= Contact-line pinning
A <contact line> is pinned when its position remains fixed while the surrounding drop shape or <contact angle> changes. Surface heterogeneity or deposited material can sustain such a state over a range of angles. <Evaporation> from a pinned drop then requires compensating liquid motion and can generate outward solute transport associated with the <coffee-ring effect>. A model with a freely receding line and fixed <contact angle> has a different <boundary> <constraint> and need not predict the same flow direction.
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