Periodic Phase-Separation During Meniscus-Guided Deposition

The meniscus-guided coating (MGC) of a binary fluid mixture containing a solute and a volatile solvent that undergoes spinodal decomposition is investigated numerically. Motivation is the evaporation-driven deposition of material during the fabrication of organic thin film electronics. 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 is found. This transition originates from the competition between the injection of the solution into the film and diffusive transport that cannot keep up with replenishing the depletion of solute near the domains. The critical velocity of the transition is determined by the ratio of two 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 diffusivity of the solute and the coating velocity. For coating below the critical velocity, the domain size and deposition wavelength are proportional to a solute depletion length. This competition in the mass transport is inherent in any kind of unidirectional deposition of demixing solutions and the findings should therefore apply to many coating techniques and forced demixing processes.