Nanomaterials 2022, 12, 2600. https://doi.org/10.3390/nano12152600 www.mdpi.com/journal/nanomaterials Review Control of the Drying Patterns for Complex Colloidal Solutions and Their Applications Saebom Lee 1,† , Tiara A. M. 2,3,4,† , Gyoujin Cho 2,3,4,5, * and Jinkee Lee 1,3, * 1 School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Korea; leesb@skku.edu 2 Department of Biophysics, Sungkyunkwan University, Suwon 16419, Korea; tiara.am@gmail.com 3 Institute of Quantum Biophysics, Sungkyunkwan University, Suwon 16419, Korea 4 Research Engineering Center for R2R Printed Flexible Computer, Sungkyunkwan University, Suwon 16419, Korea 5 Department of Intelligent Precision Healthcare Convergence, Sungkyunkwan University, Suwon 16419, Korea * Correspondence: gcho1004@skku.edu (G.C.); lee.jinkee@skku.edu (J.L.) † These authors contributed equally to this work. Abstract: The uneven deposition at the edges of an evaporating droplet, termed the coffee-ring ef- fect, has been extensively studied during the past few decades to better understand the underlying cause, namely the flow dynamics, and the subsequent patterns formed after drying. The non-uni- form evaporation rate across the colloidal droplet hampers the formation of a uniform and homo- geneous film in printed electronics, rechargeable batteries, etc., and often causes device failures. This review aims to highlight the diverse range of techniques used to alleviate the coffee-ring effect, from classic methods such as adding chemical additives, applying external sources, and manipulat- ing geometrical configurations to recently developed advancements, specifically using bubbles, hu- midity, confined systems, etc., which do not involve modification of surface, particle or liquid prop- erties. Each of these methodologies mitigates the edge deposition via multi-body interactions, for example, particle–liquid, particle-particle, particle–solid interfaces and particle–flow interactions. The mechanisms behind each of these approaches help to find methods to inhibit the non-uniform film formation, and the corresponding applications have been discussed together with a critical comparison in detail. This review could pave the way for developing inks and processes to apply in functional coatings and printed electronic devices with improved efficiency and device yield. Keywords: coffee-ring effect; evaporation; interfacial flow; deposition patterns 1. Introduction Deposition patterns from an evaporating droplet containing solute or colloidal par- ticles leave a ring-like stain at the droplet periphery, termed the “coffee-ring effect”. Col- loidal solutions, which consist of nano/microparticles dispersed in solvents, are useful for a variety of technological applications such as printing [1–4], coating [5–15], micropattern- ing [16–18] and bio arrays [19–21] owing to their functionalities. For example, quantum dots (QDs) are semiconductor nanoparticles with tunable optical properties depending on their size and photochemical stability and have been proven suitable for the light-emit- ting display industry [22,23]. Mixtures containing silver, graphene nanoparticles, carbon nanotubes (CNTs), etc. are used in the fabrication of electronic devices due to their high conductivity, compatibility with substrates and excellent mechanical properties [24–28]. During the manufacturing process, the printed film displays non-uniform 3D morphol- ogy after solvent evaporation due to the coffee-ring effect, leading to low-resolution pat- terns and challenges in reproducing similar patterns, subsequently decreasing the perfor- mance of the device. Therefore, it is important to understand the dynamics of dispersed Citation: Lee, S.; A.M., T.; Cho, G.; Lee, J. Control of the Drying Patterns for Complex Colloidal Solutions and Their Applications. Nanomaterials 2022, 12, 2600. https://doi.org/10.3390/ nano12152600 Academic Editor: Alexander Wittemann Received: 5 July 2022 Accepted: 25 July 2022 Published: 28 July 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).