Dynamics of Droplet Absorption into Fibrous Materials

This project studies the mechanism of droplet penetration into a non-woven fibrous material. Understanding these dynamics is the first step in designing novel hierarchical materials where a polymeric solution is sprayed on a fibrous mat. There are two common regimes of droplet absorption: Constant Contact Area, where the drop base stays the same, and Constant Contact Angle, where the drop base retracts as liquid is absorbed. 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 solution was chosen to attain suitable surface tension and viscosity conditions. The total absorption time, and the evolution of contact area and drop volume with time, were measured. In this case, there is a combination of the two regimes: the drop spreads, reaches a plateau, and then retracts. To characterize the fibrous material, capillary rise experiments were done with a wetting liquid to determine the mean pore radius, and with the glycerol-ethanol solution to determine the contact angle. Dynamics of vertical liquid penetration are fitted with the Lucas-Washburn equation to obtain the permeability. The change in drop volume as a function of time was compared to this model.