Types of foam

Spherical foams:

• It has more continuous phase
• Example shaving foam

Polyhedrical foams:

• It has thin continuous phase
• Example beer foam

Spherical foams might transfer to polyhedrical

Transient foams:

• A foam that is not stable tends to collapse quickly
• Examples: detergents, beer, champagne

Stable foams

• A foam that is stable doesn’t tend to collapse quickly
• Examples: styrofoam, bread etc. (since continuous phase is a solid)

Foams

• Associative liquids → interface with air → needs stabilization by surface active molecules

• Surface costs energy! (high surface energy)

• Gas is hydrophobic since there is low inter-molecular interaction

• Pure liquids do not foam

• Creation of foams

– Whipping/vigorous stirring

– Through small pores

– In situ (gas generation)

Characterization of foams

• Overrun- how efficient it is at foaming (volume of foam/volume of liquid)

• Density (volume fraction of gas in foam) - weigh the foam

• Bubble size (easiest is microscopy)

• Yield value – solid properties of foam (how much do you have to tip foam to make it flow)

Stability of foams- The stability of foams is affected by

• Draining
• Coalescence
• Disproportionation

Drainage- liquid between bubbles will drain because of gravity

• Fast without surface film (adsorbed layer of surfactant)
• Slower with surface film
• Rate of drainage depends on the viscosity of the liquid phase (by increasing viscosity (adding thickening agent)→ draining rate will decrease) - for example maräng

Increased curvature of small bubble → increase Laplace pressure

Coalescence

Presence of solid particles:

• Penetrating particles destabilize → rupture of interface
  • Typically particles that are not wetted by the liquid phase
• Non-penetrating particles stabilize → increase in viscosity of the liquid phase

Presence of spreading oils destabilizes aqueous interfaces:

• Spreading along the interface occurs if: γ_water/air > γ_oil/water + γ_oil/air
• Air/water interface collapses

Thick adsorbed layers are more stabilizing → more resistant to deformation.

Large bubbles are less stable against coalescence → more easily deformed

Disproportionation (Ostwald ripening)

• Small bubbles “disappear” and large bubbles grow
• Driven by the Laplace pressure.
• Individual gas molecules dissolve from small bubbles and diffuse to large bubbles.

R=bubble radius

s=solubility of gas

γ=surface tension

A=area of film (per volume gas phase)

Henry’s law:

Higher K → lower solubility x

Flow behaviour

Gel formation

• A gel is a system with a considerable yield value (overcome yield value → to make it flow)

• Requires a continuous 3D-network of polymers or particles

Yield stress

• Above the yield stress the system flows.

• Below the yield stress the system appears solid (i.e. gel)

Viscous flow can be very low

• Relaxation from one state to another can be very slow

– The time for observing viscous flow can be very long

– Very viscous ”liquids” can appear solid (depending on how long observe it)

Types of gels

• Particle gels

• Polymer gels

– Covalently linked

– Linked through physical interaction

• Ion bridges

• Hydrophobic interaction

• Partial crystallization

• Helix formation

• Lamellar structures of surfactants and lipids (cosmetic products like creams → incorporates into the skin)

Gel formation in polymer solutions

Viscoelasticity

• Oscillatory deformation with rheometer

• Particle tracking (lower diffusion→ higher viscosity) - microscopic technique

G’= ”in phase” shear modulus = storage modulus = elastic response=”solid-like” contribution.

G’’=”out of phase” shear modulus = loss modulus = viscous response=”liquid-like” contribution.

Transitions between liquid-like and solid-like flow behaviour in colloids

• Pronounced elasticity

• Concentrated (high volume fraction) or strongly interacting dispersions

• Emulsions

• Polymeric or particle gels

• Absence of thermodynamical equilibrium (meta-stable)

• Arrested states (often described as)

Molecular and colloidal glasses

Molecular glass: If we increase temperature, it becomes more mobile

Colloidal glass: Doesn’t depend on temperature

Colloidal glasses- It is formed when colloidal particles are densely packed in a suspension to the point where they form a disordered, amorphous solid-like structure.

Transition occurs at different volume fractions depending on:

• Attractive interaction → attractive glasses

• Repulsive interaction → repulsive glasses

Repulsion: are forced together even though there is repulsion between them

Attraction: occurs at a much lower volume fraction since it flocculates

The one top, takes up more space, they can’t get closer than that

Effect of particle size on viscosity and glass transition

Infinite viscosity means solid behaviour

Long range repulsion

The surface area is much larger for smaller particles than for larger ones

Colloidal glasses

• Non-equilibrium

• For repulsive glasses:

• ”Cage effect” - one particle is caged by other particles

• φ_g≈0.58

• For attractive glasses:

• Transition occurs at lower φ

Guinness (foam):

Since it smooth and coherent foam → repulsive forces

φ_g≈0.58

d≈150 nm → smaller makes it more creamy

N₂ is much lower solubility in water compared to CO₂ - the foam is more stable against Ostwald ripening

Dapier (gel):

Cross-linked polymer

Water is absorbed onto negatively cross-linked polymer and forms layer of counter ions → drives surfaces apart (electrostatic repulsion)

CaCl2 makes the diaper to drip. Increasing valence of ion → smaller layer of counter ions → surface can come closer to each other. Not sufficient electrostatic repulsion →