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Surface is a very important branch of science that touches all facets of our lives. Fundamental understanding of the interactions between a liquid droplet and a solid surface, such as wetting, spreading, adhesion, and de-wetting, is not only crucial to the science itself, but also of tremendous values to many seemingly unrelated applications. In our daily activities such as cleaning and washing, surface active materials known as detergents are commonly used to aid detachments of dirt and stain from a solid surface initiating the cleaning process. Although we have been taken the process for granted, there is a lot of science involved in washing and cleaning, e.g., the type of detergent used, its efficiency, cost, and the impact to human and environment. Even as simple as ink writing with a fountain pen, Kim and co-workers [1] found that the line width of the ink image on paper depends on the speed of the pen as well as the physicochemical properties of both ink and paper. Controlling ink wetting and spreading is crucial in avoiding clogging and ink spreading. Similarly, mastering and controlling liquid–solid interactions have become critical skills in many industries, such as in the design and manufacturing of paints, stain/soil-resistant textiles and clothing, anticorrosion surfaces for bridges and other metal structures, antifouling coatings for ships and marine structures, anti-icing surfaces for power line, airplane and roof-top, mining, and petroleum fracking. Whether it is surface design or process development, knowledge in manipulating liquid–surface interaction in the “right” place at the “right” time is imperative in microfluidic device [2, 3], oil/fluid transportation [4], and printing and coating [5, 6], just to name a few. In printing, understanding the wetting and de-wetting of ink on different printing surfaces is critical to the quality of the final print. As a society, we have been practicing offset printing for more than a century, and the entire print process is a good illustration on the importance of ink–surface interaction [7]. A schematic for the offset printing process is shown in Fig. 1.1.

Fig. 1.1
figure 1

A schematic of the offset printing process

Full size image

In a very simple term, the offset printing process involves (1) wetting of the plate cylinder with a fountain solution image wise through the dampening system, followed by (2) inking the plate cylinder with the inking system, (3) transfer of the ink image from the plate cylinder to an offset blanket, and (4) fixing it on paper. In the first step, the desirable outcome is to have the fountain solution first wets and then pins on the surface of the plate cylinder. While wetting is required for the formation of the wetted images, a successful pinning of the contact line on the plate cylinder is needed to ensure image integrity and resolution control. Offset inks are acrylate materials, and they will be repelled by areas that are wetted with the fountain solution. On the other hand, ink will be attracted and adhered to the oleophilic areas on the plate cylinder surface. Controlling ink adhesion in the oleophilic region and repelling it from the fountain solution wetted areas are critical to the image quality of the output. Afterward, the transfer of the ink image to the offset blanket and then paper is more straightforward as these surfaces are selected based on their relative affinity to the ink materials. Although printing (on papers) is an industry in decline, printing has evolved and has become a manufacturing technology for printed electronics, flexible and wearable devices, ceramics, textiles, solar cells, and many others [8–20]. Arias et al. [19] showed that balance between pinning and overspreading of printed liquid ink on the solid surface is very important in defining the position, resolution, and size in the fabrication of thin-film-transistor array. Jetted ink drop is also known to form the so-called coffee ring stain due to un-optimized spreading and drying, which is detrimental to the quality of the printed image and ultimately the performance of the printed device [21]. The need of characterizing surfaces and understanding liquid–surface interaction continues to play a critical role in the modern technology arena [8].

Studies of surface and liquid–surface interaction can be traced back to Thomas Young’s work two centuries ago [22]. In his legendary article entitled “An Essay on the Cohesion of Fluids,” he described his study of wetting of glass by water and mercury, wetting of various metal surfaces by mercury and the capillary effect. He descriptively stated that the angle of contact between a liquid on a solid surface is the result of a mechanical equilibrium among the three surface tensions at the contact line. They are the liquid, solid and liquid–solid interfacial surface tensions . This has become the famous Young’s equation in the literature. Today, surface is a multidiscipline subject and is studied by chemists, physicists, and engineers both theoretically and experimentally worldwide. Research topics range from fundamental understanding of the wetting, spreading, adhesion, and de-wetting phenomena to their applications in materials, surface coatings, and device innovations. Unfortunately, there have been continuous confusion and faulty intuition about the measurements, data interpretation, and definitions in the surface literature. One of the reasons is due to the multidiscipline nature in this field, where researchers with a very diverse background are investigating surfaces together. Another reason, which was pointed out by Gao and McCarthy, is their insufficient basic surface training in school [23, 24]. This flaw in surface science has been recognized by expert researchers. In 2009, Gao and McCarthy published an excellent article titled “Wetting 101°” where they discussed the faulty perception and used demonstrative examples to illustrate the correct basic concepts [24]. While we are certainly benefitted from the article and references therein, the faulty perception and confusion are continuing unfortunately.

