Langmuir 1999, 15, 7829-7835
7829
Contact Angle Measurements on Fibers in the Environmental Scanning Electron Microscope L. M. Jenkins and A. M. Donald* Polymers and Colloids Group, Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, U.K. Received January 21, 1999. In Final Form: June 16, 1999 A method has been developed for the condensation of droplets onto individual textile fibers in the environmental scanning electron microscope. It can be seen how the drops formed surround the fibers as long as the fiber surface is smooth. In this case they adopt an unduloid shape. The images produced of the droplets allow measurement of contact angle and have many advantages over corresponding images from the optical microscope. When the fiber surface is roughened, e.g., by crenelations, the droplet shape changes and contact angle measurement is much harder.
Introduction The wetting behavior of a textile material is an important factor in determining the applications for which it is used. It will also affect many manufacturing processes, including dyeing and finishing, where movement of water both in and around the fiber assemblies is required. For this reason, development of fast and accurate methods for the assessment of water-fiber interactions is needed for the study of fiber assemblies. Wetting is defined as the initial behavior of a material when brought into contact with a liquid, with wettability the potential of a surface to interact with liquids of specified characteristics.1 Spontaneous wetting is defined as the migration of liquid over a solid surface toward thermodynamic equilibrium. The extent of movement will be determined by the competition between solid-liquid adhesion forces and liquid-liquid cohesive forces.2 In complete wetting, the interaction between liquid and solid is strong, and the liquid will spread out as much as it can.3 Spreading of water will occur when the material contains, e.g., hydrophilic groups to attract the water molecules. The attraction means that the water-solid interactions are of lower energy than interactions between water molecules. In partial wetting, the surface prefers to remain dry, so the liquid does not spread. It covers only part of the material at equilibrium and forms a droplet assuming a finite angle with the surface. The contact angle that the droplet edge makes with the solid surface is often used as a comparative measure of a material’s wettability. Some degree of hydrophilicity will lead to a contact angle 90°, and decreases it if originally acute.
rs ) At/Aa ) cos θa/cos θ
(2)
where rs ) roughness factor; At ) true surface area (including roughness); Aa ) apparent surface area; cos θa ) measured contact angle; cos θ ) true contact angle. On a flat surface, a droplet of water has the ability to spread infinitely in all directions to minimize surface free energy, and its equilibrium shape is ultimately a thin film if the contact angle approaches 0°.10 On a smooth, cylindrical fiber, minimization of surface energy is achieved by droplet formation only. The water spreads until it surrounds the fiber, reaching equilibrium in an unduloid shape (Figure 2).5,11 For the droplets formed on flat surfaces, there is a constant mean curvature of the drop meniscus at all points in the absence of gravity effects. For the unduloid droplets formed around fibers, the water passes through two oppositely signed arcs of curvature quite close to the surface. The contact angle is that which the water makes closest to the fiber surface, before the change of direction.12 The unduloid droplet is completely symmetrical, and the contact angle can therefore be measured from above, as well as from the side. The shape itself places a limit on the contact angle values that can be observed. It is formed when the drop is large compared with the diameter of the fiber, and when the contact angle is
