The wetting phase and non-wetting phase refer to how two immiscible fluids, ones that don’t mix, like water and oil interact with a solid surface in a confined space, such as pores or a capillary tube. The distinction depends on which fluid has a stronger affinity for the solid surface, driven by forces like surface tension and adhesion.
Wetting Phase
The fluid that preferentially spreads over or sticks to the solid surface. It tends to coat the surface, forming a thin film or being drawn into small spaces (like pores or tubes) due to strong adhesive forces between the fluid and the solid. The angle ( \(\theta\) ) where the fluid meets the solid is typically less than \(90^{\circ}\) (often close to \(0^{\circ}\) for strong wetting). A smaller angle means the fluid spreads out more.
Non-Wetting Phase
The fluid that does not preferentially adhere to the solid surface and is displaced by the wetting phase. It tends to bead up or resist entering small spaces, as it has weaker adhesive forces with the solid compared to the wetting phase. It occupies the space left by the wetting phase. The contact angle is typically greater than \(90^{\circ}\), meaning the fluid forms droplets rather than spreading out.
