The amazing accidental physics of coffee grounds
We sell two different 'puck simulators', which are just portafilter baskets with a tiny, tiny hole drilled in them by an electron beam drill.
A single hole at 0.2mm in diameter lets through about 0.8ml/s (similar to a thick espresso) while a 0.3mm hole lets through twice as much as that, similar to what you'd want from making espresso with a light roasted bean. https://decentespresso.com/basket
Let that sink in for a moment.
A typical espresso puck lets through the equivalent of a 0.2mm to 0.3mm hole of water. That's over a 58mm size, which is 26 square cm, almost exactly 4 square inches.
Think of how amazingly effective coffee are at resisting water under great pressure. Only the equivalent of a 0.2mm of water makes it way through all that coffee. Not that many materials work so well.
Another mind blowing thing about espresso is to appreciate just how much pressure is on those coffee grounds. 9 bar is about 140 pounds of pressure per square inch. As there are 4 square inches of area on a coffee puck, that's about equivalent to 3 adult male ballet dancers “en pointe” on that coffee puck.
It's truly amazing to me that this tasty roasted fruit happens to optimally give its soluble material up at these low flow rates, and can this same material can create these flow rates under these tremendous pressures. Change the pressure down to (say) 3 bar, with a coarser grind, and espresso isn't as tasty.
Try to make a tasty extraction from ground cocoa, powdered cinnamon, or …. anything else.
Espresso blows my mind sometimes.
-john
Updated 2021/11/07