Contact angle measurement is commonly used to characterize a surface and to study various wetting and de-wetting phenomena . While the measurement is simple, the interpretation is not. This point has been noted by many surface investigators in the past, e.g., Pease in 1945 [25], Morra et al. [26] in 1990, Kwok and Neumann [27] in 1999 and more recently by Marmur [28] as well as Strobel and Lyons [29]. Prior to data interpretation, one has to make sure that the measurement apparatus and procedures are impeccable. Over the years, many have voiced concerns over surface preparation and conditioning, measurement procedure and technique, and data analysis [26, 30–35]. It is therefore imperative for the community to have a set of common measurement protocol or guideline, so that inter laboratory data can be compared. Discrepancy in conclusion can be rationalized without concerns of experimental setups or procedures.

In view of the ongoing discussions on measurement procedures, data interpretation, terminologies, and definitions, the surface community would be benefitted for an overview of the past conversations and a recent account of the resolution. The objective of this book is to provide a coherent, easy to understand, fundamental picture on surface wetting and characterization . In Chap. 2, we first summarize the best practices in static and dynamic contact angle measurements. This may be served as a standard protocol for surface researchers and practitioners in the future. From there, data collected in different laboratories can be compared without casting doubt to each other. Some of the fundamental measurement issues, such as drop size, drop dispensing procedure, lab ambient condition (temperature and relative humidity), and method of calculating the contact angle will be discussed. This is followed by discussions of the concept of wetting, first on smooth surfaces in Chap. 3 and then on rough surfaces in Chap. 4. Important concepts such as (1) the Young’s angle is a result of a mechanical equilibrium, not thermodynamic equilibrium, at the three phase contact line, (2) the liquid droplets are all in their metastable states in their static, advancing and receding positions, and (3) contact line, not contact area determines the contact angle, will be articulated with conclusions that are supported by both experimental and theoretical data. The rationale for the shortfall in both the Wenzel and Cassie–Baxter analysis of wetting on rough and porous surfaces is summarized. Recent results on factors that govern wettability and wetting dynamics of liquid on rough surfaces are overviewed. Visuals on the distortion of the contact line by surface roughness as well as the structure of the liquid–solid–air composite interfaces are reported. Chapter 4 also includes discussions on the design principles for superhydrophobic and superoleophobic surfaces as well as the challenges related to technology implementation.

Due to simplicity of the contact angle measurement, it has become a popular tool to characterize the property of a surface or probing liquid–solid interactions . However, the literature is full of conflicting information owing to the difficulty in correctly interpreting contact angle data. Chapter 5 is devoted to provide experimental data to shrine light into this specific issue. Evidence is provided that advancing and receding contact angles are measurement of surface wettability and adhesion , respectively. The stickiness of liquid on surface can be predicted from the sliding angle. A recommendation for surface characterization is provided. As for contact angle hysteresis, the difference between advancing and receding contact angle, it is shown to relate to adhesion and stickiness qualitatively only. More work is needed to clarify the origin of contact angle hysteresis as well as its role in surface characterization. As a fundamental book, it is hard not to discuss the “pains” we have in surface definitions and terminologies as well as early work on the use of contact angle to determine the surface energy of solid, where the practice and its usefulness have been constantly challenged. In Chaps. 6 and 7, updates on the definitions for hydrophilicity/hydrophobicity, oleophilicity/oleophobicity, and other related terminologies will be provided. The different methodologies used to determine surface energy will briefly be reviewed. Fundamental issues will be discussed, and a path to move forward is shared. We are not taking side in these discussions, rather updating the readers with the latest experimental data and the current status. This book will conclude with a brief look back on the evolution of surface science, followed by a summary of some of the basic concepts as a reminder for new or young researchers in this field. It is our hope that surface science will prosper when researchers in the next-generation are armed with improved basic concepts and fundamentals.