Quantification of Marangoni flows and film morphology during solid film formation by inkjet printing

We visualized experimentally the internal flow inside inkjet droplets of polystyrene–anisole solution during solid film formation on substrates at room temperature. The effects of contact angle and evaporation rate on the internal flow and film morphology were quantitatively investigated. The transport process during film formation was examined by measuring the relationship between internal flow and film morphology, which provided three remarkable findings. First, self-pinning and the strength of outward flow on the free surface under 2.3 Pa s determined film morphology. The solute distribution, corresponding to rim areas in ring-like films and a convex trough in dot-like films, had already developed at self-pinning. Second, the mass fraction at self-pinning close to the contact line converged to one, regardless of the film morphology. This implies that self-pinning is independent of parameters such as the contact angle and evaporation rate. Third, at room temperature, the solutal Marangoni numbers were 20–30 times larger than the thermal ones. Thus, the outward flow on the free surface caused by the solutal Marangoni effect dominates in droplets before self-pinning. The solutal Marangoni number at self-pinning and thickness variation at the center of the film displayed a good relationship for droplets with different contact angles and evaporation rates. This suggests that film morphology can be technically controlled by solutal Marangoni number at room temperature.