I’m frustrated by the carafe of our coffee-maker. If you pour the hot coffee out, there is an angle at which the coffee will pour perfectly-anything less (or more), and the coffee will no longer pour straight out-instead, it will dribble out and flow down the side of the carafe. Why is this? The coffee will dribble at low angles (this I understand), but why will it also flow along the carafe, at angles greater than the optimum?
My guess is that there is a fight between adhesive forces (that cause the coffee to stick to the surfaces of the carafe) and inertial forces caused by the momentum of the flowing coffee. A low angles, there is a low velocity and adhesion wins. At higher angles there is a higher velocity and momentum wins, so the liquid comes off the spout cleanly.
At very high angles it’s less clear–is there some obstruction that might slow down the flow at high angles? For some containers, there is a critical angle above which you no longer get smooth flow, because there is no easy way for air to get in to fill the void left by the emptying liquid. I dunno if that’s the case here. It could be something as simple as a lip under the lid of the carafe getting in the way.
I share your frustrations. it didn’t used to be this way, When I first noticed the dribble issue with my coffee carafe a few years back, I got an old one from the attic (the coffee maker itself died of old age) it seemed to me that there was a subtle change in the pour spout, seemed the new one was less pointy and had raised funnelling edges that were lower (comparably non-existant actually). I just chalked it up to stupid design with an eye to saving .000503 cents worth of glass in the manufacture of each coffee carafe
When I had a carafe that did that, it was the kind with a hinged plastic splash-guard lid. I found the dribble didn’t happen as long as I flipped the lid up out of the way.
There could be issues of air pressure. The plastic lid forms a seal over the coffee. If it were an airtight seal, the the coffee wouldnt flow freelyl. Obviously it isn’t air-tight, but it could be tight enough that the air pressure inside the carraf is less then the air pressure in the room, making it harder for the coffee to flow out.
Is that logical?
I expect at higher angles the flow is so fast it lifts from the carafe surface before the edge, leaving room for “stray” flow to dribble underneath the “real” flow.
The lid idea may have some merit but I have noticed this same problem with non-lidded measuring cups as well.