Dynamical density functional theory for the evaporation of droplets of nanoparticle suspension

We develop a lattice gas model for the drying of droplets of a nanoparticle suspension on a planar surface, using dynamical density functional theory (DDFT) to describe the time evolution of the solvent and nanoparticle density profiles. The DDFT assumes a diffusive dynamics but does not include the advective hydrodynamics of the solvent, so the model is relevant to highly viscous or near to equilibrium systems. Nonetheless, we see an equivalent of the coffee-ring stain effect, but in the present model it occurs for thermodynamic rather the fluid-mechanical reasons. The model incorporates the effect of phase separation and vertical density variations within the droplet and the consequence of these on the nanoparticle deposition pattern on the surface. We show how to include the effect of slip or no-slip at the surface and how this is related to the receding contact angle. We also determine how the equilibrium contact angle depends on the microscopic interaction parameters.Comment: 35 pages, 10 figure

The Influence of Performance Level, Age and Gender on Pacing Strategy During a 100-km Ultramarathon

The aim of this study is to analyse the influence of performance level, age and gender on pacing during a 100-km ultramarathon. Results of a 100-km race incorporating the World Masters Championships were used to identify differences in relative speeds in each 10-km segment between participants finishing in the first, second, third and fourth quartiles of overall positions (Groups 1, 2, 3 and 4, respectively). Similar analyses were performed between the top and bottom 50% of finishers in each age category, as well as within male and female categories. Pacing varied between athletes achieving different absolute performance levels. Group 1 ran at significantly lower relative speeds than all other groups in the first three 10-km segments (all P < 0.01), and significantly higher relative speeds than Group 4 in the 6th and 10th (both P < 0.01), and Group 2 in the 8th (P = 0.04). Group 4 displayed significantly higher relative speeds than Group 2 and 3 in the first three segments (all P < 0.01). Overall strategies remained consistent across age categories, although a similar phenomenon was observed within each category whereby ‘top’ competitors displayed lower relative speeds than ‘bottom’ competitors in the early stages, but higher relative speeds in the later stages. Females showed lower relative starting speeds and higher finishing speeds than males. ‘Top’ and ‘bottom’ finishing males displayed differing strategies, but this was not the case within females. Although pacing remained consistent across age categories, it differed with level of performance within each, possibly suggesting strategies are anchored on direct competitors. Strategy differs between genders and differs depending on performance level achieved in males but not females

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

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

Evaporation-Induced Branched Structures from Sessile Nanofluid Droplets

We investigate the formation of branched nanoparticle aggregates resulting from the evaporation of sessile nanofluid droplets of the copper water-based nanofluids experimentally. Both symmetric and asymmetric drying patterns were found as the sessile droplet evaporated. A kinetic Monte Carlo (KMC) approach is developed to explain the drying process in a circular domain, representing the top view of a drying sessile droplet. It is found that the lattice-gas-based Monte Carlo model can describe the nanoparticle self-assembly into a solid highly branched aggregate. While the chemical potential function is coupled to the nondimensional spherical droplet size during evaporation, the results reveal that the fingering contact line instabilities can emerge under a given condition and force the formation of a branched nanoparticle structure. The pattern comparison shows that the simulation results have a qualitative agreement with the experiments. The parameter study shows that the model parameters, such as domain diameter, chemical potential distribution, particle interaction energy, and so on, have significant influence on the resulting patterns

Elimination of the Coffee-Ring Effect by Promoting Particle Adsorption and Long-Range Interaction

A Monte Carlo model has been developed to investigate the transition from the coffee-ring deposition to the uniform coverage in drying pinned sessile colloidal droplets. The model applies the diffusion-limited aggregation (DLA) approach coupled with the biased random walk (BRW) to simulate the particle migration and agglomeration during the droplet drying process. It is shown that the simultaneous presence of the particle adsorption, long-range attraction, and circulatory motion processes is important for the transition from the coffee-ring effect to the uniform deposition of finally dried particles. The absence of one of the specified factors favors the coffee-ring deposition near the droplet boundary. The strong outward capillary flow on the latest evaporation stage can easily destroy the entire particle pre-ordering at the early drying stages. The formation of a robust particle structure is required to resist the outward flow and alter the coffee-ring effect

Evaporation-Induced Branched Structures from Sessile Nanofluid Droplets

We investigate the formation of branched nanoparticle aggregates resulting from the evaporation of sessile nanofluid droplets of the copper water-based nanofluids experimentally. Both symmetric and asymmetric drying patterns were found as the sessile droplet evaporated. A kinetic Monte Carlo (KMC) approach is developed to explain the drying process in a circular domain, representing the top view of a drying sessile droplet. It is found that the lattice-gas-based Monte Carlo model can describe the nanoparticle self-assembly into a solid highly branched aggregate. While the chemical potential function is coupled to the nondimensional spherical droplet size during evaporation, the results reveal that the fingering contact line instabilities can emerge under a given condition and force the formation of a branched nanoparticle structure. The pattern comparison shows that the simulation results have a qualitative agreement with the experiments. The parameter study shows that the model parameters, such as domain diameter, chemical potential distribution, particle interaction energy, and so on, have significant influence on the resulting patterns

Evaporation-Induced Branched Structures from Sessile Nanofluid Droplets

We investigate the formation of branched nanoparticle aggregates resulting from the evaporation of sessile nanofluid droplets of the copper water-based nanofluids experimentally. Both symmetric and asymmetric drying patterns were found as the sessile droplet evaporated. A kinetic Monte Carlo (KMC) approach is developed to explain the drying process in a circular domain, representing the top view of a drying sessile droplet. It is found that the lattice-gas-based Monte Carlo model can describe the nanoparticle self-assembly into a solid highly branched aggregate. While the chemical potential function is coupled to the nondimensional spherical droplet size during evaporation, the results reveal that the fingering contact line instabilities can emerge under a given condition and force the formation of a branched nanoparticle structure. The pattern comparison shows that the simulation results have a qualitative agreement with the experiments. The parameter study shows that the model parameters, such as domain diameter, chemical potential distribution, particle interaction energy, and so on, have significant influence on the resulting patterns

Evaporation-Induced Branched Structures from Sessile Nanofluid Droplets

We investigate the formation of branched nanoparticle aggregates resulting from the evaporation of sessile nanofluid droplets of the copper water-based nanofluids experimentally. Both symmetric and asymmetric drying patterns were found as the sessile droplet evaporated. A kinetic Monte Carlo (KMC) approach is developed to explain the drying process in a circular domain, representing the top view of a drying sessile droplet. It is found that the lattice-gas-based Monte Carlo model can describe the nanoparticle self-assembly into a solid highly branched aggregate. While the chemical potential function is coupled to the nondimensional spherical droplet size during evaporation, the results reveal that the fingering contact line instabilities can emerge under a given condition and force the formation of a branched nanoparticle structure. The pattern comparison shows that the simulation results have a qualitative agreement with the experiments. The parameter study shows that the model parameters, such as domain diameter, chemical potential distribution, particle interaction energy, and so on, have significant influence on the resulting patterns