A novel contact angle measurement technique by analysis of capillary rise profile around a cylinder (ACRPAC)

Abstract

A new experimental technique and its computational scheme to determine the contact angles of capillary rise profiles around a cylinder are presented. In the experiment, a carefully coated conic glass cylinder was inserted vertically and slowly into a tested liquid. Then the precise image of the partial capillary rise profile of the liquid around the conic cylinder was acquired and digitized by applying computer image processing and analysis techniques. From the digitized profiles of the liquid-vapour interface and the conic cylinder, the local inclination angle β and the local radius Rc, of the conic cylinder at the three-phase contact circle were calculated directly. Furthermore, an objective function was constructed, which expresses the discrepancy between the physically observed capillary rise profile and the theoretically predicted curve, i.e. the curve representing a solution of the Laplace equation of capillarity. The contact angle of the capillary rise profile on the conic cylinder was used as an adjustable parameter in optimizing the objective function and determined once the minimum objective function had been achieved. The accuracy of the measured contact angles is approximately 0.1°. In addition to the local gravity, the densities of the liquid and vapour phases and the liquid-vapour surface tension, the input requirement is the digital information of the partial capillary rise profile which is provided by a specially designed computer image analysis program. This method was tested by measuring contact angles of four n-alkane liquids around cylindrical glass fibres coated with FC725. The measured contact angles are in very good agreement with those determined by the Wilhelmy plate technique. Finally, the present technique was also applied to study the dependence of contact angles on the geometry of the conic cylinder, i.e. on cos ó/Rc. Contact angles of the four n-alkane liquids on a conic glass cylinder coated with FC725 were measured at different positions along the cylinder. The results were interpreted in terms of the line tension effect. The calculated line tensions were positive and of the order of 1 μJ m−1, which is consistent with the published data for similar solid-liquid systems obtained by using the sessile drop method. In particular, the contact angle without line tension effect θ∞ for a given solid-liquid system can be measured directly by this method. The validity of the derived contact angle θ∞ and line tension σ was also confirmed by means of the axisymmetric drop shape analysis (ADSA) technique. This novel technique is particularly suitable to the study of the wetting and spreading phenomena of a liquid on fibres, as most fibre surfaces are rough and their shapes may deviate considerably from those of right circular cylinders. A general user-oriented computer program to implement the technique was developed.