Surface Science Techniques
The book describes the experimental techniques employed to study surfaces and interfaces. The emphasis is on the experimental method. Therefore all chapters start with an introduction of the scientific problem, the theory necessary to understand how the t
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Contact Angle and Wetting Properties Yuehua Yuan and T. Randall Lee
Abstract This chapter highlights a variety of techniques that are commonly used to measure contact angles, including the conventional telescope-goniometer method, the Wilhelmy balance method, and the more recently developed drop-shape analysis methods. The various applications and limitations of these techniques are described. Notably, studies of ultrasmall droplets on solid surfaces allow wetting theories to be tested down to the nanometer scale, bringing new insight to contact angle phenomena and wetting behavior.
1.1 Introduction The topic of wetting has received tremendous interest from both fundamental and applied points of view. It plays an important role in many industrial processes, such as oil recovery, lubrication, liquid coating, printing, and spray quenching [1–6]. In recent years, there has been an increasing interest in the study of superhydrophobic surfaces, due to their potential applications in, for example, self-cleaning, nanofluidics, and electrowetting [7–12]. Wettability studies usually involve the measurement of contact angles as the primary data, which indicates the degree of wetting when a solid and liquid interact. Small contact angles ( 90°) correspond to high wettability, while large contact angles ( 90°) correspond to low wettability. This chapter will begin with an introduction of the fundamental science behind wetting and contact angle phenomena, followed by a comprehensive description of the various techniques used to measure contact angles, as well as their applications and limitations in terms of the geometric forms of solid samples. Most of the techniques can be classified into two main groups: the direct optical method and the indirect force method. Calculations based on measured contact angle values yield an important parameter—the solid surface tension, which quantifies the Y. Yuan · T.R. Lee (B) Department of Chemistry, University of Houston, 4800 Calhoun Road, Houston, TX 77204-5003, USA e-mail: [email protected] Y. Yuan e-mail: [email protected] G. Bracco, B. Holst (eds.), Surface Science Techniques, Springer Series in Surface Sciences 51, DOI 10.1007/978-3-642-34243-1_1, © Springer-Verlag Berlin Heidelberg 2013
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Y. Yuan and T.R. Lee
Fig. 1.1 Illustration of contact angles formed by sessile liquid drops on a smooth homogeneous solid surface
wetting characteristics of a solid material. The criteria of calculating solid surface tension based on experimental contact angle values will be discussed. Finally, the most up-to-date contact angle measurement techniques will be presented and discussed.
1.2 Theoretical Background 1.2.1 Surface Tension and Contact Angle—Young’s Equation Consider a liquid drop resting on a flat, horizontal solid surface (Fig. 1.1). The contact angle is defined as the angle formed by the intersection of the liquid-solid interface and the liquid-vapor interface (geometrically acquired by applying a tangent line from the contact point along the liquid-vapor interface in the droplet profi
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