Fei Duan

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.

Sessile nanofluid droplet drying.

Nanofluid droplet evaporation has gained much audience nowadays due to its wide applications in painting, coating, surface patterning, particle deposition, etc. This paper reviews the drying progress and deposition formation from the evaporative sessile droplets with the suspended insoluble solutes, especially nanoparticles. The main content covers the evaporation fundamental, the particle self-assembly, and deposition patterns in sessile nanofluid droplet. Both experimental and theoretical studies are presented. The effects of the type, concentration and size of nanoparticles on the spreading and evaporative dynamics are elucidated at first, serving the basis for the understanding of particle motion and deposition process which are introduced afterward. Stressing on particle assembly and production of desirable residue patterns, we express abundant experimental interventions, various types of deposits, and the effects on nanoparticle deposition. The review ends with the introduction of theoretical investigations, including the Navier-Stokes equations in terms of solutions, the Diffusion Limited Aggregation approach, the Kinetic Monte Carlo method, and the Dynamical Density Functional Theory. Nanoparticles have shown great influences in spreading, evaporation rate, evaporation regime, fluid flow and pattern formation of sessile droplets. Under different experimental conditions, various deposition patterns can be formed. The existing theoretical approaches are able to predict fluid dynamics, particle motion and deposition patterns in the particular cases. On the basis of further understanding of the effects of fluid dynamics and particle motion, the desirable patterns can be obtained with appropriate experimental regulations.

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.

Dynamic viscosity measurement in non-Newtonian graphite nanofluids

The effective dynamic viscosity was measured in the graphite water-based nanofluids. The shear thinning non-Newtonian behavior is observed in the measurement. On the basis of the best fitting of the experimental data, the viscosity at zero shear rate or at infinite shear rate is determined for each of the fluids. It is found that increases of the particle volume concentration and the holding time period of the nanofluids result in an enhancement of the effective dynamic viscosity. The maximum enhancement of the effective dynamic viscosity at infinite rate of shear is more than 24 times in the nanofluids held for 3 days with the volume concentration of 4% in comparison with the base fluid. A transmission electron microscope is applied to reveal the morphology of aggregated nanoparticles qualitatively. The large and irregular aggregation of the particles is found in the 3-day fluids in the drying samples. The Raman spectra are extended to characterize the D and G peaks of the graphite structure in the nanofluids. The increasing intensity of the D peak indicates the nanoparticle aggregation growing with the higher concentration and the longer holding time of the nanofluids. The experimental results suggest that the increase on effective dynamic viscosity of nanofluids is related to the graphite nanoparticle aggregation in the fluids.

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.

Three-dimensional patterns from the thin-film drying of amino acid solutions

Experimental atomic force microscopy (AFM) images show the dried-in patterns from amino acid solutions which can be in the form of dots or networks. The three-dimensional lattice-gas Kinetic Monte Carlo (KMC) model is applied to simulate the formation of dot-like and network-like particle structures from the evaporating thin films of solutions. A sigmoidal jump in the chemical potential value is implemented to obtain dual-scale structures with the grain size distribution peaking at two distinctive values. The simulated and experimental results are qualitatively comparable.

Fractal Patterns in the Nanofluidic Sessile Droplet Drying

The two-dimensional lattice-gas kinetic Monte Carlo model is used to simulate the process of a nanofluidic sessile droplet drying with a moving contact line in this study. A new modeling approach is implemented by introducing the two-dimensional circular simulation domain in order to operate with the top view of spherical cap of a drying droplet. The non-uniform effective chemical potential function is applied to the model, taking into account the thickness profile of a droplet. Although our simulation is two-dimensional, this modification mimics to some extent the three-dimensional nature of a mesoscopic sessile droplet. Hence we introduce a new, pseudo-3D modification of Monte Carlo simulation model, i.e., a 2D model with additional dependence of the chemical potential on the local droplet thickness. In result, the evaporation-induced nanoparticle self-assembly led to the formation of a fractal-like structure, which is observed in simulation runs. Interestingly, the fractal-shaped patterns obtained in numerous simulations are comparable to the experimentally recorded images of dried nanoparticle structures. The difference in average fractal dimensions of simulated and experimental images is estimated to be less than 5%. It is suggested that the lattice-gas model provides an easy way to reproduce the formation of branched structures from nanoparticles during the drying of spherical sessile droplets.Copyright

Switching On and Off the Coffee-Ring Effect in Drying Sessile Nanofluid Droplets

The diffusion-limited cluster aggregation (DLCA) model has been implemented in a three-dimensional (3D) domain with a shape of an approximately spherical cap for simulating the drying process of a sessile nanofluid droplet. The droplet evaporation is investigated with the pinned three-phase line, resulting in shrinking contact angle and outward capillary flow. The cluster-cluster aggregation between the particles is taken into account in the model, and the transition from the uniform deposition to the coffee-ring pattern is established by altering the sticking probability parameter. The results of the simulation turn to be consistent with the experimental observation. The influence of the parameters, such as particle volumetric concentration and relative domain size, are studied.Copyright

Thermal Conductivity Affected by Temperature and Particle Size in Al2O3-Water Nanofluids

The effective thermal conductivity of Al2 O3 -water nanofluids has been measured using a transient hot wire method. Experimental results demonstrate that the thermal conductivity of Al2 O3 nanofluids increases linearly with increasing nanoparticle concentration. Adding 5 vol % of Al2 O3 nanoparticles in water increases the effective thermal conductivity of the nanofluids by 20%. Thermal conductivity of Al2 O3 nanofluids increases with an increase of temperature. The enhancement is around 1.7% at 15 °C in comparison with around 16% at 55 °C in a 1 vol % nanofluid. The particle size is another important parameter for the effective thermal conductivity. The increase of thermal conductivity reduces from 30% to 10% as the particle sizes increase from 10 nm to 35 nm. The increase of the effective thermal conductivity starts as the particle size increases above 35 nm, reaching about 27.5% in the nanofluid with the particle size at 150 nm.Copyright