Surface effects including interfacial tension, wettability, contact angles and dispersibility

Abstract

The subject of chapter 3 was emulsions, i.e. the liquid—liquid interface, particularly from a chemical-surfactant point of view, whereas chapter 6 was about gels, and chiefly involved molecular structures. This chapter is mainly concerned with the understanding of physical rather than chemical processes, e.g. wetting and dispersing. In the food industry, this relates to such problems as dissolving milk powder in water, or the dispersion of cocoa solids in milk. This chapter therefore includes details of the physics involved where more than two phases meet, one of which is a solid. When two phases come together they form an interface and four types are readily defined: liquid—gas, liquid—liquid, liquid—solid and solid—gas. The first three of these are examined in detail, as being crucial to food processing. The important physical parameters, such as surface tension, surfactant efficiency and contact angle are considered, both as to how they can be measured, the instrumental methods themselves and how the data can be used. Pitfalls, due to making the wrong measurement, are also noted. The theory of wetting is very complicated, but a résumé is included as an aid to understanding the processes which are taking place. The chapter concludes by examining some of the physical parameters which are important when dispersing powders in liquids.