Connecting monotonic and oscillatory motions of the meniscus of a volatile polymer solution to polymer transport and deposit morphology

Ofer Manor, Chemical Engineering, Technion, Haifa, Israel
Mohammad Abo-Jabal, Chemical Engineering, Technion, Haifa, Israel

We study the interplay of the evaporative receding motion of the meniscus of a polymer solution and the resulting pattern deposition of the polymer by vaporizing a solution of poly-methyl-methacrylate (PMMA) in toluene inside a micro-chamber. We observe qualitatively different deposition patterns when varying the molecular mass of the polymer, the initial polymer concentration, and the temperature and dimensions of the micro-chamber. The motion modus of the three-phase contact line between the solution, its vapor, and the substrate correlates directly with the modus of deposition. Both modi change with the Peclet number, characterizing the ratio between rate of polymer convection which results from solvent evaporation and the rate of polymer diffusion, which is connected to the molecular mass of the polymer. At low Peclet numbers the contact line moves monotonically and deposition is continuous, producing a homogeneous deposit layer. At larger Peclet numbers, we find an oscillatory motion of the contact line; the deposition is then periodic, producing a stripe pattern. The oscillatory motion differs from the well known stick-slip motion reported in the absence of confinement. The confinement reduces the evaporation rate of the solvent, which may support periodic sequences of advancing and receding contact line motions that overall de-wet the substrate. This oscillatory motion is attributed to the opposing influences of evaporation and Marangoni forces. The Marangoni contribution results from the dependence of the surface tension of the solution on polymer concentration. The occurring qualitative transitions in the deposition of the polymer are studied in detail and are systematically presented in the form of morphological phase diagrams.