On various occasions in this Blog, I've mentioned that a beer was available "on nitro." What does that mean, you might be asking? Most beers are carbonated with carbon dioxide to varying levels typical of the style of beer or the brewer's whim. An English brown ale might have relatively low carbonation while a west coast pale ale usually has very high carbonation. Carbonation can be either artificial or natural. With artificial carbonation, the most common method, CO2 is forced into the beer and occurs rather quickly. The amount of carbonation is easily controlled and consistent.
Natural carbonation is often referred to as "bottle conditioned" where the carbonation occurs naturally through reactions that occur in the bottle or keg after bottling/kegging. A bottle conditioned beer is metabolically active because it has live yeast remaining in it, an essential element for natural carbonation. A small amount of additional malt or other fermentables (i.e. sugars) are added to the fermented beer just before bottling. (Commercially, there's a way to do it without adding extra fermentables, but I'm speaking from a homebrewer's perspective.) Once in the bottle, the yeast remaining in suspension consumes the additional malt/sugars with the byproduct being CO2 gas (and a little alcohol). Because the bottle/keg is sealed, the pressure builds up and the CO2 gas dissolves in the beer and the beer is "carbonated." Yeah, yeah, I know. Get on with the "nitro" thing.
Typically, pressurized CO2 is used to push the beer from the keg to the tap. When a beer is "on Nitro" the gas used to push the beer is a mixture of nitrogen and CO2 in roughly a 75/25% mix. The nitrogen displaces quite a bit of the dissolved CO2 in the beer and creates a thick, creamy head (think Guinness) and a very smooth, creamy mouth feel. The different style tap you see is necessary because the pressure used to serve beers on nitro is much higher. There is a restricter plate/screen in the tap that the beer is forced through which causes the nitrogen to break out of solution and create the thick, creamy head.
The cascading bubbles you see when the beer is first poured are the nitrogen bubbles. Because much of the CO2 is displaced, a beer on nitro often tastes a bit flatter. I find that it also tends to temper the flavors and bitterness somewhat. At the start of my craft brew drinking days, serving beer on nitro seemed reserved for stouts and porters. But in today's wide open world of beer experimentation you'll see practically any style served up on nitro from time to time. The three pictures here show Kettlehouse's Disc Down Oatmeal Brown on nitro as it transitions from the initial pour with the cascading bubbles to the thick creamy head.