Dynamics of droplet absorption into fibrous materials

This project studies the mechanism of droplet penetration into a non-woven fibrous material. Liquid droplet interaction with fibrous materials is important in composite materials and biomedical applications like bio-sensing and tissue engineering. Understanding these dynamics is the first step in designing novel hierarchical composite materials where a polymeric solution is sprayed on a fibrous mat. In our experiments, droplets of a glycerol-ethanol solution were placed on the fibrous material and their absorption was recorded with a video camera. The liquid is a model for polymeric solutions which would be used in the future to produce secondary structures after solvent evaporation. The total absorption time, and the evolution of contact area and drop volume with time, were measured. There are two common regimes of droplet absorption: Constant Contact Area and Constant Contact Angle. In this case, there is a combination of the two regimes: the drop spreads, reaches a plateau, and then retracts. The fibrous material was characterized by measuring the porosity, fiber size distribution, and the mean pore capillary radius, contact angle and in-plane permeability by capillary rise experiments. A semi-empirical model was used to calculate trans-planar permeability using the mean fiber radius and porosity. A simple model based on Lucas-Washburn dynamics describing the porous material with two parameters (permeability and capillary radius), and describing droplet absorption as pure in-plane penetration with the maximum droplet area, shows good agreement with experimental results.