Silica Droplets Research Articles

Modeling the evaporation and drying of two-component slurry droplets

In this paper, we present a mathematical model and numerical simulation for the evaporation and drying of a liquid droplet containing suspended solids, relevant to processes such as spray drying and spray pyrolysis in the food and pharmaceutical industries. The model comprises three stages: first is the evaporation of the liquid droplet consisting of solid particles, followed by the second stage starting with the formation of a porous crust around a wet-core region and, finally, the third stage with sensible heating of the dry particle. Using a finite difference method with a moving grid, we account for the moving interface between the crust and wet core. Our model incorporates spatial temperature variations and is validated against experimental data on colloidal silica droplet drying, showing good agreement. We examine model assumptions and analyze the impact of drying conditions on drying rate and final particle morphology. Along with the temperature and velocity of the drying gas, we also find that the shape of suspended solid particles inside the droplet and assuming continuum flow of vapor through the crust influence drying quality. Finally, we develop a regime map to predict whether the final particle will be solid or hollow based on operating conditions.

Bioinspired silica-based sol–gel micropatterns on aluminium for humid air condensation

Several patterned coatings with a hybrid organic-inorganic nature were deposited on metallic substrates by exploiting the dewetting of a sol–gel bilayer. The hybrid coatings, inspired by the exoskeleton of a desert beetle, consisted of hydrophilic silica droplets on a hydrophobic CH3-silica bottom layer. The patterned morphology was tuned by changing the initial solution concentration, which resulted in substantial changes in the size and the density of distribution of the hydrophilic droplets. The reproducibility of the dewetting process on metallic substrates was confirmed, together with its scalability over large area substrates. The real-life application of the patterned coating for atmospheric water harvesting was tested in a custom-made apparatus, which demonstrated that the patterned coating led to a higher collected mass during condensation from humid air compared to the bare aluminium substrate. The patterned coating was proven to maintain its structure after the humid air exposure, confirming the robustness of the sol–gel-based materials.Graphical abstract

Toughening of soda-lime-silica glass by nanoscale phase separation: Molecular dynamics study

The low fracture toughness of oxide glasses is a key limitation for many of their applications. Inducing and controlling nanoscale phase separation in oxide glasses has been proposed as a potential toughening strategy, as, unlike many alternative extrinsic toughening approaches, it allows one to retain the optical transparency. Using molecular dynamics simulations, we here investigate the toughening mechanism in soda-lime-silica glasses with embedded glassy nanoscale silica droplets. This system is chosen as a model for the experimental structure of phase-separated soda-lime-silica glass, which is attractive considering its existing commercial use and the ease of inducing phase separation. We calculate the fracture toughness of glass structures containing nanodroplets of varying sizes and with different precrack positions, revealing that the glassy silica droplets toughen the material. The simulations show that crack propagation is impeded by crack arrest, crack deflection and diversion, and stress field alteration, ultimately increasing the fracture toughness. Our findings thus shed light on the toughening mechanism due to phase separation, with important implications for the experimental design of oxide glasses with controlled nanoscale phase separation.

Drying silica-nanofluid droplets

Exsiccation of suspension droplets generates characteristic ring-like stains. The homogeneity of such coffee rings is disturbed by subsequent cracking. Placing silica droplets on copper and stainless steel substrates, the ensuing drying and cracking chronology is investigated. The formation of the coffee ring and its cracking are visualised through microscopic imaging and videotaping. Based on the cracking chronology, three crack generations are identified. The evolution of the 1st crack generation is studied in detail. Two fracture types – logarithmic spiral and straight radially oriented – characterise these initial cracks. While the first type is due to deposition delamination, the straight cracks are induced by the interplay of capillary pressure and shear stress between coating and substrate. Most noteworthy is our finding that the cracking speed of the straight cracks is one order of magnitude larger than that of the spiral cracks. The pattern of the 1st generation cracks defines the final crack network. Our findings provide a comprehensive understanding of the crack pattern formation following the desiccation of silica droplets. This study offers details on nanofluid exsiccation on metal substrates, thereby opening routes for depositing nanoparticles on heated and non-heated surfaces for various engineering applications.

Drying Kinetics and Particle Formation from Dilute Colloidal Suspensions in Aerosol Droplets.

