Viscosity affected by nanoparticle aggregation in Al2O3-water nanofluids

An investigation on viscosity was conducted 2 weeks after the Al2O3-water nanofluids having dispersants were prepared at the volume concentration of 1-5%. The shear stress was observed with a non-Newtonian behavior. On further ultrasonic agitation treatment, the nanofluids resumed as a Newtonian fluids. The relative viscosity increases as the volume concentrations increases. At 5% volume concentration, an increment was about 60% in the re-ultrasonication nanofluids in comparison with the base fluid. The microstructure analysis indicates that a higher nanoparticle aggregation had been observed in the nanofluids before re-ultrasonication

Monte Carlo modeling of nanofluid drying patterns

Nanofluids are a new kind of fluids engineered by dispersing nanoparticles in base fluids. After full drying of nanofluid, nanoparticles are left on a substrate and can self-organize into different structures. This process can be employed as a convenient way to generate useful micro-devices or new materials. Better understanding of nanofluid drying patterns will lead to cost-saving optimizations in applications such as printing, coating, medical research, etc. Two different modeling approaches are combined together to investigate drying nanofluid patterns and qualitatively predict the shape and structure of the resulting particle aggregates. The first model is based on the Kinetic Monte Carlo (KMC) simulation and is developed to predict the drying patterns in an open two-dimensional (2D) domain, in which the dewetting front shrinks from the edge towards the center. The model behavior is investigated independently in open domains of two shapes: square and circle. The simulation in the open square domain reveals that the dispersed particles can be deposited into an isotropic branched structure which remains frozen after full evaporation of the base fluids. The prediction of fractal particle aggregation in the square domain is confirmed in the experiments with drying water-based nanofluids under the Scanning Electron Microscope. The KMC approach in the open circular domain simulates the formation of fractal-like structures during evaporation of the water-based nanofluid sessile droplets in the experiments. We found that the lattice-gas based Monte Carlo model can theoretically predict the nanoparticle self-assembly into a solid fractal-like aggregate after a sessile droplet is fully dried. The simulated patterns show an agreement with the experimental counterparts. Furthermore, if the chemical potential function is coupled with the solvent thickness profile of a sessile droplet, the results reveal that fingering contact line instabilities can emerge under a given condition and lead to the formation of branched nanoparticle structure. The second model, based on Diffusion Limited Aggregation (DLA) approach, accounts for particle pre-aggregation in the bulk of drying nanofluids. We mathematically extend a classical DLA by developing a stochastic model of drying the pinned sessile droplet which includes cluster-cluster aggregation (DLCA) as well. The dried pattern has been shown to change from coffee-ring-like configuration to uniform-like one if the particle sticking probability substantially increases. In addition, the model is upgraded to include the different drying conditions, such as sole outward capillary flow (1 flow model), inward and outward flows (2-flow model), and particles' interactions.The KMC and DLCA models are then combined to describe the full process of sessile droplet drying: the DLCA model is used to simulate the initial stage of droplet evaporation, capillary flows and particle pre-aggregation, while KMC is used to simulate the final stage of the thin-film dewetting. The cases of broken coffee rings, fractals inside the rings and drying holes have been studied. The three-dimensional (3D) DLCA model is consequently developed. The 3D simulation provides a realistic view of drying of nanofluid droplets and also includes the cluster-cluster aggregation and the capillary flow. The influence of droplet base diameter, contact angle, particle sticking probability and volumetric concentration on the final 3D pattern has been investigated in the presented study. Finally, the simulation results are supported by experimental work with drying nanofluid droplets, and quantitative comparisons of results are provided. The future work aims to model more complicated conditions, such as non-uniform local evaporation, shrinking three-phase line in the three-dimensional domain, etc. More detailed experiments are planned, including the study of 3D patterns.DOCTOR OF PHILOSOPHY (MAE

Evaporation-induced formation of fractal-like structures from nanofluids

After the nanofluids are fully dried, the self-assembled nanoparticles can form various structures on the substrate. The fractal-like patterns are among them. The two-dimensional Kinetic Monte Carlo model is developed to predict the drying patterns of the nanofluids in an open domain, where the dewetting front shrinks from the edge toward the center. The simulation reveals that the initially dispersed particles can be deposited into an isotropic branched structure which remains frozen after full evaporation of the base fluid. The well-developed fractal-like particle aggregates are different from the fractal cavities obtained in the previous closed domain simulation. The present prediction of the fractal particle aggregation is verified by the experiments with the water-based nanofluids. The images taken using a scanning electron microscope prove that the evaporation-induced branched microstructures are formed by the nanoparticles as the water is totally dried

Noise attenuating performance of metasurfaces with regular Euclidean tiling and uniformly embedded Helmholtz resonators

