CONTACT-ANGLE DETERMINATION TOGETHER WITH SHAPE AND VOLUME OF SESSILE DROP

The modified Young-Laplace equation ((4) and (5)) describes the shape of a sessile drop on a plane surface. The equation includes the gravitational effect and has been developed using the variational minimum energy principle [9]. Only numerical solution of Eq. (4) may be obtained. At first the constant l (A) should be reasonably estimated. In spite of lack of an analytical solution of Eq. 4, the rigorous Eq. (14) connecting the l constant, volume V and radius r of a drop base with physicochemical parameters of the system (K, theta) has been developed in the paper. Thus for given theta, V and r one will calculate the constant l (lambda) and consequently the exact drop profile. Among the theta, V and r values the measurement of contact angle is the most subjective [2]. By a comparison of experimental and calculated profiles the accuracy of the contact angle value measured using the photographic-geometrical procedure may be evaluated. When they differ too much the value of theta may be changed to match a calculated profile to the real drop profile. A comparison of calculated and experimental profiles of water drops on graphite and paraffin wax has been presented (Figs. 5, 6). The measured and improved by calculation (matching profile) contact angle values for graphite and paraffin wax are respectively: 63-degrees and 104-degrees. The technique employing the presented procedure and image analysis will be the subject of a separate study.