Contact Angle: A Definition
The angle formed at a point on the line of contact of three phases, of
which at least two are condensed phases, by the tangents to the curves
obtained by intersecting a plane perpendicular to the line of contact with
each of three phases. One of the phases must be a liquid, another phase
may be solid or liquid and the third phase may be gas or liquid. Experimentally,
when a liquid is contacted with a solid and another phase, it is usually
observed that the contact angle does not reach its equilibrium value instantaneously.
The same is true when liquid in contact with a solid is displaced. Therefore,
one distinguishes an advancing and a receding contact angle respectively.
Contact angle can be modified by the addition of surfactant.
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