It is a brush that can draw a drop on the screen.
It is a set of water and water pens with a special shape, such as a drop-down pen with a standard round droplets.

I use it like a glass window in an upper image, but I think that I can use it for the expression of the rain water and the sweat.

When you want to make more realistic, you can express water droplets easier than draw from scratch and add a highlight and shadow like the image below.

It may be hard to see the water droplets if you do not vacuuming the shadow in overlay or soft light, especially when you want to use it for the color of the skin such as thin color.

The ink is overlay by default, but if you want to blur or erase partially as shown in the sample, draw the ink in a new overlay layer.