Wetting of Solid Surfaces: Fundamentals and Charge effects ☆ Luuk K. Koopal ⁎ Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreyenplein 6, 6703HB Wageningen, The Netherlands abstract article info Available online 4 July 2012 Keywords: Wetting Work of adhesion Critical surface tension Surfactant adsorption Protonic surface charge Electrowetting Electronic surface charge Contact angle saturation Wetting is important when solids are brought in contact with a liquid and for movement of micro-drops on solids. The Young equation that describes the wetting of a solid and the relations between adhesion and wet- ting and vapor adsorption and wetting are discussed first. The characterization of low-energy surfaces is dis- cussed as these surfaces are important for electrowetting on a dielectric (EWOD) and the characterization methods reveal that both dispersion and acid–base interactions are important for wetting. Surfactant solu- tions are very suitable for modifying the wetting behavior; the distinct differences between low- and high-energy surfaces in relation to surfactant adsorption are discussed. The effects of surface charges on the wetting behavior are considered for both adsorbed charges and charges due to an applied electric poten- tial (electrowetting). The physical interpretation of Young-Lippmann equation, which describes EWOD, is critically examined and contact angle saturation is briefly discussed. © 2012 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2. Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.1. Young's law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.2. Wetting and adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.3. Wetting and vapor adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3. Characterization of low-energy solids by wetting with pure liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.1. Zisman-method and the critical surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.2. Wu-method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.3. Fowkes-method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.4. Van Oss-method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4. Wetting of solids with surfactant solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.1. Effect of surfactants on the contact angle on low- and high-energy surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.2. Lucassen-Reynders plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 5. Wetting and surface charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.1. Wetting and adsorbed surface charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.2. Wetting and electronic surface charge: electrowetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 5.3. Electrowetting and contact angle saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 1. Introduction Wetting is an important subject in interface science with many prac- tical applications. The work of Ralston [1–8] and his former coworkers [9–13] can be used as an example to illustrate the various practical aspects of wetting. Ralston's interest in mineral flotation has led to his early wetting studies: wetting of model heterogeneous surfaces [1], contact angle measurements on powders [2], and adsorbed surface charge effects on wetting [3]. In later work the kinetic aspects of wetting have been given attention [e.g., 4,5]. Applying an external voltage across an interface also affects the contact angle, this is called electrowetting. Recently, electrowetting of low-energy surfaces is booming and also here Ralston [e.g., 6–8] and his former coworkers [9–13] have made sig- nificant contributions. Most applications of electrowetting are based on Advances in Colloid and Interface Science 179–182 (2012) 29–42 ☆ Dedicated to the occasion of John Ralston's 65th anniversary. ⁎ Tel.: +31 317482629; fax: +31 317483777. E-mail address: luuk.koopal@wur.nl. 0001-8686/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2012.06.009 Contents lists available at SciVerse ScienceDirect Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis