Happy St. Patrick’s Day!
This great card is brought to you by my Aunt, who never fails to spread the St. Patrick’s Day cheer. I photographed it by some clover on the stone wall in my front yard which just seemed like the right thing to do.
I thought I’d share some science relevant to today and there’s nothing more relevant to St. Patrick’s Day than Guinness. Besides the Guinness company supporting equal rights, they also make great beer.
But there was long a mystery surrounding the legend: bubbles in a glass of Guinness — or any stout — sink as the beer settles. Feel free to check the facts at your favorite pub today.
It wasn’t until a few years about that mathematicians at the University of Limerick (of course) figured why.
Bubbles in stout are comprised of both carbon dioxide and nitrogen gas, while other beers just contain the carbon dioxide. The nitrogen is what give stout a different taste than other beers and also leads to smaller bubbles that sink as the beer settles. Just as with any beer, the bubbles are lighter than the beer. So naturally you might assume that they’ll float to the top, not sink. But that’s not what we see.
It turns out that the secret lies in the shape of the glass and the distribution of the bubbles. As the Guinness is poured, the bubbles do start to rise straight up, moving away from the outward-sloping sides of the glass. Most of the volume of the glass is in a cylinder from the bottom of the glass straight to the top, so most of the bubbles rise here in the center of the glass, leaving fewer bubbles along the sides.
As the bubbles rise of the center, it forces the beer along the edges of the glass down to fill in the vacated space. So when we look at the side of the glass, we see bubbles that have been caught in this downward flow.
The mathematicians used models of fluids and bubbles in different shaped glasses to figure this out. They found that if they modeled the beer in a upside-down pint glass (such that the narrow part was at the top) they saw the opposite kind of flow, where the bubbles rose along the sides of the glass and sank in the middle.
Here’s the original paper if you want to get technical. Either way, share your new knowledge and impress the whole pub as you’re sipping your stout in honor of St. Patrick’s Day.