THE EFFECT OF DROP SIZE ON CONTACT-ANGLE OVER A WIDE-RANGE OF DROP VOLUMES

Contact angle measurements for the air/water/polyethylene and for the oil/water/quartz systems have been carried out over a wide range of drop (bubble) sizes, from 60 µm to 2 mm. Two distinctly different experimental techniques were used: placement of the drop (bubble) with a microsyringe, the static technique, and collision of the drop (bubble) with the surface due to the buoyant force, the dynamic technique. A nonlinear relationship between the cosine of the contact angle measured through the aqueous phase (cos Θ) and the reciprocal radius of the drop base (1/r) was obtained. The lack of linearity for this range of drop sizes cannot be attributed to a variation in line tension which should be constant. Nevertheless, a pseudo line tension can be calculated and it was found that negative pseudo line tensions on the order of -1 × 10-8 N(-1.2 × 10-8 -6.6 × 10-8 N) were obtained for small drops or bubbles (drop volume less than 0.1 µl, drop base diameter less than 0.3 to 0.4 mm). The pseudo line tensions increased when the drop (bubble) volume increased above 0.1 µl, reaching a maximum of about -1 × 10-6 N (-0.6 × 10-6 to -1.4 × 10-6 N). Thus, it was found that the contact angle for small drops (bubbles) changes significantly from that measured for large drops (bubbles) which may be an important fundamental aspect in certain phase separation phenomena. © 1993 Academic Press, Inc.