Contact Angle, θ
(find instruments that measure contact angle here)
When a liquid does not completely spread on a substrate (usually a solid), a contact angle (θ)
is formed, which is geometrically defined as the angle on the liquid
side of the tangential line drawn through the three phase boundary
where a liquid, gas and solid intersect, or two immiscible liquids and
solid intersect.
Summary/Description:
Contact angle is a quantitative measure of the wetting of a solid by
a liquid. It is the angle formed by the liquid at the three phase
boundary where a liquid, gas (or a second immiscible liquid) and solid
intersect. It is a direct measure of interactions taking place between
the participating phases (gas/liquid/solid or liquid/liquid/solid). The
contact angle is determined by drawing a tangent at the contact where
the liquid and the solid intersect.
 
The shape of the drop and the magnitude of the contact angle are
controlled by three interaction forces of interfacial tension of each
participating phase (gas, liquid and solid). In an ideal situation the
relation between these forces and the contact angle can be described by
the Young's equation and is often also referred to as Young's contact
angle. However, often non-ideal conditions due environmental, roughness
and chemical heterogeneity effects leads to deviations from this
relationship. Many other theoretical approaches based on the Young's
equation have therefore been developed to account for these non-ideal
contributions. The non-ideal contact angles are referred to as apparent contact angles.
It is often necessary to distinguish between the Static (Equilibrium) Contact Angle (θe) and
Dynamic Contact Angles (DCA) i.e. Advancing Contact Angle (θa) and Receding Contact Angle
(θr). When θr α θa the system is said to exhibit contact angle hysteresis.
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