THE INFLUENCE OF DROPLET VOLUME AND CONTACT-ANGLE ON LIQUID SURFACE-TENSION MEASUREMENTS BY AXISYMMETRICAL DROP SHAPE ANALYSIS-PROFILE (ADSA-P)

The axisymmetric drop shape analysis-profile (ADSA-P) technique was evaluated with respect to the influence of contact angle and droplet volume. The system was implemented on our standard contact angle measuring apparatus employing a Vidicon video camera and a 512 x 256 pixels framegrabber. For calibration of the x and y magnification factors a perfectly spherical ball was used of approximately the same dimensions as the liquid droplet under study. Droplets of water, formamide and hexadecane with volumes varying from 2-200-mu-l were put on FEP-Teflon and PMMA. Erroneous surface tensions were found by ADSA-P when too small or too large droplet volumes were employed. Ideal volumes to be used were dependent on the contact angle and amounted in our system to 5-10-mu-l for theta <40-degrees, 20-50-mu-l for 40-degrees <theta <90-degrees and 50-100-mu-l for theta > 90-degrees. Statistical evaluation of results obtained for the same droplet (i.e. water on Parafilm), digitized 16 times using two different sets of five calibration frames, indicated that digitization of the liquid profile is the largest source of variation. Typical standard deviations over 16 images of the same droplet are +/- 0.9 mJ m-2 in surface tension and +/- 1.5-degrees in contact angle. However, in summary it is concluded that ADSA-P is a powerful tool for simultaneous determination of liquid surface tension and contact angle, even when implemented using a relatively simple image analyser.