  We numerically investigate the meniscus-guided coating of a binary fluid
mixture containing a solute and a volatile solvent that phase separates via
spinodal decomposition. Motivation is the evaporation-driven deposition of
material during the fabrication of organic thin film electronics. We find a
transition in the phase-separation morphology from an array of droplet-shaped
domains deposited periodically parallel to the slot opening to isotropically
dispersed solute-rich droplets with increasing coating velocity. This
transition originates from the competition between the hydrodynamic injection
of the solution into the film and diffusive transport that cannot keep up with
replenishing the depletion of solute near the solute-rich domains. The critical
velocity separating the two regimes and the characteristic length scale of the
phase-separated morphologies are determined by the ratio of two emergent length
scales: (i) the spinodal length, which implicitly depends on the evaporation
rate and the properties of the solution, and (ii) a depletion length
proportional to the ratio of the tracer diffusivity of the solute and the
coating velocity. For coating below the critical velocity, an array of
droplet-shaped domains is deposited periodically parallel to the slot opening,
with the domain size and deposition wavelength proportional to a solute
depletion length. As the competition in the mass transport is inherent in any
kind of unidirectional deposition of demixing solutions, our findings should
apply to a broad range of coating techniques and forced demixing processes.