Industrial processes such as spray drying of pharmaceutical and food products often involve the drying of aerosol droplets containing colloidal suspensions into powdered microparticles of desired properties. The morphology and surface properties of the final dry products/microparticles obtained after the drying process are strongly influenced by the parameters of the initial aerosol droplet composition and the drying conditions. In particular, the final dry microparticle morphology can be dependent on the dimensionless Péclet number (Pe), which expresses the relative competition between the diffusion of the dispersed particles within the droplet and the rate of solvent loss via evaporation. In this work, we examine how control over the gas phase drying conditions and initial aerosol droplet composition can be used to influence the aerosol droplet drying kinetics in the gas phase for a range of Péclet numbers. We used a single-particle levitation instrument, the electrodynamic balance, to measure the drying kinetics of colloidal silica droplets (0.10-0.60% v/v) under controlled gas phase drying conditions of temperature (263-326 K) and relative humidity (0-90%) and obtained Péclet numbers ranging from 4.05 to 184.5. We demonstrate that, for aerosol droplets with initially dilute feed colloid concentrations and within the constant evaporation regime, the starting composition does not strongly influence the solvent evaporation rate with the included nanoparticles (NPs) acting as spectators. However, the gas phase drying conditions, temperature, and relative humidity, directly influence the droplet temperature via evaporative cooling as well as the droplet drying kinetics and the final dry microparticle properties. With a priori knowledge of the droplet drying kinetics from the single droplet measurements, we further demonstrate the possibility of tailoring the morphology of the dried microparticles. Dried silica microparticles collected at Pe = 23.8 had dense spherical morphologies, while those at the highest Pe = 180.0 had crumpled surface morphologies with a transition in morphology between these limiting Pe values. Our results extend the fundamental understanding of the mechanisms controlling the drying of aerosol droplets in colloidal suspensions across a wide range of application areas extending from spray drying to the drying of respiratory fluid droplets containing bacteria and viruses and the drying of atmospheric aerosol droplets.

Coffee‐ring‐effect‐induced water scale formation of silicic acid‐containing droplet

AbstractThe drying behavior of silicic acid‐containing water droplets on a glass substrate was investigated by using as‐prepared aqueous silica solutions of varying concentrations to simulate water scale‐like formation. The droplets of aqueous silica solution were found to form circumferential solid silica deposits (water scale). Additionally, the drying behavior of the silica droplets was observed under varying temperature and humidity. Optical microscopy images of the deposited material showed thicker deposits in the inner part of the circumference, indicating the so‐called rush‐hour effect and induced by the coffee‐ring effect. Thus, the formation of water scale observed in daily life can be attributed to the coffee‐ring effect. Scanning electron microscopy confirmed that the as‐prepared water scale consisted of polymerized silica spheres. The hardness of the water scale depended on the amount of silica deposited and the initial concentration of the silica solution. Thus, the formation of water scale in daily life was concluded to occur by two mechanisms: formation of silica spheres by silica polymerization and agglomeration‐deposition by the coffee‐ring effect.

Modeling analysis on the silica glass synthesis in a hydrogen diffusion flame

Silica glass ingot synthesis by flame hydrolysis deposition (FHD) is an important approach to obtain high-purity synthetic silica glass with high refractive index homogeneity. Using the precursor of silicon tetrachloride (SiCl4), the silica ingot (SiO2) can be synthesized in the oxy-hydrogen diffusion flame through a set of kinetic reactions and subsequent cooling. The homogeneity of the synthetic silica glass is influenced by the temperature/diameter equality of silica droplets and the residue of radicals (e.g. OH) in the synthetic silica. In this study, the mixing system of oxy-hydrogen diffusion flame and silica droplets in a furnace were modeled by a Euler–Lagrange multi-phase model, where the interphase exchange of mass, momentum and energy between the gas phase molecules and liquid phase silica droplets during the formation and transportation of silica droplets were included. The silica droplets were tracked by a Lagrangian formulation that includes the discrete phase inertia, hydrodynamic drag, the force of gravity and the dispersion due to turbulent eddies. The molecular gas phase reactions are described by a simple kinetic mechanism involving the major species during the formation of molecular SiO2, with kinetic and thermodynamic parameters taken from the literature when available.The effect of initial equivalence ratio on the flame structure was analyzed in the study, where the high-temperature region is uniform over the glass ingot and the OH residue is limited at unity equivalence ratio. The probability distributions of silica droplet diameter and temperature were analyzed by employing a developed droplet growth model, where the condensation rate is controlled by the in situ SiO2 vapor concentration and the local flow-condition-determined mass transfer rate. The maximum diameter of silica droplets in the furnace is 1.39×10−4m, and the percentage of silica droplets decreases with the increasing of diameter. Two high probability regions were observed for the droplet temperature distribution, the temperature below 1500K accounts for 40% of the total droplet number, and the temperature range between 3500 and 4500K accounts for 58%. The droplet diameter on the ingot cap distributes in the range from 4×10−5 to 9×10−5m with approximately probability of 85%. The distribution of droplet temperature on the ingot cap is much uniform, with more than half (67%) of the droplets has the temperature between 3100 and 3200K.

