A NOVEL CONTACT-ANGLE MEASUREMENT TECHNIQUE BY ANALYSIS OF CAPILLARY RISE PROFILE AROUND A CYLINDER (ACRPAC)

A new experimental technique and its computational scheme to determine the contact angles of capillary rise profiles around a cylinder are p resented. In the experiment, a carefully coated conic glass cylinder w as inserted vertically and slowly into a tested liquid. Then the preci se image of the partial capillary rise profile of the liquid around th e conic cylinder was acquired and digitized by applying computer image processing and analysis techniques. From the digitized profiles of th e liquid-vapour interface and the conic cylinder, the local inclinatio n angle beta and the local radius R-c, of the conic cylinder at the th ree-phase contact circle were calculated directly. Furthermore, an obj ective function was constructed, which expresses the discrepancy betwe en the physically observed capillary rise profile and the theoreticall y predicted curve, i.e. the curve representing a solution of the Lapla ce equation of capillarity. The contact angle of the capillary rise pr ofile on the conic cylinder was used as an adjustable parameter in opt imizing the objective function and determined once the minimum objecti ve function had been achieved. The accuracy of the measured contact an gles is approximately 0.1 degrees. In addition to the local gravity, t he densities of the liquid and vapour phases and the liquid-vapour sur face tension, the input requirement is the digital information of the partial capillary rise profile which is provided by a specially design ed computer image analysis program. This method was tested by measurin g contact angles of four n-alkane liquids around cylindrical glass fib res coated with FC725. The measured contact angles are in very good ag reement with those determined by the Wilhelmy plate technique. Finally , the present technique was also applied to study the dependence of co ntact angles on the geometry of the conic cylinder, i.e. on cos beta/R -c. Contact angles of the four n-alkane liquids on a conic glass cylin der coated with FC725 were measured at different positions along the c ylinder. The results were interpreted in terms of the line tension eff ect. The calculated line tensions were positive and of the order of 1 mu J m(-1), which is consistent with the published data for similar so lid-liquid systems obtained by using the sessile drop method. In parti cular, the contact angle without Line tension effect theta(infinity) f or a given solid-liquid system can be measured directly by this method . The validity of the derived contact angle theta(infinity) and line t ension sigma was also confirmed by means of the axisymmetric drop shap e analysis (ADSA) technique. This novel technique is particularly suit able to the study of the wetting and spreading phenomena of a liquid o n fibres, as most fibre surfaces are rough and their shapes may deviat e considerably from those of right circular cylinders. A general user- oriented computer program to implement the technique was developed.