The Shape of a Water Droplet
The shape of a water droplet depends on the surface it rests on. Its shape can be changed by modifying the surface. The side-by-side photos below show two droplets of equal volume on two different surfaces of the same material. On the ordinary surface, the droplet forms a hemisphere. On the modified surface, the droplet forms a sphere.
Hydrophobic
Ordinary Surface
Water Contact Angle: 90º
Superhydrophobic
Modified Surface
Water Contact Angle: 150º
The Evaporation of a Water Droplet
The shape of a water droplet and the rate at which it evaporates depend on the surface it rests on. The time-lapse video below shows the droplets on the surfaces described above. Although their volumes are equal, their surface areas are not.
Water molecules at the droplet’s surface, in contact with the ambient air, have fewer hydrogen bonds tethering them to the liquid phase and therefore may leave the liquid more readily than molecules not at the surface. Of the two equal-volume shapes, the sphere has a smaller surface area than the hemisphere. The sphere has fewer of its molecules in contact with the ambient air and, in a subsaturated environment, has a longer lifetime than the hemisphere. In this instance, the spherical droplet lasts more than 35% longer (more than 7 minutes longer) than the hemispherical droplet.