Tracing a single particle in an expending pipe in a fluid

I would like to module an finite pipe inside a fluid that expands in time and the effect this expansion has on a near by particle.

In the attached illustration, the wired tube is the pipe, and the blow arrows mark the movement of the pipe. the black disk remains stationary during the expansion, so the gap between the the disc and the pipe grows with the expansion. all of this is inside a fluid.

I would like to trace the motion of a particle ( the red arrows).

Will appreciate any help on how to build and execute this.