LENS SIZE DEPENDENCE OF CONTACT-ANGLE AND THE LINE TENSION OF THE DODECANE-WATER-AIR SYSTEM

Line tension is calculated by applying the contact line quadrilateral relation to a dodecane-water-air liquid lens system, when two contact angles are measured through an image analysis scheme. To complement pr evious measurements on individual lenses [1], contact angles were dete rmined as a function of lens diameter. In order to exclude the possibi lity of any fluid mechanics or other dynamic effects, the change of le ns size was performed at three different rates. No apparent rate depen dence of contact angle and line tension can be observed for the three speeds. By reformulating the quadrilateral relation, the lens-size dep endence of contact angles can be used in the calculation of line tensi on. To compute line tension with this scheme, three derivatives need t o be obtained in addition to two contact angles and three surface tens ions. No computational advantage can be gained over the direct applica tion of the quadrilateral relation; however, the approach used here pr ovides a potentially important approximation in term of a single surfa ce tension and a lens-size dependent contact angle. For the water-dode cane system studied here, the line tension values calculated, by the q uadrilateral relation, its reformulation and the simplified quadrilate ral relation, are negative and between -1 and -4 x 10(-6) J m(-1). (C) 1997 Elsevier Science B.V.