Abstract
The most influential parameter on the behavior of two-component flow in porous media is “wettability”. When wettability is being characterized, the most frequently used parameter is the “contact angle”. When a fluid-drop is placed on a solid surface, in the presence of a second, surrounding fluid, the fluid-fluid surface contacts the solid-surface at an angle that is typically measured through the fluid-drop. If this angle is less than 90°, the fluid in the drop is said to “wet” the surface. If this angle is greater than 90°, the surrounding fluid is said to “wet” the surface. This definition is universally accepted and appears to be scientifically justifiable, at least for a static situation where the solid surface is horizontal. Recently, this concept has been extended to characterize wettability in non-static situations using high-resolution, two-dimensional digital images of multi-component systems. Using simple thought experiments and published experimental results, many of them decades old, it will be demonstrated that contact angles are not primary parameters – their values depend on many other parameters. Using these arguments, it will be demonstrated that contact angles are not the cause of wettability behavior but the effect of wettability behavior and other parameters. The result of this is that the contact angle cannot be used as a primary indicator of wettability except in very restricted situations. Furthermore, it will be demonstrated that even for the simple case of a capillary interface in a vertical tube, attempting to use simply a two-dimensional image to determine the contact angle can result in a wide range of measured values. This observation is consistent with some published experimental results. It follows that contact angles measured in two-dimensions cannot be trusted to provide accurate values and these values should not be used to characterize the wettability of the system.
Highlights
This paper will discuss various considerations of the interface between a gas, a liquid, and a solid surface.The fundamental principle of a force balance between the gas and the liquid, if they are separated by a curved surface, is that the difference in pressure across that surface, ∆PP, is given by∆PP = σσllll (RR1 + RR2) (1)Here σσllll is the interfacial tension between the gas and the liquid, and RR1 and RR2 are the two radii of curvature of the surface
Σσssss is the interfacial tension between the solid and the gas, σσssss is the interfacial tension between the solid and the liquid, and θθ is the angle that the liquid-gas surface makes with the solid surface, termed the
If θθ when measured through the liquid is found to be much less than 90 ̊, the system is said to be strongly “liquid wet”; a typical liquid wet system is water/ air/ glass
Summary
This paper will discuss various considerations of the interface between a gas, a liquid, and a solid surface. These three equations are all that is required to demonstrate the characteristic behaviors of contact angles. If θθ when measured through the liquid is found to be much less than 90 ̊, the system is said to be strongly “liquid wet”; a typical liquid wet system is water/ air/ glass. For very simple cases, such as a drop of water on a horizontal solid surface, this definition of wettability is universally accepted. This definition has become dogmatic and it has been extended to mean that the contact angle directly defines the wettability in all situations. This paper does demonstrate that interpreting wettability from direct contact angle measurements is not always a meaningful activity
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