Periodic phase-separation during meniscus-guided deposition

Abstract: 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.