Imaging an Oil Droplet under a Latex Film

Abstract

An age-old dream in the coatings industry has been to develop a paint capable of coating over oily surfaces without first carefully cleaning the surface and still provide protection and coverage. The wettability of the solid surface plays an important role here, as it does in many other areas of technology such as lubrication, detergency, and oil recovery. As a small first step in this direction, we consider the example of a latex clearcoat applied over a surface containing a thin film of oil. From a paint perspective, one asks the questions: Where does the oil end up in the coating? Does it remain at the substrate? Is it displaced to the film surface or does it become dispersed in the coating? From a surface science perspective, one asks different questions. Can one measure the contact angle between micrometer-size droplets of the oil on the substrate, and can one detect the influence of the latex dispersion and the dry film it forms on the properties of the oil droplets remaining at the substrate surface? In this communication, we show that many of these questions can be answered by laser scanning confocal fluorescence microscopy1-6 (LSCFM) if the oil in question is treated with a trace amount of an oil-soluble fluorescent dye. Experiments on the behavior of oil-like films cast onto nonwetting substrates have been reported previously. Wilkinson et al.7 studied the wetting and spreading of two liquids in contact with a solid substrate, including the case of oil on glass in the presence of water. They examined immiscible liquids (oil and water) and macroscopic oil drops and found that as the water films became thinner, rupture often occurred, leading to the formation of stable oil drops surrounded by bare substrate. Bass and Sharma8 used atomic force microscopy (AFM) to measure the forces between oil drops immersed in a NaCl solution and a solid surface of either glass or mica. They found that an oil droplet adhering to the substrate more easily ruptured thin aqueous films on glass than on mica. They speculated that the higher surface roughness of the glass (2 orders of magnitude higher than that of the mica) led to the more facile rupture of the aqueous film. Redon et al.9 studied the evolution of a thin (20 μm) film of a nonvolatile liquid forced to spread onto a nonwetting substrate. Since all films are metastable below a certain thickness controlled by gravity effects (about 1 mm),10 thin films dewet by nucleation and growth of dry patches. The rate of this process depends primarily on the oil/ substrate contact angle. In another set of experiments by this group, the dewetting of a liquid A deposited by solvent evaporation on an immiscible nonwettable liquid B was followed. The dewetting evolved by amplification of capillary waves to form a cellular pattern of A droplets on the surface of liquid B.11 The problem we address is significantly more complex. We are interested not only in the nature of the oil droplets formed on the surface of a substrate but how those droplets are affected when a dispersion of film-forming latex is spread over the substrate and allowed to dry. We employ triolein, a liquid triglyceride, as the oil, quartz as the substrate, and poly(butyl methacrylate) (PBMA) as the film-forming latex. Triolein serves as a model for oils and greases that might appear on a wall to be painted. It is also a model for lubricant traces on the surface of an aluminum sheet that is to be coated. PBMA is an excellent model for a variety of acrylic dispersions that form films at or near room temperature.12,13 Typical latex films are 30 to 60 μm thick. The latex films we prepare are coated over quartz with oil droplets on the surface that are significantly thinner. We wish to understand how the coating affects the contact angle between the oil droplets and the quartz substrate. Contact angles define the tendency of a nonwetting fluid to either adhere to the surface or separate from it, and can be measured by a variety of techniques.14-16 Traditional methods for determining contact angles cannot be used to study a system consisting of micrometer-size oil droplets on a substrate coated with a solid but transparent polymer film. To characterize these oil droplets, we need a technique with 3D imaging capability and submicrometer resolution. Laser scanning confocal microscopy1-4 fulfills these requirements and allows us to image the oil droplets inside the polymer film. This technique is widely used in the biological sciences5,6 and has been used to image domain shapes and sizes in a variety of solid polymer blend systems.17-25