I'm specifically talking electronics, where the usefulness runs out very quickly... As soon as you get to EMI/EMC there is no real liquid analogy (so that rules

I'm specifically talking electronics, where the usefulness runs out very quickly... As soon as you get to EMI/EMC there is no real liquid analogy (so that rules out basically any reasonably complex custom electronic design you're going to get certified to sell as a product). Parasitic effects of traces, ESR, characteristic impedance, etc. are similarly problematic (people try to come up with stuff like springs or flexible tubes for characteristic impedance but again it's just more confusing in practice). So that rules out anything decently high speed digital (like, say, routing DDR3/4). That's of course not to mention RF, antennas, transmission lines etc. where all bets are off.