After the success of our original Saturn Wine Glasses, we set off to apply it's spill-proof geometry to another bar glass, the Saturn Rocks Glass. At a frustrating point during our development of the rocks glass, we realized that we didn't fully understand the why the original Saturn Wine Glasses work as well as they do. For the longest time, we thought it was just the ledge that kept our glass from spilling, but it turns out it is SO MUCH MORE THAN THAT. Somewhere around version 5 of the our new rocks glass, we had to fully analyze the physics of our wine glasses, and here's what we found.
Center of gravity:
The exact location of the glass’ center of gravity is super important. When the glass is tipped over, a line pointing straight down from the c.o.g. must land between the two points where the cup contacts the table. If the c.o.g. is too low, the cup will just pop back upright when it's knocked over. Too high, and it’s at risk of falling all the way over.
Adding liquid to this system makes things more complicated. It changes the center of gravity, which must remain within those two contact points even when there's ice and liquid sloshing around. Luckily, We realized we only had to make sure that the c.o.g. stays nicely between the contact points in the cup's most extreme state: a full glass of whiskey + ice, knocked over hard enough to make some liquid slosh over the lip. Our solution was to move the two contact points as far apart as possible to widen the "window" for a successful catch, and make the bottom thick enough to bring the c.o.g. lower.
This one was really surprising! When we video'd the wine glasses to understand why they work so well, we realized the liquid was moving very little when the glass is knocked over. Our rocks glass prototypes seemed to almost throw liquid out the top.
It turns out the flat bottoms and sides that normally make a rocks glass look like a rocks glass were almost paddling the liquid out as it fell. Imagine paddling a kayak— flat surfaces push water hard and move a lot of volume. If you roll a barrel of water, the round surface moves around the liquid and causes very little turbulence.
It seems almost obvious to us now that a hemisphere bowl causes the least sloshing.
Minimizing sloshes even more:
The last thing that really helped keep the liquid from spilling was changing the sides from angled out, to angled in. In the end, a difference of only 6 degrees drastically reduced the momentum of liquid sloshing in a tipped glass.
Since a rocks glass can't have spherical sides to really minimize sloshing, those straight sides are inevitably going to move the liquid around a bit. When the sides where angled out, the sloshes were prone to keeping their momentum and sliding out the mouth of the glass, which wasn't just bad because you lost a few drops, but it was bad because all that moving liquid could really throw the whole thing off balance. By angling the sides inward (even though it was only a few degrees), those surfaces helped push any sloshing liquid back toward the center of the glass, drastically reducing it's momentum!
Such subtle, functional details! By making the form follow such strict functional and physical requirements, we’ve designed another shape that is super pleasing, balanced, and a pleasure to drink from.
So that's the gist of it. Yay science!
A big thank you to our generous supporters on Kickstarter! Without them, this project would never have been possible.