Wetting and Drying of Fibers

Abstract

Given the ubiquity of fibrous media in engineered systems, it is both important and useful to understand the wetting and drying of liquids on fibers. Previous studies on the wetting of fibers have mostly considered totally wetting liquids, but for practical applications in industry, it is important to understand the wetting of fibers with partially wetting liquids. We investigate experimentally the wetting behavior of dodecane, a partially wetting liquid, on two fiber configurations: two rigid parallel fibers and two rigid fibers crossing at an angle. On parallel fibers, the liquid can exist in either a drop or an extended column state; on crossed fibers, the liquid can exist in a mixed morphology, where a drop lies at the end of a column, in addition to the drop and column states. The morphology adopted by the liquid on parallel fibers depends on the inter-fiber distance, volume of liquid and fiber radius, while the morphology adopted by the liquid on crossed fibers depends on the angle between the fibers, volume of liquid and fiber radius. We use our experimental results to verify an analytical model described in the literature for the wetting of parallel fibers with partially wetting liquids. We adapt an analytical model developed for the wetting of crossed fibers with totally wetting liquids to the case of partially wetting liquids and verify the model with experimental results. For both fiber configurations, we identify regions where each morphology prevails based on the global geometry of the system and volume of liquid. The drying of fibers has not received as much attention as the wetting of fibers. While the evaporation of liquid on two parallel fibers has been studied, no studies to date have considered the drying dynamics of liquid on crossed fibers. We investigate experimentally the evaporation of volatile silicone oil on crossed fibers by measuring the evolution of the length of liquid with time as evaporation occurs. A liquid that initially adopts a drop or mixed morphology when placed on the fibers switches to a column state as the volume of liquid is reduced with evaporation. We observe distinct trends in the evolution of length with time for each of the three possible morphologies on crossed fibers and choose to focus in particular on the drying dynamics of liquid in the column state. By rescaling the time evolution of column length using as length and time scales respectively the initial length and total evaporation time, the data collapses toward a single curve that can be described by a scaling law with an exponent approximately equal to 1/2. We propose an analytical model to explain the experimental results.