Physical aspects - Foam formation
7 important questions on Physical aspects - Foam formation
What are 3 differences between emulsions and foams?
- Air bubbles are much larger than emulsion droplets (mm vs um)
- Density difference between dispersed and continuous phase is much larger in foam than in emulsion
- Solubility of gas in water is usually much higher than oil in water
This all leads to faster creaming/disproportionation (Ostwald ripening in emulsions)
What is needed for the production of foam?
- Low surface tension --> creation of a lot of surface area, surface active agents needed
- Fast diffusion to interfaces (surface coverage)
What does the energy input in a foam determine?
- Foam height
- Foam volume
- Bubble size
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What is the difference between the foam formation with a globular or flexible protein?
A flexible protein (e.g. Gelatin) has a low foamability at pI, and higher foamability further from pI.
What does the large density difference between air and liquid in foam cause?
Which 2 different types of air flow can occur in foams and what is the result?
- Disproportionation --> within the foam
- Coalescence --> outside
The air bubbles will grow and the entrapped air volume in the foam will decrease.
What is the difference between foam formers and foam stabilizers?
- Foam formers
- Small molecules: fast diffusion
- Large hydrophobic groups: hydrophobicity, composition of proteins
- Foam stabilizers
- Large molecules (proteins): thick interface, more stable
- Protein coverage (effect of pH, salt)
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