatively evaluated by means of scanning electron microscopy. The freeze-drying behavior of spray-frozen particles was experimentally investigated by measuring the histories of product temperatures and numerically studied by developing an analysis model based on the finite volume method in a fixed grid system.     

Numerical and experimental investigation of convective drying in unsaturated porous media with bound water

The heat and mass transfer in an unsaturated wet cylindrical porous bed packed with quartz particles was investigated theoretically for relatively low convective drying rates. Local thermodynamic equilibrium was assumed in the mathematical model describing the multi-phase flow in the unsaturated porous media using the energy and mass conservation equations to describe the heat and mass transfer during the drying. The drying model included convection and capillary transport of the free water, diffusion of bound water, and convection and diffusion of the gas. The numerical results indicated that the drying process could be divided into three periods, the temperature rise period, the constant drying rate period and the decreasing drying rate period. The numerical results agreed well with the experimental data verifying that the mathematical model can evaluate the drying performance of porous media for low drying rates. The effects of drying conditions such as the ambient temperature, the relative humidity, and the velocity of the drying air, on the drying process were evaluated by numerical solution.     

A multiphase DNS approach for handling solid particles motion with heat transfer

In the current work we propose a multiphase DNS method capable of resolving the motion of solid particles coupled with heat transfer effects. The method is based on solving a shared set of momentum and energy balance equations for the carrier phase and the particulate phase. Individual particles are tracked using a number of volume fraction advection equations. The proposed method is in very good agreement with the available data in the literature for the following cases: isothermal particle motion (in the presence of walls and other particles), natural convection around a stationary particle and solid particles motion accompanied with heat transfer effects. In addition, we show that the method is inherently capable of handling deformable particles (i.e. droplets and bubbles) co-existing with solid particles. The method is thus well suited to deal with challenging multiphase systems, such as diesel spray combustion with soot formation, spray drying with particle nucleation, and biological treatment of waste water.     

Large Eddy Simulation of the spray formation in confinements

The particle and powder properties produced within spray drying processes are influenced by various unsteady transport phenomena in the dispersed multiphase spray flow in a confined spray chamber. In this context differently scaled spray structures in a confined spray environment have been analyzed in experiments and numerical simulations. The experimental investigations have been carried out with Particle-Image-Velocimetry to determine the velocity of the gas and the discrete phase. Large-Eddy-Simulations have been set up to predict the transient behaviour of the spray process and have given more insight into the sensitivity of the spray flow structures in dependency from the spray chamber design.     

Comparison of near-infrared spectroscopy and DEM simulations for tracing spatial dispersion of water during mixing

The aim of this work was to validate a wet mixing process, in which a liquid spray is used to impregnate particles during mixing. The experimental results obtained using a bladed-mixer with a near-infrared sensor were compared with the results obtained using a 1:1 discrete element method simulation. The porous particles used in both cases absorbed the sprayed liquid for a process time of about 18 min. Multiple sensors attached to the mixer wall continuously monitored the liquid contents of passing particles. The sensors were modeled in the simulation and the resulting signals were analyzed and compared with the experimental results. We show that the algorithms used for spray and liquid absorption can be used to predict the moisture distribution inside granular materials in chemical and pharmaceutical processes. Such simulations can help to save money, e.g., in resource-intensive experimental plans and equipment design studies, and by varying material parameters.     

Coating of finely dispersed particles by two-fluid nozzle

Particle coatings are used extensively to generate dispersed solids with well-defined properties, e.g., to protect active ingredients, with most coating processes using core particles of a diameter larger than 200 μm. This work contributes to the development of a coating process for fine dispersed particles (diameter less than 50 μm) by combining two particle-formulation processes, namely, coating and spray drying. The feasibility of the operation is based on and demonstrated by the innovative application of a two-fluid nozzle. Experiments were conducted by using glass particles as core particles and sodium benzoate as the coating agent. The coating of finely dispersed particles is achieved by the spraying of particles and coating solution as a homogeneous suspension. The aim is to create droplets with only one contained particle at the nozzle outlet. After evaporation of the water in the droplet, a thin solid film is built on the particle surface. The suspension viscosity was measured and compared with empirical equations from the literature. The liquid-film thickness on the particle surface was calculated to predict the building of a uniform coating layer or agglomerates. In this study, the feasibility of pneumatic transport through the nozzle and an investigation of the process were illustrated. The agglomeration fraction and degree of coating of the particle surface were analyzed optically by scanning electron microscopy. In this way, the influence of different processes and suspension parameters on the product quality were determined.     

ELECTROSTATICALLY SUPPORTED MIXING OF FINE GRAINED PARTICLES

The processing of fine-grained particles with diameters between 1 and 10 microns is difficult due to strong van-der-Waals attraction forces. In order to improve the handling properties, the fine-grained particles, i.e. host-particles,are coated with various nanoparticles, i.e. guest-particles. The mixing of fine-grained powders is influenced by particle-particle interactions. If these forces are distinctively used, both interactive and ordered mixtures can be produced.These particle mixtures consist of composite-particles that have new physical properties. These modified properties d epend strongly on the coating process, the diameter- and mass-relationship of the guest- and the host-particles. The properties of the composite-particles can systematically be adjusted to the requirements of industrial applications. For example, a laboratory bubbling fluidized bed can be used to describe the conveying behavior of the functionalized host-particles. Applications for the functionalized particles are in the pharmaceutical and the powder coating industries,e.g. enhanced dry powder inhalers and thin lacquer films. The present research compares three different mixing/coating processes. The composite-particles are characterized by TEM, SEM and with their fluidization characteristics. The coating process itself is monitored by the electrostatic charge of the particles.