Vital Apparatus Vent
Vacuum tubes are an aesthetic element of Portal 2. They are used throughout Aperture to distribute objects, like cubes, turrets, and gel. They can be used to add visual interest to levels, and should be used around droppers.
The cubes and turrets that fly through the tubes are an animation. Look in the model folder "props_vac_anim".
To make gel flow through tubes create an info_particle_system and set the particle to blob_curve_orange for propulsion gel or blob_curve_blue for repulsion gel (These particles are not listed anywhere so i do not know the name for the conversion gel particle). Then create an info_target entity, name it, check the "Always transmit to client (ignore PVS)" flag and set the info_target as control point 1 of the info_particle_system you created before. The Gel will now go straight from the info_particle_system to the info_target.
The distance between those two entities will determine how thick the gel flow is.