Kinetics of particle settling during solvent evaporation

Simulations based on the Discrete Element Method have been carried out in order to study the kinetics of 100 nm particle deposition during solvent evaporation. The solvent properties were considered to be similar to water and the particles possessed a density of 1370 kg/m3. The computer model considered the Brownian motion, van der Waals interactions (Hamaker constant =1.0x10-20 J) and hydrodynamic resistance function. The evaporation rate was simulated based on a liquid pool evaporation model, which is a function of the area exposed, liquid properties, temperature and liquid saturation vapour pressure. As the area of liquid interface exposed is directly related to the evaporation rate, a discretised pixel map of the surface of the liquid was created. The decrease in the level of the liquid due to evaporation reduced the space available for the particles to move with the consequence that the aggregation rate increased. In addition, the decrease in the level of the liquid forced the particles to settle. When the height of the liquid was comparable to the height of the sediment, the evaporation process slowed down due to the reduction in the surface area available for evaporation. These findings foster a deeper understanding into the aggregation and deposition of nanoparticles and provide the first step to the analysis of the structure of the formed sediments which is our ultimate goal.