Spreading dynamics of a droplet upon impact with a liquid film containing solid particles.

This study examines how a deionized water droplet behaves when it centrally collides with a liquid film containing TiO2 nanoparticles at low impact velocities, aiming to understand how nanoparticles affect droplet spreading, in particular its maximum spreading diameter. Typically, we found that both the spreading velocity and dynamic contact angle of the droplet would be similarly affected by increasing TiO2 nanoparticle concentration. During retraction, the droplet's dimensionless spreading diameter oscillates, with more pronounced oscillations at higher nanoparticle concentrations. Moreover, both the droplet's maximum dimensionless rebound height and dynamic contact angle show similar trends with increasing TiO2 nanoparticle concentration. Interestingly, we proved that the influence of the solid-liquid interaction (Stokes force) on the fluid during the spreading process accounts for less than 2% of the surface energy when the droplet reaches its maximum spreading diameter, indicating a negligible effect on droplet spreading. We hypothesize that the droplet's initial energy is fully converted into surface energy and viscous dissipation at maximum spreading diameter, which involves viscous dissipation both between the fluid and the solid wall surface and the fluid and solid particle surface. Based on this, we developed a model for predicting the droplet's maximum spreading diameter that includes parameters associated with the solid particles. Compared to models in the literature that do not consider the effect of solid particles, our model aligns more closely with experimental data. The results indicate that adding solid particles leads to increased viscous dissipation, which in turn reduces the droplet's maximum spreading diameter.

[Perspectives in researches on grassland ecology for the early 21st century in China].

Grassland degeneration is a prominent problem in China. More in-depth studies should be carried out on the key problem--grassland degeneration for Chinese grassland ecologists in the early 21st century. Some hot research fields were restoration ecology, interface ecology, grazing ecology, health diagnoses and evaluation for grasslands. Among them, restoration ecology was the foundation for restoring degenerative grasslands; interface ecology involved with the point of contact for analyzing degenerative grasslands; grazing ecology was the effective way to control degenerative grasslands; diagnoses of grassland health benefited accurately appraising degenerative levels of grasslands; and evaluation for grassland was also considered as an estimation on services and benefits of grassland ecosystems.