Milk is a complex mixture containing various types of colloids. The primary colloids present in milk are:1. Emulsion: Milk is an oil-in-water O/W emulsion, where fat globules are dispersed in an aqueous phase. The fat globules are stabilized by a membrane consisting of proteins and phospholipids, which prevents them from coalescing and separating from the water phase.2. Sol: Milk also contains a sol colloid, which consists of soluble proteins, mainly casein micelles, and whey proteins. Casein micelles are large, spherical structures composed of individual casein molecules held together by calcium phosphate nanoclusters and hydrophobic interactions. Whey proteins are soluble globular proteins that remain in the aqueous phase after casein coagulation.The particle interactions in milk are mainly governed by the following forces:1. Electrostatic forces: The casein micelles and whey proteins carry a net negative charge due to the presence of carboxylate and phosphate groups. This negative charge leads to electrostatic repulsion between the particles, preventing them from aggregating and maintaining the stability of the milk.2. Steric forces: The adsorbed layer of proteins and phospholipids on the surface of fat globules provides steric stabilization, preventing the fat globules from coming too close and coalescing.3. Van der Waals forces: These weak attractive forces can act between all types of particles in milk, including fat globules, casein micelles, and whey proteins. However, the repulsive forces mentioned above usually dominate, maintaining the stability of the milk.4. Hydrophobic interactions: These interactions play a crucial role in the formation and stability of casein micelles. The hydrophobic regions of casein molecules tend to cluster together, minimizing their exposure to water and contributing to the formation of the micelle structure.In summary, milk is a complex colloid system containing both emulsion and sol colloids, with fat globules and casein micelles being the primary particles. The stability of milk is maintained by a balance of electrostatic, steric, van der Waals, and hydrophobic forces acting between the particles.