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Chapter 2

2.6. Effects of clay solid wettability and rag layer formation

Wettability of clay solid can also affect emulsion stability. Wettability is the
preference of one fluid to spread on or adhere to a solid surface in the presence of
other immiscible fluids ^[33]. For water-oil-clay system, wettability of clay depends
on the structure and surface property of clay, the composition of oil and water, and
temperature. Wettability can be characterized by measuring the contact angle θ of
oil and water on clay surface, which is shown in Figure 2.8 ^[34].

The equilibrium contact angle is defined by Eq. 2.20.

σ_owcos^ = σ_oj-σ_w                                             [2.20]

θ: equilibrium contact angle.

σ_ow: interfacial tension between oil and water phases.

σ_ws: surface energy between water and substrate.

σ_os: surface energy between phase oil and substrate.

Figure 2.8 Force balance at three phase contact line

Clay has very small particle size (about 1-10 μm), thus it is hard to measure

contact angle directly as shown in Figure 2.8. Other methods need to be used to

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