The Capillary Contact Angle
It was recognized by the initiators of capillarity theory that the underlying hypotheses leading to the notion of contact angle are satisfied only under very particular circumstances, usually related to purity and cleanliness of the materials and smoothness of the bounding surfaces. Even so, reproducible experimental determinations of contact angle are notoriously difficult to obtain. There remains considerable disagreement as to the root causes of the difficulty, but a body of literature has arisen describing a resistance to motion of the contact line (or “hysteresis”) as evidenced by a discrepancy between “advancing” and “receding” contact angles. Even on this point there remains disagreement. For an expository survey, see ; experiments with discordant, seemingly conflicting, results are reported in [1, 45, 104, 121, 188]. We remark also the comments in [4, p. 609].
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