MEASUREMENT OF SMALL CONTACT ANGLES FOR SESSILE DROPS

The contact angle (θ) of a sessile liquid drop on a horizontal solid surface can be calculated from the drop volume and the radius of the contact circle at the liquid/solid interface. A simple apparatus which allows simultaneous estimation of these two parameters is described, and tests of the method for two systems are reported. The first system is a 3.25 mole liter-1 solution of 1-propanol in water on paraffin wax. The advancing (θa) and receding (θr) contact angles at 20°C are found to be (59.5 ± 1.0)° and (54.3 ± 0.3)°, respectively, in good agreement with the literature values and those found by direct measurement. The second system chosen is cyclohexane (a volatile liquid) on cleaved mica at 20°C. Two mica sheets were used. Mean contact angles of cyclohexane on the first mica sheet are θa = (7.45 ± 0.10)°, θr = (6.99 ± 0.12)°. For cyclohexane on the second sheet the mean contact angles are θa = (6.48 ± 0.31)°, θr = (5.56 ± 0.06)°. The difference between advancing and receding contact angles is statistically significant (P < 0.01) for both sheets. Other methods of comparable accuracy exist for θ {greater-than or approximate} 30°, but the accuracy of most of these methods diminishes rapidly if θ {less-than or approximate} 30°. If calculation of the contact angle from the spacing between interference fringes is not appropriate, then estimation from drop volume and contact circle radius becomes the method of choice if θ <~ 30°. © 1979.