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If a liquid wets a surface, it tends to spread and cover that surface. Observed on a microscopic scale, the edge of the liquid has a characteristic shape. A knife edge shape indicates wetting, while a beaded edge shape indicates nonwetting. This is shown quantitatively in [[:file:wettability_fig1.png|Figure 1]], which shows a drop of water surrounded by oil and contacting a solid surface. If the edge of the drop forms an acute angle (θ<sub>e</sub> 5.)
If a liquid wets a surface, it tends to spread and cover that surface. Observed on a microscopic scale, the edge of the liquid has a characteristic shape. A knife edge shape indicates wetting, while a beaded edge shape indicates nonwetting. This is shown quantitatively in [[:file:wettability_fig1.png|Figure 1]], which shows a drop of water surrounded by oil and contacting a solid surface. If the edge of the drop forms an acute angle (θ<sub>e</sub> 5.)
Several techniques used to measure this angle are described by Adamson.<ref name=pt05r2 /> The measurements are generally made on a polished surface that simulates the reservoir material. For sandstones, glass slides or polished quartz are often used. Polished marble is usually chosen to simulate a carbonate reservoir. An adaptation of the technique uses a drop of liquid confined between two surfaces.<ref name=pt05r39>Craig, F. F., 1971, The reservoir engineering aspects of waterflooding: Dallas, TX, Society of Petroleum Engineers Monograph No. 3, 120 p.</ref>
Several techniques used to measure this angle are described by Adamson.<ref name=pt05r2 /> The measurements are generally made on a polished surface that simulates the reservoir material. For sandstones, glass slides or polished [[quartz]] are often used. Polished marble is usually chosen to simulate a carbonate reservoir. An adaptation of the technique uses a drop of liquid confined between two surfaces.<ref name=pt05r39>Craig, F. F., 1971, The reservoir engineering aspects of waterflooding: Dallas, TX, Society of Petroleum Engineers Monograph No. 3, 120 p.</ref>
Contact angle measurements can be precise, but even for ideal systems, measurements can show significant variation. Variations are related to surface preparation, equilibration of the solid and liquids, and surface roughness.
Contact angle measurements can be precise, but even for ideal systems, measurements can show significant variation. Variations are related to surface preparation, equilibration of the solid and liquids, and surface roughness.