I replaced all 1 1/4" pipes and caps with new pipes and caps made of stainless steel to prevent contamination of the ink with rust which was a problem before. 

Now the ink is only flowing through stainless steel pipes and caps, 

brass + plastic fittings,

polyamide tubes

and aluminum valves. 

I will keep an eye on contamination of the ink by the other materials that are in contact with it and maybe replace more of them with stainless steel parts if needed.

Next up is designing and 3D printing a printhead with a LED and wiring for the piezo rings (which I could separate from the metal parts of the ultrasonic transducer) and repeating the ink-splitting experiment.