= Solution
\b[The <contact angle> is measured through the liquid between the solid surface and the tangent to the liquid–vapor interface at the <contact line>.] The right panel of the wetting diagram labels that angle and the three interfacial tensions.
At $0^\circ$, the ideal macroscopic limit is <complete wetting>: a film spreads rather than remaining a finite-angle cap. Angles between $0^\circ$ and $90^\circ$ are conventionally called hydrophilic for <water>; the drop wets the surface relatively well. At $90^\circ$ the tangent is vertical. Between $90^\circ$ and $180^\circ$ the <water> drop is conventionally hydrophobic, with increasingly weak wetting. The exact $180^\circ$ limit represents vanishing macroscopic contact in an ideal model, not a routine finite-area sessile drop on a real solid.
The categories describe <water> under specified conditions. Roughness, contamination and contact-line pinning can make advancing and receding angles differ from each other and from the ideal equilibrium angle.
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