Drag
Applies drag to the linear velocity of particles, which slows down the particles at every frame.
This tool is usually used when a 
Force is applied to particles because forces make particles move faster and faster. Drag helps slow down the particles so that they do not infinitely increase their speed. Drag can also be used to simulate things moving through water or other high density spaces.
Using Drag
Click the
Drag button on the Drive Particles tab.Click
the 
particle system you want to be affected and press Enter to confirm your selection.Specify the node settings in the operation controls toolbar or parameter editor.
For specific parameter help see the Drag node help.