Designing a better raincoat
Water-repellent clothing is designed to keep water droplets out while letting humidity escape. Hydrophobic fabric meets both requirements, but in striking a balance between them, it fails to repel water completely. When and how that failure occurs is the subject of a new study led by Lorenzo Botto and Rafael Castrejón-Pita at Queen Mary University of London. The researchers treated nylon fabric to make it hydrophobic, and then they used a high-speed camera to record how the droplets—made of water and glycerol—behaved as they struck the fabric.
Each 1.5 mm droplet exhibited one of three types of impact dynamics. The droplet speed and fabric pore size determined the impact type. The figure shows each type’s progression. In the first row, a slow droplet is completely stopped by fabric with small pores. In the third row, a quickly moving droplet penetrates fabric with large pores, with filaments of liquid passing through and breaking into small droplets. For other combinations of pore size and droplet speed, a third type of behavior emerged, shown in the middle row, in which some liquid goes through the fabric but then retracts back into the droplet.
The researchers then developed a simple model to predict when any liquid droplet will go through a nonwettable fabric. They determined that the critical pore size is inversely proportional to the impact speed, and the proportionality constant depends on the droplet’s size, viscosity, and surface tension. For raindrops, the critical pore size is about 100 µm. The model can also be applied more broadly for creating fabrics to protect against more hazardous liquids. (G. Zhang et al., Soft Matter, 2018, doi:10.1039/c8sm01082j ; thumbnail image: Giuseppe Milo, CC BY 2.0 )
