[EvilFrog (OP)]

what caused the water or other substance move down the concentration gradient? Is it a force or others?

[Chemistry4me]

No, it just happens because the movement of the particles are all random.

[EvilFrog (OP)]

that mean the particle will move automatically until equilibrium achieved?

[Chemistry4me]

I'm not sure what you mean.

[Bored chemist]

Given a chance (ie as long as there's nothing stopping them) concentration gradients diaspear.
I guess you could call it automatic- it happens without external intervention.

[Nizzle]

It has to do with Brownian motion in water. http://en.wikipedia.org/wiki/Brownian_movement