What controls the net direction of molecules?

Answer: Molecular diffusion

Molecular diffusion is the process of mass transfer as a result of a concentration difference in a mixture.

This process is related to the thermal movement of the particles and their velocity, which is a function of the temperature, the viscosity of the fluid and the size of the particles. This is how diffusion controls the net direction of the molecular flow from a region of higher concentration to one of lower concentration, as long as there is a difference (or gradient) of concentration.