Wetting of solid surfaces: fundamentals and charge effects.

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 wetting and vapor adsorption and wetting are discussed first. The characterization of low-energy surfaces is discussed 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 solutions 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 potential (electrowetting). The physical interpretation of Young-Lippmann equation, which describes EWOD, is critically examined and contact angle saturation is briefly discussed.