Consider the Void

Nothing keeps your coffee hot like… nothing! The secrets of your vacuum insulated flask.

It’s a cold winter morning and you’re about to rush out the door. You don’t have time to sit and drink the hot, caffeinated beverage you just made, so you grab your trusty metal travel mug and pour a serving inside.

If you keep the lid tightly closed, a vacuum-insulated tumbler or water bottle (think Stanley, Hydro Flask, or Yeti) can keep a drink hot for several hours. You probably don’t spare a thought for how a mere mug is able to retain its inner temperature so miraculously as you bundle up and grab your phone, wallet, and keys. You just know that it works.

But now that you have a moment, let’s think about what makes a vacuum—an empty space where even the air has been sucked out—such a good insulator. And to talk about that, we get to learn about how heat travels from one place to another.

Heat is the transfer of thermal energy, and thermal energy is the energy of molecular motion. Molecules are constantly wiggling, tumbling, or bouncing around. The higher the temperature of a material, the faster its molecules move (and the more energy they have). When these high-powered molecules bump into each other, the faster ones transfer some of their energy to the slower ones.

If you put a hot block of metal in contact with a cold block of metal, the game of atomic bumper cars begins! Right where the blocks touch, energy starts to flow from the hot metal to the cold metal. This flow of energy from hot to cold is called conduction, and although it happens slowly, it will naturally continue until the two objects are the same temperature.

Like a hot mug of tea or coffee, steaming in a chilly room: at first, the air around the mug will feel warmer, but not for long. The thermal energy of your favorite brew is gradually transferring into the surrounding air; this will continue until the drink comes to room temperature.

Unless, of course, you protect it with an insulating barrier!

Vacuum-insulated drink containers are double-walled; there is a thin space between the inner compartment which holds the beverage and the outer shell. During the manufacturing process, a vacuum pump pulls out as much air as possible from between these layers before they are sealed together at the neck. Now there is a space completely empty of molecules between your drink and the atmosphere.

No molecules means no molecular bumper cars. No molecular collisions means no heat transfer. 🤯 That’s the simple concept of how vacuum insulation works.

Of course, in real life, things are never perfect. In fact, a perfect vacuum is impossible to achieve. Even if it were, energy could still “jump” across the vacuum via radiation (think infrared light), although this problem is very minimal with shiny materials like stainless steel.

The most heat leakage actually occurs through the cap of the container – you may have noticed it gets quite warm after a while, even while the metal sides stay cool. Theoretically, we could keep the liquid hot much longer if we eliminated the cap altogether to completely enclose the inner container within vacuum insulation. But what would be the point in keeping our beverage hot if there was no way to drink it? 😆

Next time you take a scalding sip from your travel mug, you can appreciate the insulating power of nothing.

⚠️UPDATE: a Seattle-based law firm has filed a class action lawsuit against Stanley after many consumers with at-home test kits discovered many of their “Quencher” mugs reacted positively for lead. Stanley confirmed that their manufacturing process seals their famous vacuum-insulated cups with a lead pellet at the base, after which the area is covered in stainless steel and, in their words, “inaccessible” (unless this steel barrier falls off or is removed).

Stanley has called the lawsuit “meritless” and advised that their products have a lifetime warrantee, so if you have any issues with this seal, you can return the mug for a replacement.

To learn more, click the links in this post or reach out in the Comments field below with your questions. Thank you for your thoughts and feedback. 🙏🙏🙏

About Anne Hylden 3 Articles
Anne Hylden holds an M.S. in Inorganic Chemistry and is pursuing an M.A. in Science Writing. She taught math and chemistry for twelve years before turning to freelance writing. When she's not thinking about science, she enjoys drinking tea, walking her dog around Northwest Philly, and laughing with her partner Bob.

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