No abstract

The solid surfaces used in evaporation studies of nanoparticle sessile droplets usually exhibit significant surface roughness, causing significant pinning of the three-phase contact lines and producing different types of nanoparticle deposits, from single and multiple coffee rings (formed at the initial pining of triple contact lines) to central bumps. Here we used nanometer-scale smooth hydrophobic surfaces to investigate the evaporation of sessile water droplets containing silica nanoparticles and organic pigment nanoparticles. We observed a new type of coffee ring deposits which were not formed at the initial pinning but at the later pinning. We referred them to as the inner coffee ring deposits (ICRDs). The radius of ICRDs was smaller than the radius of the initially pinned contact area and increased with increasing concentration of added salts and nanoparticles and with increasing contact angle hysteresis of hydrophobic surfaces. We also observed different dendrite deposit patterns inside ICRDs. We argue that all the deposit patterns are due to the second pinning of the three-phase contact lines, which occur when the forces on particles are balanced. The hypothesis is further supported by the transient changes of the dynamic contact angles and contact base area radius. The contact angle hysteresis, the particle concentration, and the colloidal interaction forces such as the electrical double-layer forces play a vital role in determining the size and patterns of ICRDs and the evaporation kinetics of nanoparticle sessile droplets.

The effect of nanoparticles on the evaporation of a sessile droplet into air is still controversial. Unlike insoluble surfactants which reduce the droplet evaporation rate, here we show that the presence of nanoparticles and the increase of their concentration lead to an increase in the overall rate of diffusive evaporation and, consequently, a decrease of the droplet lifetime. The nanoparticles accumulating at the droplet edge due to the well-known coffee-ring effect pin the three-phase contact line for an extended time and maintain a large air-water interface area, leading to the increased evaporation rate. We provide a full analytical prediction for the lifetime of a sessile droplet evaporating by the combined pinned-receding mode. A master equation and a master diagram for the droplet lifetime of the combined mode are obtained and experimentally validated, and explain the effect of nanoparticles on increasing the global evaporation rate and decreasing the droplet lifetime.

The evaporation of sessile droplets with a constant base radius (pinning mode) and a constant contact angle (depinning mode) has been experimentally observed. Here we analyzed the effect of substrate hydrophobicity on the lifetimes of evaporating droplets for the two modes. Theoretical predictions were obtained and compared with available experimental results. The theoretical analysis and experimental results show that linear methods of extrapolating limited experimental data for a transient droplet contact angle and base radius overpredict the droplet lifetime. Likewise, the linear extrapolation of limited experimental data for transient droplet volume underpredicts the droplet lifetime. Correct methods of extrapolating limited experimental data for transient droplet parameters are described, discussed, and validated. The new methods removed inconsistencies in the previous theory and experimental analysis. Master equations and master curves for the droplet lifetime for the two evaporation modes are obtained and experimentally confirmed.

The formation of water-stable aggregates in finely textured and polymineral magnetite Fe ore tailings is one of the critical processes in eco-engineering tailings into soil-like substrates as a new way to rehabilitate the tailings. Organic matter (OM) amendment and plant colonization are considered to be effective in enhancing water-stable aggregation, but the underlying mechanisms have not yet been elucidated. The present study aimed to characterize detailed changes in physicochemistry, Fe-bearing mineralogy, and organo-mineral interactions in magnetite Fe ore tailings subject to the combined treatments of OM amendment and plant colonization, by employing various microspectroscopic methods, including synchrotron-based X-ray absorption fine structure spectroscopy and nanoscale secondary ion mass spectroscopy. The results indicated that OM amendment and plant colonization neutralized the tailings’ alkaline pH and facilitated water-stable aggregate formation. The resultant aggregates were consequences of ligand-promoted bioweathering of primary Fe-bearing minerals (mainly biotite-like minerals) and the formation of secondary Fe-rich mineral gels. Especially, the sequestration of OM (rich in carboxyl, aromatic, and/or carbonyl C) by Fe-rich minerals via ligand-exchange and/or hydrophobic interactions contributed to the aggregation. These findings have uncovered the processes and mechanisms of water-stable aggregate formation driven by OM amendment and plant colonization in alkaline Fe ore tailings, thus providing important basis for eco-engineered pedogenesis in the tailings.