Droplet Aggregation

Because they are constantly moving, due to Brownian motion, gravity or mechanical

agitation, emulsion droplets frequently collide with each other. Then, they can either

move apart or aggregate. There are two main types of aggregation: flocculation and

coalescence. Flocculation is the aggregation of droplets that keep their physical

properties; coalescence is the aggregation of droplets that merge together. Flocculation

may be reversible (weak flocculation) or irreversible (strong flocculation) while

coalescence is irreversible23,45.

Emulsion droplets are surrounded by the continuous phase in which the droplets are

formed and dispersed. As droplets move close to each other, a thin layer, usually called

thin film, of the continuous phase is formed between the droplets. As long as this film

exists, there is no droplets contact, due to hydrodynamic resistance induced by the

presence of the thin film44,50,51. Droplet aggregation can only occur when the liquid is

Chapter 2. Literature Survey

The thickness of the thin film depends on the nature of the colloidal and hydrodynamic

interactions in the system17,23,44,45,52,53. The thicker the film is, the weaker the collisions

are. The thin film forms a barrier between the droplets. The rupture of the film requires

a certain level of energy, which determines the nature of the collisions. At high energy

barrier, there is no aggregation and the droplets will move apart; at slightly lower

energy barrier, droplets are weakly flocculated, as the thin layer still exists; at low

energy barrier, droplets are strongly flocculated with a very thin film existing between

the droplets. In last instance, the energy barrier is so low that the film is broken, which

results in droplets coalescence.

2.3.3.1

Flocculation

Droplet flocculation has antagonistic effects in terms of emulsion stability. It is usually

considered as an instability phenomenon. The formation of droplets flocs in the

emulsion has an influence of the creaming rate47,54,55. In dilute emulsions (Figure 2-2a),

flocs, which do not or only little interact with each other, tend to increase the creaming

velocity, since bigger particles are more subject to gravitational effect. Moreover, the

presence of flocs tends to increase the emulsion viscosity which may not be wanted for

some food products. In other hand, the possibility to create a network of flocs in the

emulsion (Figure 2-2b) may be an advantage to modify or control the texture of some

products. Understanding of flocculation is of great importance in order to control the

texture and structure of emulsions. Mathematical models that take into account the

phenomena occurring during droplet flocculation (collision frequency and collision

efficiency) in order to predict the effect of flocculation on emulsion stability have been

Chapter 2. Literature Survey

Figure 2-2: Structure of flocculated droplets in (a) dilute emulsion and (b) concentrated emulsion.

There are several methods to control flocculation, depending on the final products. The

choice of one of these methods is dictated by the components of the emulsion, and the

nature of the emulsion to develop (texture, structure, appearance, etc.)23. The most

efficient way to control the rate and extent of flocculation is to regulate the colloidal

interactions between droplets (steric, electrostatic, hydrophobic, etc.).

2.3.3.2

Coalescence

Coalescence is the merging of two or more droplets to form a larger single droplet and

results in the formation of a layer of oil at the top of emulsion (in the case of O/W

emulsions)57. This can only occur when the thin film separating two droplets is

ruptured. When droplets move close to each other, they may be deformed and the

surface of the droplets may be flattened17,50,58. Because the surface area between

droplets in contact increases due to the deformation, droplets are more likely to

coalesce. The rate at which the thin film ruptures is also of great importance to predict

coalescence56,59. Emulsion droplets are moving constantly and so collision time may be

Chapter 2. Literature Survey

break, coalescence is likely to occur. The film rupture mechanisms are largely

dependent on the continuous phase properties and on the properties of emulsifiers

adsorbed at the droplets. Indeed, emulsifier molecules form a layer, usually called

interfacial membrane, around the droplets that protects them from rupture. These

mechanisms are very complicated and almost unique for each emulsion as they strongly

depend on the system properties23.

Preventing droplet coalescence is a major issue in emulsion stabilisation. A few

methods have been developed to control or even prevent coalescence. As coalescence

depends both on the colloidal and hydrodynamic interactions between the droplets, and

the physic-chemical properties of the components used in the emulsion (particularly the

continuous phase and the emulsifier), reducing or preventing droplets contact and

interfacial membrane rupture are the two points on which efforts have to be made to

prevent coalescence60.

Droplet coalescence is mostly prevented by the presence of emulsifiers adsorbed at the

oil-water interface. Surfactants’ ability to prevent coalescence depends on their physic-

chemical properties. For example, the presence of charged (positive or negative)

emulsifiers at the droplet interface induces electrostatic repulsion between the droplets,

which tends to prevent droplet contact. There are many different emulsifiers. Surface

active agents (Surfactants) are the most common emulsifiers. They are very efficient to

reduce interfacial tension and prevent droplet contact. Proteins have been shown to

provide long term stability against coalescence61,62. Even though the presence of

proteins in the emulsion decreases the interfacial tension, it tends to be higher than in

Chapter 2. Literature Survey

interface and may provide strong electrostatic repulsive forces between droplets. Fine

divided solid particles also provide very good stability against coalescence14,63,64. Solid

particles adsorb at the interface and form a solid layer that prevents coalescence. Due to

the nature of this emulsifier, the stabilisation mechanisms are quite different from those

of emulsions containing surfactant or protein (this will be discussed section 2.5).