Intermolecular and interparticle forces
An Example: Milk
Milk is an example of a colloidal dispersion, which illustrates several key features common to other such colloids. On its own, fresh whole milk segregates into a cream layer floating on top of a fat-depleted liquid. However homogenization was developed in France around 1900 to overcome this problem. By forcing hot milk through a surface of small nozzles, turbulence in the fluid tears the 4-micron fat globules into smaller particles closer to a micron in size. The original membrane surrounding the globules is insufficient to cover the greatly increased surface area of the globules. Since they are hydrophobic, they attract casein proteins from the surrounding liquid, which weights them down. The combination of smaller particle size and greater density allows Brownian motion to keep the particles in suspension.
Aggregation is another phenomenon that can lead to phase separation in a colloid. In the case of milk, additional ingredients or a change in acidity can cause the globules to stick together and separate from the liquid. This can happen with the addition of an acid, such as lemon juice. The astringent tannins in beverages like tea and coffee make this process more likely (which could be why one rarely adds both milk and lemon juice to tea).
To read more about the gastroscience of milk, see On Food and Cooking by Harold McGee (in the section "Unfermented Dairy Products") or Chapter 4 ("Colloidal dispersions") of Soft Condensed Matter by Richard A. L. Jones.
Flow properties from molecular energies
Forces near surfaces
- Bulk phases are characterized by density, free energy and entropy – not by forces.
- Molecular forces average out.
- Not so at surfaces.
(Modern) forces near sufaces
- (a) This potential is typical of vacuum interactions but is also common in liquids. Both molecules and particles attract each other.
- (b) Molecules attract each other; particles effectively repel each other.
- (c) Weak minimum. Molecules repel, particles attract.
- (d) Molecules attract strongly, particles attract weakly.
- (e) Molecules attract weakly, particles attract strongly.
- (f) Molecules repel, particles repel.
Interactions from molecular attraction
- (a) A molecule near a flat surface.
- (b) A sphere near a flat surface.
- (c) Two flat surfaces.
Derjaguin Force Approximation
Where W(D)is the energy of interaction of two flat plates.