Metrological study of measuring the interfacial tension of liquids by the method of a rotating drop

An improved procedure for measuring the interfacial tension of liquids by the rotating drop method is considered. In the improved method it is proposed to determine the coordinates of the points of the contour of the rotating drop from the digital image using iterative calculation of the coordinates of the points of the corresponding contour of the rotating drop. The coordinates of the points of the drop shape arc calculated by the numerical method of Rungc-Kutta integration of the 4th order with step correction. The sources of uncertainty of measuring the interfacial tension of liquids are determined by the improved method of a rotating drop and the device that realizes it. Uncertainties of input values are the following: uncertainty of measuring the difference of densities of investigated liquids, uncertainty of measuring the frequency of rotation of the tube with liquids, uncertainty of coordinates of points and radius of the rotating drop contour in the image, uncertainty of calculated coordinates of points and radius of rotating drop contour. A mathematical model for estimating measurement uncertainty has been developed. It is shown that the procedure has a higher sensitivity to the uncertainties of the optical magnification of the glass tube and the conversion factor of the optical system of the device. It is also shown that the uncertainty of the calculation of the coordinates of the points and the radius of the contour of the rotating drop by the Runge-Kutta method can be neglected. The value of the measurement uncertainty estimate was calculated on the example of measuring the interfacial tension of 0.02% aqueous solution of "Stinol" at the boundary with kerosene. The obtained value of the expanded uncertainty confirms the effectiveness of the method of calculating the interfacial tension of liquids using the set of coordinates of the points of the drop shape.