Sound-attenuation capabilities of a certain class of subwavelength sound barriers based on wall-embedded Helmholtz resonators and a ventilation duct are studied theoretically and numerically. A simple analytical expression for the broadband integral transmission attenuation is introduced for this type of metasurface structures, which indicates that the sound blocking performance depends only on the main operational frequency, thickness and ventilation capacity of the structure. This result derived from the lumped parameter theory implies that the trade-off between the barrier performance and its physical footprint is inevitable and provides a practical guidance for the design strategies with specific targets and limitations. Detailed finite element modelling (FEM) parameter study is performed both to validate the theoretical predictions and test the limitations of simple lumped parameter approach. It was found that the agreement between the analytical and finite element results is very good for the structure thickness greater than 30 mm in a 300–1000 Hz frequency range. However, at smaller thickness values the FE results start to diverge from the predictions and also become dependent on the specific cell shape, which suggests that more advanced analytical approach is required for thinner metasurfaces.Agency for Science, Technology and Research (A*STAR)The authors are grateful for the financial support from A*STAR Science and Engineering Research Council under AME Individual Research Grant (IRG) 2018 Grant Call (Project No. A1983c0030)

Amplifying and attenuating the coffee-ring effect in drying sessile nanofluid droplets

Experiments and simulations to promote or attenuate the “coffee-ring effect” for pinned sessile nanofluid droplets are presented. The addition of surfactant inside a water suspension of aluminum oxide nanoparticles results in coffee-ring formation after the pinned sessile droplets are fully dried on a substrate, while droplets of the same suspension without the surfactant produce a fine uniform coverage. A mathematical model based on diffusion-limited cluster-cluster aggregation has been developed to explain the observed difference in the experiments. The simulations show that the particle sticking probability is a crucial factor on the morphology of finally dried structures.Published versio

Three-dimensional Monte Carlo model of the coffee-ring effect in evaporating colloidal droplets

The residual deposits usually left near the contact line after pinned sessile colloidal droplet evaporation are commonly known as a “coffee-ring” effect. However, there were scarce attempts to simulate the effect, and the realistic fully three-dimensional (3D) model is lacking since the complex drying process seems to limit the further investigation. Here we develop a stochastic method to model the particle deposition in evaporating a pinned sessile colloidal droplet. The 3D Monte Carlo model is developed in the spherical-cap-shaped droplet. In the algorithm, the analytical equations of fluid flow are used to calculate the probability distributions for the biased random walk, associated with the drift-diffusion equations. We obtain the 3D coffee-ring structures as the final results of the simulation and analyze the dependence of the ring profile on the particle volumetric concentration and sticking probability.Published versio

Crossover from the coffee-ring effect to the uniform deposit caused by irreversible cluster-cluster aggregation

The coffee-ring effect for particle deposition near the three-phase line after drying a pinned sessile colloidal droplet has been suppressed or attenuated in many recent studies. However, there have been few attempts to simulate the mitigation of the effect in the presence of strong particle-particle attraction forces. We develop a three-dimensional stochastic model to investigate the drying process of a pinned colloidal sessile droplet by considering the sticking between particles, which was observed in the experiments. The Monte Carlo simulation results show that by solely promoting the particle-particle attraction in the model, the final deposit shape is transformed from the coffee ring to the uniform film deposition. This phenomenon is modeled using the colloidal aggregation technique and explained by the “Tetris principle,” meaning that unevenly shaped or branched particle clusters rapidly build up a sparse structure spanning throughout the entire domain in the drying process. The influence of the controlled parameters is analyzed as well. The simulation is reflected by the drying patterns of the nanofluid droplets through the surfactant control in the experiments.MOE (Min. of Education, S’pore)Published versio

Effect of Surfactant on the Drying Patterns of Graphite Nanofluid Droplets

We investigate the effect of surfactant on the formation of nanoparticle aggregates that resulted from evaporation of sessile nanofluid droplets theoretically and experimentally. A Monte Carlo model is developed to explain the transition from the coffee-ring pattern to the uniform deposition in drying the pinned sessile nanofluid droplets. The model applies the diffusion limited cluster–cluster aggregation approach coupled with the biased random walk of nanoparticles. The experiments show that the addition of surfactant in nanofluids helps the formation of a coffee ring instead of the uniform domain coverage. The simulations suggest an explanation of this transition by controlling the sticking probability parameter between the particles. The simulated results statistically agree with the experimental observation of the finally dried graphite nanoparticle structures from the pinned nanofluid droplets

Dendritic nanoparticle self-assembly from drying a sessile nanofluid droplet

The pattern formation left by a drying nanofluid droplet is related to the evaporation induced particle self-assembly. The experimental results demonstrate the formation of dendritic particle deposition after the liquid phase of unpinned sessile nanofluid droplets is fully evaporated. The dried-in particle assemblies exhibit the dendritic patterns connecting the sprawling branches with a central core structure. The branched structures are formed by particles merging in the receding front. A three-dimensional lattice-gas kinetic Monte Carlo model is developed to simulate the particle self-assembling behaviour in a drying particle-laden droplet with the dewetting three-phase line. The parameter study is carried out to demonstrate the trend of the dendritic pattern formation. The various patterns are simulated by varying the chemical potentials and the interaction energies among particles, liquids, and substrates. The dendritic particle depositions are measured in three dimensions after the nanofluid droplet is completely dried. Qualitative agreement is observed between the experimental and the numerical results. Thicker branches and larger central cores are observed with an increase of particle concentrations.Agency for Science, Technology and Research (A*STAR)The authors would like to thank the Agency of Science, Technology and Research (A*STAR), Individual Research Grant (IRG), grant number A1783c0006 for the support