JOURNAL OF COLLOID AND INTERFACE SCIENCE 197, 68–77 (1998) ARTICLE NO. CS975218 A Generalized Drop Length–Height Method for Determination of Contact Angle in Drop-on-Fiber Systems Bihai Song, Alexander Bismarck, Ralf Tahhan, and Ju ¨rgen Springer 1 Institut fu ¨r Technische Chemie der Technischen Universita ¨t Berlin, Fachgebiet Makromolekulare Chemie, Strasse des 17. Juni 135, D-10623 Berlin, Federal Republic of Germany Received March 6, 1997; accepted October 9, 1997 be accurately applied to fibers (8, 9). The methods, which A generalized drop length–height method is introduced here determine the contact angle by means of the direct observa- for the determination of contact angle in the drop-on-fiber systems. tion of the formed meniscus using microscopy (10, 11), are This method overcomes some of the difficulties encountered in the associated with large subjective factors, since the liquid drop practice by applying the common maximum drop length–height surface changes steeply near the three-phase contact points method to determine the contact angle. Furthermore, with this (8). The contact angle can also be indirectly determined by new generalized method a large part of a drop profile can be using the Wilhelmy plate principle. The results obtained with used in the calculation, which reduces the statistical error of the this method lack often reproducibility (12, 13). determination and improves the reliability and accuracy of the Recently the method based on the determination of the obtained results. The method has been examined using both the theoretically simulated and the experimentally obtained drop pro- critical parameters of a drop profile (e.g. maximum drop files. The accuracy of the method has been estimated to be in order length, drop height, and the apparent contact angle of a drop of 0.5 – 1° for the drop profiles of usual experimental quality, which surface curve at its inflection point (8, 9)) received much can be obtained using common digital image processing hardware attention ( 2, 3, 8, 9, 12 ) . Its principle is due to the Laplace – and software.  1998 Academic Press Young equation (in the absence of gravity effect), which Key Words: drop-on-fiber; contact angle; wetting of fibers; rules the profile of a liquid drop on a fiber. With this method method of contact angle measurement. accurate measurement may be possible if the drops are small enough that the effect of gravity may be neglected and if the critical drop dimensions can be determined with enough INTRODUCTION precision. The former condition is usually fulfilled for fibers of small diameter ( õ50 mm in gas–atmosphere systems or The wetting of cylindrical monofilaments (e.g., fibers) by larger in two-liquid systems (9)). The practical problem is, liquids is of general scientific (1) and technological interest however, associated with the precise determination of the (2, 3). The wetting behavior of a fiber in liquid matrices drop dimensions, which is particularly difficult for microfi- (e.g. dying mixtures or polymer melts) plays an important bers ( diameter õ10 mm). Since the final results of contact role in the textile industry and in the fabrication of advanced angle are very sensitive to the errors made by determination fiber-reinforced composite materials. In these cases the sur- of drop dimensions, especially in cases of small contact face energy of the fiber is usually one of key parameters in angles, the method is still strongly subject to subjective in- determining the final interfacial properties between a fiber fluences and to statistical errors. and its matrix. Various methods such as plasma treatment On the basis of our computer-aided pendant-drop method and chemical treatment are commonly applied in industry (14, 15) we attempt to develop a reliable and convenient to modify the surface energy of the fibers and thus improve contact angle determination method for the drop-on-fiber or weaken their adhesion to the matrices. The solid surface system, by taking the advantage of using digital image pro- energy of a fiber may be determined by the contact angle cessing technique to determine drop profiles. To calculate measurement, just as in the cases of planar solid surfaces the contact angle from the extracted drop profiles we have ( 4 – 7 ) . However, the direct measurement of contact angles is extended the concept of the common maximum drop length – usually much more difficult with fibers, due to the cylindrical height method (8, 9) and introduced a so-called generalized shape and the small diameter (usually in order of 5–100 drop length – height method as will be described below. With mm), than with planar surfaces. Many common methods, this new method, the contact angle may be calculated from which may work well for flat plates, may not, however, the drop profiles very conveniently and more reliably com- pared to the original maximum drop length–height method. 1 To whom correspondence should be addressed. Furthermore, this extended method should be more suitable 68 0021-9797/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.