Physical aspects - Instability mechanisms

8 important questions on Physical aspects - Instability mechanisms

Which 4 processes affect the emulsion stability?

  1. Flocculation (aggregation)
  2. Coalescence
  3. Creaming and sedimentation
  4. Ostwald ripening (disproportionation)

What happens when flocculation occurs in emulsions?

Oil droplets cluster together
  • Low volume fraction of dispersed droplets --> aggregation leads to formation of large clusters/aggregates/flocs --> faster creaming
  • High fraction dispersed oil droplets --> lead to formation 3D space filling network, system will act as a gel

What happens when coalescence occurs in emulsions?

The merging of oil droplets. When a droplet comes into contact with another droplet, they will merge into one droplet.
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What happens when creaming occurs in emulsions?

Upward move of oil droplets due to a density difference.
  • Oil droplets are lighter than continuous phase --> oil droplets cream
  • Heavier dispersed particles --> particles will move down --> sedimentation

What happens when Ostwald ripening occurs in emulsions?

A change in droplet size. The droplets do not merge, large droplets grow into larger, small droplets become smaller.

How is Ostwald ripening in emulsions caused?

By a difference in the Laplace pressure. Small droplets have a larger pressure, larger droplets have a smaller pressure. Difference creates a diffusion of the oil, from high to low pressure, from small to larger droplet. Smaller droplets will become smaller, larger droplets become larger --> coarsening of the emulsion.

Ostwald ripening can only take place in emulsions when?

  • Emulsions with water soluble oils --> oil not soluble, cannot diffuse, no Ostwald ripening. More soluble the oil --> larger degree of Ostwald ripening, faster process
  • Emulsion need wide size distribution --> when droplets re same size, no difference in pressure, no diffusion of oil. Bigger difference in size, larger difference in pressure, larger effect on Ostwald ripening.

What is the disproportionation in foams influenced by?

  • Solubility of air in water --> air is more soluble in water compared to oil in water --> rate faster
  • Gas with different solubility --> less soluble gas, less soluble in water than air, e.g. Nitrogen, less fast disproportionation

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