Given the geometry of our lips and the physics of our drinking, is it a better design to have flared tops ? (better in the sense of ease of drinking and less spillage)
Flared and unflared designs exist in all those materials; if you want to see a lot of glassware with flared tops, just look at lab supplies. AFAICT it’s got nothing to do with ease of use, it’s mainly a matter of aesthetics. “Tulip” glasses (for which the widest point is close to the bottom) are unflared, not very comfy to drink from and a PITA to wash; they’re also extremely popular because they’re pretty. A design’s popularity doesn’t necessarily have to do with usage considerations.
It’s also about strength and stiffness. A flared shape is curved in 3D and doesn’t bend nearly as much under pressure as a 2D-curved cone. The lip is the most exposed and the most vulnerable area of the vessel to damage from applied forces, since it has the least support.
I don’t know why this was the case, but it’s quite an oldaesthetic. Maybe it comes from the shape of drinking horns? Rhytonsare frequently that shape, and they descended from drinking horns.
Why would plastics and glasses be difficult to shape this way? Also, glassblowing has been around for at least 2000 years; blown-glass vessels with flared rims have been around a long time.
One could argue that amber—polymerized tree sap—is a plastic. In that sense, plastic drinking vessels with flared rims have been around for at least 3000 years. The Hove amber cup has a rim that’s not dissimilar from the flared rim in some of your example photos.
A flared lip prevents dribbling at least partly by ensuring that the pouring threshold is “downhill” from the exterior surface of the vessel. An over-full vessel might still dribble, of course.
I guess a flared rim could keep the flow from cup to mouth laminar longer than an abrupt rim would, but I’m speculating. I don’t even know whether laminar flow is somehow advantageous at that point.