= Contact-angle hysteresis
{title2=$\theta_R\leq\theta\leq\theta_A$}
A contact line can remain pinned while the observed <contact angle> changes between receding and advancing limits. Surface roughness and chemical heterogeneity can produce this hysteresis. Slowly adding or removing liquid from a sessile drop measures advancing and receding angles separately. An observed pinned angle therefore need not be the single equilibrium angle predicted by <Young's equation> for a perfectly homogeneous surface.
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