Foam Stability

Question 1

How are foams made? (2& ex)

Answer 1

Supersaturating liquid with gas

• dissolving gas under pressure and then releasing the pressure (as in carbonated drinks)
• let gas form in situ (as in fermenting dough)
• nucleation of gas bubbles is the critical process

Mechanical formation

• injection of gas through narrow opening
• beating (formation of bubbles in wake of fast moving rod)

Question 2

Bubble formation

Answer 2

1) Smaller bubbles dissolve, bigger bubbles grow in size by diffusion of gas through continuous phase
- disproportionation or Ostwald ripening

2) Bubbles cream –> foam layer on top of bulk liquid
3) Bubbles deform one another –> polyhedral foam (with plateau borders)

Question 3

Foam stability

Answer 3

• Liquid drains from foam into bulk
• Lamallae between foam bubbles rupture, leading to coalescence
• The presence of fatty particles at an air/water interface may destabilise the system

Question 4

Foam drainage

Answer 4

• Water drains from foam under gravity

- Drainage along lamallae

Question 5

Antifoam or defoamer

Answer 5

Antifoam agent enters into the liquid lamella

• -> the lamella starts thinning –>
• -> foam lamella breaks

Question 6

Foam rupture by fat spreading

Answer 6

It is the radial movement which causes a local thinning of the film which may ultimately result in the collapse of the bubble when enough of the liquid has been squeezed away

• poor foaming of whole milk
• effect of egg yolk on beaten egg white

Question 7

Define Disproportionation
driven by ?
Equation?

Answer 7

The growth of bigger gas bubbles at the expense of the smaller ones

This is driven by the Laplace pressure
= pressure difference between inside and outside of a curved surface
= equal to twice the surface tension divided by the radius of the bubble
= pressure is greater for small bubbles

Question 8

The Laplace pressure difference leads to

Answer 8

leads to a concentration gradient of gas in the liquid between two bubbles of different sizes

Question 9

The equation to calculate the radius of the smaller bubble as a function of time

Answer 9

slide 29

Question 10

other factors of Disproportionation

Answer 10

Solubility of gas is important

CO2 is 55x more soluble than N2

Question 11

Self-acceleration

Answer 11

Pressure gradient increases as bubble gets smaller

It is this behaviour leading to the loss of the smaller bubbles which leads to foam collapse

Question 12

We have assumed constant surface tension, BUT most surfaces, especially in foods, are ____

Answer 12

viscoelastic

Question 13

Changes in surface dilation viscosity (SDV) means

Answer 13

means that the surface tension in a shrinking bubble is less than the equlibrium surface tension

• SDV is strongly dependent on the rate of compression of the surface
• this is non-Newtonian behaviour

This leads to deviations from ideal bubble behaviour and it is not possible to predict the rate of foam collapse

Question 14

Foam evaporation

Answer 14

The evaporation of water vapour from thin films may play a role in foam destabilisation

This is important in parts of the foam exposed to the external atmosphere which is usually not saturated with respect to the continuous phase

Evaporation reduces film stability by thinning the film to less than the critical film thickness