Oxidation of SiC in low-pressure CO2 plasma: Formation of silica nano-needles

Samples of silicon carbide were exposed to carbon dioxide plasma at high temperature to study thermal and chemical degradation, as well as the evolution of surface morphology. Scanning electron microscope images revealed that the samples remained fairly intact up to a temperature of approximately 1700 K, when surface modification and degradation started to occur. The oxidation at high temperature caused the formation of liquid silica droplets that were several μm in diameter at a temperature exceeding 1800 K. Sharp nano-needles were found that stretched perpendicularly from the surface at this temperature. Increases in temperature resulted in increased density of the droplets with nano-needles until the entire surface was covered with the nano-needles at a temperature of approximately 1850 K. Further increases of temperature caused rapid degradation of material and loss of the nano-needles. These results are explainable by the particularities of passive oxidation of SiC upon treatment with plasma particles.

Insight into morphology changes of nanoparticle laden droplets in acoustic field

Hollow structures with unique morphologies form due to particle agglomeration in acoustically levitated nanofluid functional droplets when subjected to external heating. The final diameter of the structure depends only on the ratio of agglomeration to evaporation time scales for various nanoparticle laden droplets, and not on the type of the suspended particles. These time scales depend only on nanoparticle concentration. This valuable information may be exploited to form microstructures with desired properties from ceramic compounds. Phase diagrams for alumina and silica droplets indicate the transition from a bowl to ring structure depending on concentration.

Simulating the structural evolution of droplets following shell formation

Building on the new droplet drying framework developed by the authors in their previous work Handscomb et al., Chem. Eng. Sci. 64(4) 628–637 and Chem. Eng. Sci. 64(2) 228–246 this paper develops physically motivated criteria for dynamically deciding the appropriate structural sub-model to use at each stage of the drying process. Such criteria create a spatially resolved mechanistic droplet drying model which is capable of simulating multiple dried-particle morphologies based on evolving droplet composition and drying conditions. The new criteria are used in conjunction with the previously described model framework to simulate colloidal silica droplets, investigating the relationship between suspended particle size and dried-particle morphology.

Heat and mass transfer of single droplet/wet particle drying

A theoretical model, which considers the fully unsteady character of both heat and mass transfer during the drying of single droplet/wet particle, is presented. The model enables prediction of pressure and fraction distributions of air–vapour mixture within the capillary pores of the wet particle crust. The simulations of the drying process of a single silica droplet under different conditions show a permanent rising of pressure within the capillary pores, but the corresponding vapour fraction remains less than unity. The comparison between the drying histories of the silica droplet, predicted by the present model with the data, calculated by the model which assumes a quasi-steady-state mass transfer and linear pressure profile within the capillary pores, shows inconsiderable differences between the droplet/wet particle temperature and mass time-changes. At the same time, the present model predicts pressure build-up and temperature rising within the particle wet core. However, in the studied cases the temperature of the wet core temperature does not exceed the liquid saturation temperature and therefore no boiling of liquid within the particle wet core is observed.

Effect of the Cl − content on the formation and dissolution of precipitates in a soda–lime–silica glass

The glass with composition 13Na 2O–11CaO–76SiO 2 (mol%) undergoes subliquidus phase separation via a binodal mechanism. Below the binodal temperature, T b, the glass separates into two amorphous phases, silica-enriched droplets and a silica-poor matrix. Small-angle X-ray scattering (SAXS) is used to study the formation process of the droplet phase at 600 °C, as well as, the inverse process where the precipitates dissolve after an increase in temperature. A small amount of 0.47 mol% Cl − in the glass causes a drastic change of the kinetics and thermodynamics of the phase separation process in comparison with the pure glass: the phase separation is accelerated and the maximum volume fraction of the silica droplet phase is increased. The results of dissolution experiments demonstrate that due to the Cl − content T b increases by about 60 K. So, the main effect of Cl − in the glass is interpreted as a shift of the miscibility gap to higher temperatures resulting from a higher thermodynamic driving force.

Effect of fining with sodium chloride on the phase separation of a soda lime silica glass

Abstract The effect of the technologically important procedure of fining with NaCl on the liquid-liquid phase separation of the 13Na2O11CaO76SiO2 (mol%) glass in the metastable region of the miscibility gap has been studied by measuring the progressive changes of the small angle X-ray scattering (SAXS) intensities that characterize the phase separation process at a constant temperature of 600°C. It has proved that a small quantity of chloride remains in the glass after fining. This small content of chloride causes a dramatic change of kinetics and equilibrium conditions for the phase separation process in the glass in comparison with a pure soda lime silica glass: the phase separation is accelerated considerably and the maximum volume fraction of the silica droplet phase is increased. The effect of chloride is interpreted as a shift of the miscibility gap position by 45 K up to higher temperatures. This explanation is confirmed by measurements of the binodal temperatures of the pure glass and the glass contaminated with chloride. Viscosity measurements show that chloride does not significantly alter the conditions for diffusion in the glass. Thus, the effect of chloride on the phase separation is mainly attributed to a rise of the miscibility gap.