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Monte Carlo Simulation of Evaporation Driven Self-Assembly in Suspension of Colloidal Rods

The vertical drying of a colloidal film containing rod-like particles was studied by means of kinetic Monte Carlo (MC) simulation. The problem was approached using a two-dimensional square lattice and the rods were represented as linear $k$-mers (i.e., particles occupying $k$ adjacent sites). The initial state before drying was produced using a model of random sequential adsorption (RSA) with isotropic orientations of the $k$-mers (orientation of the $k$-mers along horizontal $x$ and vertical $y$ directions are equiprobable). In the RSA model, overlapping of the $k$-mers is forbidden. During the evaporation, an upper interface falls with a linear velocity of $u$ in the vertical direction and the $k$-mers undergo translation Brownian motion. The MC simulations were run at different initial concentrations, $p_i$, ($p_i \in [0, p_j]$, where $p_j$ is the jamming concentration), lengths of $k$-mers ($k \in [1, 12]$), and solvent evaporation rates, $u$. For completely dried films, the spatial distributions of $k$-mers and their electrical conductivities in both $x$ and $y$ directions were examined. Significant evaporation-driven self-assembly and orientation stratification of the $k$-mers oriented along the $x$ and $y$ directions were observed. The extent of stratification increased with increasing value of $k$. The anisotropy of the electrical conductivity of the film can be finely regulated by changes in the values of $p_i$, $k$ and $u$.