Unevenness of Thin Liquid Layer by Contact Angle Variation of Substrate during Coating Process

During a thin film application, the surface of the coating liquid applied to the substrate becomes uneven because of the geometry of the substrate, viscosity of the coating liquid, surface tension, and its contact angle with the substrate. The surface is particularly uneven at the edge corner portion of the substrate and is thicker than the average coating thickness. This study used the volume-of-fluid (VOF) method to examine the surface unevenness of the coating liquid in terms of the contact angle of the substrate surface and sides. After the coating liquid was evenly applied to the substrate, the maximum height of the uneven region of the coating liquid at the edge of the substrate increased as time passed. The point of maximum height moved away from the edge corner portion of the substrate. The coating liquid applied to the substrate with a contact angle less than 90° exhibited a pinning effect in which the contact point was fixed at the edge. The surface unevenness was more pronounced in the absence of the pinning effect than in its presence, due to the effects of the viscosity of the coating fluid and the surface energy of the substrate.