Drop size dependence of contact angles of oil drops on a solid surface in water

Abstract

Contact angle is a fundamental quantity in colloid and surface science. Of all the methods employed to measure the contact angles, direct measurement from sessile drops is probably the most popular approach. However, it has long been found that the measured contact angle is not unique for a given solid–liquid–fluid system. There are two types of contact angle multiple-value phenomena — the drop size dependence of contact angles and the contact angle hysteresis. In the literature, the contact angle dependence on the liquid drop size for the solid–liquid–vapour systems has been studied extensively. In this paper, the contact angle dependence on the oil drop size for a solid–oil–water system is measured by applying the axisymmetric drop shape analysis (ADSA) technique. More specifically, a natural sedimentation method is employed to deposit drops of a silicone oil with a density slightly higher than that of water onto the hydrophobic FC725 coated glass slide in the deionized ultra filtered (DIUF) water. It has been observed that the measured equilibrium contact angle decreases by approximately 7.3° as the equilibrium base radius of the silicone oil drop increases from 0.0205 to 0.4684 cm. According to the modified Young equation, the measured contact angle changes are then interpreted in terms of the so-called line tension effect. The line tension of the solid–oil–water system is found to be 0.82 μJ m −1, which is very close to those for similar solid–oil–air systems. Unlike other conventional drop formation methods, the natural sedimentation method used in this study can eliminate all the appreciable mechanical disturbance and vibrations. Furthermore, this deposition can generate liquid drops in a much larger range of drop size such that the line tension effect becomes much more pronounced.