Capillary-Induced Assembly of Janus Particles in Drying Films*

We perform mesoscale simulations to study the self-assembly and deposition of amphiphilic Janus particles in a drying thin film. Many-body dissipative particle dynamics is applied to model the evaporation of particle-laden films. For charged particles, electrostatic interactions are calculated using a finite-difference Poisson solver, which takes account for the dielectric contrast across the liquid-vapor interface. Given the unique anisotropy of Janus particles, we probe their translational and rotational dynamics in a thin film and subsequent assembly after adsorption to the interface. The simulation provides quantitative insight into the interplay between the electrostatic interactions with substrate, image force due to dielectric discontinuity, and interparticle electrostatic and capillary interactions. Janus particles assemble into a large-scale open fractal structure with identical orientation. In contrast, homogeneous particles form only isolated small aggregates having random orientation. These structures agree well with the deposits observed in experiments. Simulation further indicates the dominant force is capillary attraction due to the interface pinning at rough Janus boundaries.