Instability mechanisms - creaming and sedimentation

Question 1

When occurs creaming and sedimentation?

Answer 1

When is a difference in the density between the dispersed phase and the continuous phase. Oil droplets lighter than water and the droplets move up(creaming). For example with proteins, sedimentation.

Question 2

Predict the rate of sedimentation or creaming:

Answer 2

1. Gravitational force (down)
2. Buoyancy force( up) (negative )
3. Friction force ( up ) (negative)
   The direction of the droplet is down.

Question 3

What is steady state?

Answer 3

Constant velocity. Fg + Fb + Ffric=0

Despejar la ecuación hasta obtener la velocidad fo a particle in a continuous phase.

Question 4

The expression of the creaming sedimentation velocity depends on three different factors? When is going to decrease the velocity?

Answer 4

1. Density difference between the particle and the continuous phase. (density dispersed phase- density liquid)
2. Size of the particles.
3. Viscosity of the continuous phase.
   * When the density difference is smaller *When the radius of the particle is smaller. *When the viscosity of the continuous phase is larger.

Question 5

Conclusion of the density, difference between sediment and cream:

Answer 5

1. For density of the dispersed phase > density liquid–> v>0, sediment.
2. For density dispersed phase< density liquid–> v<0, cream

Question 6

In some products the dispersed particles are not free to move, but they can be trapped in a network . Give an example and what happen:

Answer 6

Network with polysaccharides, and this network provides a certain yield stress. When it rest do not move, when the sample is stirred or shaken, the network breaks or yields and the droplets are able to start to move.
Dressing with herbs or other plant particles.

Question 7

Which forces acts in the yield stress:

Answer 7

1. Gravitational force
2. Buoyancy force
3. Elastic force: proporcional to the contact between the particles and the continuous matrix.
   Sediment: formula sheet
   Cream: formula sheet
   This expression only provides information on when the droplets move, but not how fast they move.

Question 8

• Sediment (settle) is relevant for …….

* Cream is relevant for ……

Answer 8

• Sediment: dispersion. Eg. chocolate milk, cocoa particles are dispersed in the thickened milk.
• Cream: emulsions. Eg. milk products, dressing ,etc.