Meassure activity of particles in an video (Optical Flow)

Hello all,

I have a video and I want to quantify the movement in the image.
I am observing particles flowing through a channel and I want to measure approximately how much contamination with particles is in this flow.

I am used to PTV algorithms, but I don’t want to separate and count all the particles because they are too numerous and I want to speed up the processing time.
Is there an algorithm that can be applied to such problems?

My first approach was to subtract two consecutive images and quantify the difference, but I am not satisfied with the result.