Size-dependent lens angles for small oil lenses on water

Line tension is the excess energy associated with unit length of a three-phase contact line and it has long been of interest, in part because if sufficiently large, it can affect various processes of industrial and biological importance. Most recently, interest has centred on the magnitude and sign of experimentally determined values. Reported line tensions in systems with liquid alkanes in contact with aqueous phases include values from about +10⁻¹⁰ to 10⁻⁹ N on the one hand, and −10⁻⁶ N on the other. If the actual values of line tension lie close to the lower end of the spectrum quoted above, the influence on many systems of interest will be negligible. The higher values, however, could lead to pronounced effects. A study to determine line tension in the three-phase contact line around lenses of dodecane resting on a water subphase is presented. The method involves measuring, by interferometry, the variation of lens angle with the contact line radius. In order to bring the angles into a convenient range for measurement (around 2°), small amounts of dodecanol (ca. 2 mmol dm⁻³) have been added to the dodecane. The line tension is found to be +1.6±0.3×10⁻¹¹ N. The magnitude and sign of the line tension is discussed in terms of surface forces.