CONTACT-ANGLE MEASUREMENTS AND CONTACT-ANGLE INTERPRETATION .1. CONTACT-ANGLE MEASUREMENTS BY AXISYMMETRICAL DROP SHAPE-ANALYSIS AND A GONIOMETER SESSILE DROP TECHNIQUE

Low-rate dynamic contact angles and static advancing contact angles of 13 liquids were measured, respectively, by axisymmetric drop shape an alysis (ADSA) and a conventional goniometer technique, on two copolyme rs, poly(propene-alt-N-(n-propyl)maleimide) and poly(propene-alt-N-(n- hexyl)maleimide); both have polar groups. In the case of the former te chnique for measuring contact angles (at very low velocity of the thre e-phase contact line), very complex contact angle responses were obser ved for some solid-liquid systems. In a specific case, slip and stick contact angle behavior occurs where the contact angle increases steadi ly by as much as 35 degrees at constant three-phase contact radius and subsequently decreases sharply due to a sudden jump in the three-phas e contact line. Thus, circumspection is necessary in the decision whet her or not the experimental contact angles can be used to interpret su rface energetics in conjunction with Young's equation and whether the solid-liquid systems violate the basic assumptions made in all surface energetics approaches. It was shown that if one omits the inconclusiv e contact angle measurements, the values of gamma(lv) cos theta change smoothly with gamma(lv) for these copolymers in a pattern already wel l-known for nonpolar surfaces. Goniometer and ADSA contact angle measu rements were shown to be essentially identical for solid-liquid system s which have constant contact angles. In the specific case of the slip /stick mechanism using a goniometer for the contact angle measurements , the observed static advancing angle corresponds to the maximum angle of the entire slip/stick behavior, as registered by the automated axi symmetric drop shape analysis. Thus, conventional goniometer measureme nts may produce a mixture of meaningful and meaningless contact angles , with no criteria to distinguish between the two.