BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
The term dispersion is also used to define the process itself which requires the solid particles to be
coated by the liquid.
Consider a drop of liquid on a flat solid surface. Three interfaces and three interfacial tensions ( γ ) are
involved – solid/air (S/A), solid/liquid (S/L), and liquid/air (L/A). At equilibrium, it can be shown
that,
cosθ
γS⁄ A–
γ S⁄
L
= --------------------------
γL A ⁄
If the contact angle (θ) is greater than or equal to 90°, the liquid is “nonwetting” with respect to the
solid. The liquid tends to “bead” on the solid surface as water does on wax (see Figure 5-12). When
the contact angle is 0°, the liquid completely wets the solid and, in fact, will spontaneously “spread”
on the surface (i.e., water on glass). Surfactants lower the liquid/air surface tension (L/A), as
discussed earlier, and reduce the contact angle. Surfactants often adsorb at the solid/liquid surface as
well. This adsorbed layer alters the surface of the solid with respect to the liquid.
Figure 5-12
Liquids on Solid Surfaces
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 5-15