Stratification and skin-layer formation in evaporating droplets containing bidisperse colloids

The evaporation of colloidal dispersions in droplets is characterized by particle accumulation at the air–liquid interface, often leading to the formation of particle stratification and a skin layer. To better understand these dynamics, we simulate a bidisperse suspension with a particle size ratio of 3 confined within a shrinking spherical boundary, focusing on how particle transport, governed by the Péclet number (Pe), influences microstructure development. Two complementary simulation approaches, the Langevin dynamics (LD) method and the lattice Boltzmann (LB) method, were used to evaluate the role of hydrodynamic interactions (HIs) between colloids on droplet drying. At high Pe, the LD simulations exhibit a “small-on-skin” pattern, while the LB simulations show a mixed distribution. However, at moderate Pe, the LD simulation maintains a “small-on-skin” structure, whereas the LB simulation shows a “large-on-skin” configuration. Notably, a mechanically stiff skin layer formed by particle clustering emerges only in the LB simulations, highlighting the key role of HIs in stress buildup and skin formation. These findings demonstrate that HIs play a crucial role in determining stratification patterns and cluster formation during droplet drying, and thus provide new insights into supraparticle assembly.