Reverse particles?

Hello!.
I´m triying to make a vortex particle system, but i can´t make the particles go to the other way round. Something like this for example: https://pa1.narvii.com/6428/b0a712f5dc2cfc32b80fe1eadfd6d9a20df724b6_hq.gif
Is there any form to reverse the effects?
Thank you.