Rheology of dispersed systems

Question 1

When the emulsions of oil droplets can be hard spheres and soft spheres?

Answer 1

• Highly viscous oil (hard spheres)
• Stabilized by thick protein layer or layer of macromolecules (hard spheres)
• Not well stabilized (soft spheres).

Question 2

Two types of dispersions of spheres:

Answer 2

1. Suspension of hard spheres.

2. Emulsions of soft oil droplets

Question 3

Viscosity depends on :

Answer 3

Viscosity of the solvent (dispersed phase)

Volume fraction= volume dispersed phase/total volume

Question 4

Hard spheres in a Newtonian fluid, applies to:

Answer 4

• suspensions of solid particles

* emulsions of highly viscous oils, stabilized by thick protein layer or layer of macromolecules

Question 5

Hard spheres in a Newtonian fluid. Low volume fractions (%)? How can you measure it ?

Answer 5

*Low volume fractions (< 5%): first formula (Einstein relation)
Ubellohde: time for a liquid to pass from one point in a capillary to another point, this time is related to the viscosity. Linear relation.
*Higher volume fractions (> 5%): No linear, the particles will start to interact with each other, and this leads to an increase in the viscosity at higher volume fraction. Second formula (Krieger-Dougherty). When volume fraction max, the system starts to behaves as a solid. Monodisperse hard spheres (o.5< volum.fract.max

Question 6

Spheres with steric stabilization:

Answer 6

Low volume fractions (< 5%): Einstein formula. Some cases we do not know the volume fraction, E.g. particles are stabilized with an additional layer (polysaccharides added to the continuous phase), this increases the volume fraction–>effective volume fraction.
4Volume fraction=2.5*effective volume fraction

Question 7

Rheology of emulsions, low volume fractions and high?

Answer 7

*low volume fractions: when the droplets are not hard spheres and more soft spheres. During deformation the oil within the droplet can start to flow, shape will change. This determined by the surface tension of the interface and viscous forces. So there is NOT going to be a factor of 2.5 –>q
1<= q<=2.5. This value of q is function fo the viscosity ratio (lambda=viscosity droplet/viscosity solvent). For high values of lambda (>10) q–>2.5 –> rigid spheres. For very low values of lambda (<0.01), q–>1.

*high volume fractions (very high): system packed, droplets can take another shape. Volume fraction can be higher than the max volume fraction , can be higher than 65%. –>HIPE: high internal phase emulsion (example: mayonnaise) Hexagonal shape in foams