Citation: Zhang, Z.; Peng, F.; Kornev, K.G. The Thickness and Structure of Dip-Coated Polymer Films in the Liquid and Solid States. Micromachines 2022, 13, 982. https:// doi.org/10.3390/mi13070982 Academic Editors: Sangjin Ryu, Moeto Nagai and Seunghee Kim Received: 27 May 2022 Accepted: 17 June 2022 Published: 22 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). micromachines Article The Thickness and Structure of Dip-Coated Polymer Films in the Liquid and Solid States Zhao Zhang , Fei Peng and Konstantin G. Kornev * Department of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA; zhaoz@g.clemson.edu (Z.Z.); fpeng@clemson.edu (F.P.) * Correspondence: kkornev@clemson.edu Abstract: Films formed by dip coating brass wires with dilute and semi-dilute solutions of polyvinyl butyral in benzyl alcohol were studied in their liquid and solid states. While dilute and semi-dilute solutions behaved as Maxwell viscoelastic fluids, the thickness of the liquid films followed the Landau-Levich-Derjaguin prediction for Newtonian fluids. At a very slow rate of coating, the film thickness was difficult to evaluate. Therefore, the dynamic contact angle was studied in detail. We discovered that polymer additives preserve the advancing contact angle at its static value while the receding contact angle follows the Cox–Voinov theory. In contrast, the thickness of solid films does not correlate with the Landau-Levich-Derjaguin predictions. Only solutions of high-molecular-weight polymers form smooth solid films. Solutions of low-molecular-weight polymers may form either solid films with an inhomogeneous roughness or solid polymer domains separated by the dry substrate. In technological applications, very dilute polymer solutions of high-molecular-weight polymers can be used to avoid inhomogeneities in solid films. These solutions form smooth solid films, and the film thickness can be controlled by the experimental coating conditions. Keywords: dip coating; Landau-Levich-Derjaguin theory; polymer solutions; solid films 1. Introduction Advances in coating processes have evolved in parallel with new technological ad- vances and material development [1–4] and can be classified with respect to the processing protocols as dry processes (CVD, PVD, PECVD, etc.) [5,6] and wet processes (spray coat- ing, brush casting, spin coating, dip coating, etc.) [7–10]. Among these coating methods, dip-coating is always attractive because of its simplicity, low cost, and facile control [7,11]. Dip coating of Newtonian viscous fluids on different substrates has been studied in various applications and significant progress in the understanding of the fluid mechanics of film formation has been achieved [11–20]. Among different substrates, fibers and rods are the most challenging to coat: the curvature of the cylinder significantly influences the film deposition kinetics by altering the flow pattern in the meniscus region [21,22]. The classical Landau-Levich-Derjaguin (LLD) theory of dip coating of fibers (see Equation (10) below) can be used only in a narrow range of fiber radii, when the fiber radius R is much smaller than the capillary length l =  σ gρ l , R l ≤ 0.13, where ρ l is the density of the liquid, g is the acceleration due to gravity, and σ is the surface tension [23]. This limitation adds to the difficulties of predicting the behavior of coating films formed from complex fluids such as polymer solutions. In all wet coating processes, including dip coating, one needs to control the final thickness of the coating layer after its solidification [7,8,21,24–30]. In many cases, the coating liquid is made of rheologically complex fluids [8,29,31–34], which are difficult to handle and evenly deposit on substrates. For example, almost all ceramic precursors for ceramic coatings, such as Al 2 O 3 , TiO 2 , and mullite precursors, exhibit non-Newtonian features such as viscoelasticity [35] or nonlinear viscosity [36–38]. This complexity of the Micromachines 2022, 13, 982. https://doi.org/10.3390/mi13070982 https://www.mdpi.com/journal/micromachines