Campus Life

QUARKINESS Cryogenic tricks and treats

The science behind liquid nitrogen ice cream

Have you experienced cryogenic tricks and treats at MIT? Well, it’s not too late if you haven’t. There are activities starring cryogenics throughout CPW, and they all use liquid nitrogen as their cryogen of choice. The reason is simple: Liquid nitrogen is cheap thanks to its atmospheric abundance (in fact, liquid nitrogen is cheaper than car gas!). While other cryogenic liquids would do similar tricks, they are made from rarer gases and are correspondingly pricier.

Cryogenics relies on the super-cold, and liquid nitrogen is in fact super-cold. Liquid nitrogen, often abbreviated as LN2 (L for liquid, N2 for N2 nitrogen molecules), boils at 77°K and is way more effective than ice at cooling. It is also way more effective than ice at making ice cream.

Making LN2 ice cream is quick and delicious, and it looks awesome. Ice cream is liquid nitrogen’s call to fame among MIT undergrads, though we do appreciate its applications to, you know, research and industry.

Ice cream making by hand without liquid nitrogen can be an arduous and slow process. You must churn the ice cream a lot while cooling it. This is the usual approach of all ice cream making machines, all done for the sake of texture. Churning ice cream while it cools prevents it from crystallizing into solid blocks ­— after all, eating rock-hard ice cream would be no fun. Churning also whips the ice cream, aerating it to the fluffy and smooth consistency we love. Like any recipe that involves a lot of aeration (ever tried making whipped cream or meringues, for instance?), this takes a while, but liquid nitrogen turns ice cream making into a snap. The secret lies in liquid nitrogen’s low boiling point of 77°K.

If liquid nitrogen hits any temperature above 77°K, it starts boiling into nitrogen gas, and room temperature (~300°K) is definitely hot enough. Swing by an LN2 ice cream-making event over the course of CPW and you’ll see a lot of “steam”. Liquid nitrogen “steams” into gaseous nitrogen as it boils, just as water steams into water vapor when it boils. Same concept and same process, except liquid nitrogen boils at a much lower temperature, and thus its “steam” is correspondingly much cooler. Colder things are denser than warm things, so while steam from water rises at room temperature, the “steam” from liquid nitrogen sinks. This sinking steam is also the reason why I find liquid nitrogen and other cryogenics so aesthetically pleasing.

Like boiling water, the boiling of liquid nitrogen is not a peaceful process: it fizzles and sizzles. This bubbling action serves as a whipping and aerating mechanism perfect for ice cream making. All you need to do is create an ice cream base and pour liquid nitrogen into the base while stirring. Liquid nitrogen does all the aeration and whipping work for you, cutting down on the amount of effort you need to spend churning. Moreover, all of the liquid nitrogen evaporates, leaving you only with delicious ice cream. Yum!

Without a doubt, making liquid nitrogen ice cream is the “coolest” thing to do during CPW.