![]() High centrifugal force during spin-coating is often a cause of elongation of the mentioned structures. The solvent-depleted areas have higher surface tension and thus pull the liquid towards these areas and up, while the solvent-rich areas sink, forming valleys. The second is the surface tension gradient due to local composition differences, which manifests in solvent-rich or solvent-depleted areas 18. One is the flow of the liquid due to thermal convection. This undulation is often attributed to two interconnected phenomena. These events may lead to the unnecessary or intended waviness of the coating profile. ![]() During coating formation, a range of events attributed to the local thermal instabilities takes place: heat transfer between the bottom and top layers of the film heat transfer from the air above the layer heating the substrate by the surrounding air and local lateral temperature variance due to the thermal conductivity and heat capacity of the substrate 23. The time necessary for the morphology to stop evolving depended on the solvent evaporation rate, solution viscosity, spinning rate, and substrate thermal properties 21, 22, 23. As a result, the morphology becomes frozen in time before reaching equilibrium. Further spinning of the solution leads to gel formation, which eventually slows down the diffusion inside the film. It has been argued that such phase separation often starts in the early stages of the regime controlled by evaporation 19, 20. Changes in the solvent volume fraction and temperature often lead to unintentional or intentional liquid–liquid phase separation 19. Later, when a flat layer of the solution was obtained, the evaporation of the solvent was responsible for further thinning of this layer 18. This regime is often referred to as hydrodynamic thinning. First, the liquid solution droplet is spread hydrodynamically over the substrate. ![]() The spin coating can be divided into time regions. Thus, solvent and temperature quench occurs. Subsequently, the substrate is rotated very quickly to cover it uniformly with the solution's liquid film. During spin-coating, a droplet of a mixture of a solvent and one or two polymers is dropped onto the substrate. These processes rely heavily on the interaction dynamics between the solvent, the polymer, and the vapours in the vicinity of the surface. These methods were applied for manufacturing organic ferroelectric switches 7, light-emitting devices 8, sensors 9, 10, drug delivery systems 11, 12, biologically active surfaces 13, 14, functional nanostructured surfaces 15, 16, and membranes 17. Two widespread methods that allow the creation of a broad range of structures of polymer thin films (PTFs) are spin-coating 5 and breath figures 6. Complex morphologies compromising micro-islands and especially micropillars have gained attention due to their wide range of possible applications, such as their special wetting properties 1, 2, application in studying biofilm formation 3, or controlling stem cell differentiation 4.
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