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The Problem with Microwaving Tea:
Typically, when a liquid is being warmed, the heating source — a stove, for example — heats the container from below. By a process called convection, as the liquid toward the bottom of the container warms up, it becomes less dense and moves to the top, allowing a cooler section of the liquid to contact the source. This ultimately results in a uniform temperature throughout the glass.
Inside a microwave, however, the electric field acting as the heating source exists everywhere. Because the entire glass itself is also warming up, the convection process does not occur, and the liquid at the top of the container ends up being much hotter than the liquid at the bottom.
A team of researchers from the University of Electronic Science & Technology of China studied this nonuniform heating behavior and presents a solution to this common problem in the journal AIP Advances, from AIP Publishing.
By designing a silver plating to go along the rim of a glass, the group was able to shield the effects of the microwave at the surface of the liquid. The silver acts as a guide for the waves, reducing the electric field at the top and effectively blocking the heating. This creates a convection process similar to traditional approaches, resulting in a more uniform temperature.
Placing silver in the microwave may seem like a dangerous idea, but similar metal structures with finely tuned geometry to avoid ignition have already been safely used for microwave steam pots and rice cookers.
Journal Reference:
Peiyang Zhao, Weiwei Gan, Chuanqi Feng, et al. Multiphysics analysis for unusual heat convection in microwave heating liquid [open], AIP Advances (DOI: 10.1063/5.0013295)
(Score: 1) by shrewdsheep on Wednesday August 05 2020, @07:22AM (2 children)
As a tea drinker I'm naturally interested. TFS or the TFA seem to be sloppy on some point though.
1. If microwaving creates uniform temp but the top is hotter in the end that implies convection, right?
2. The premise of uniform temp is wrong to begin with as microwaving heats nodes in the microwave space from which local convection emerges.
3. What if I heat my water in a water cooker (as I usually do)? If I poor it into the tea pot temp is now really uniform (although there is still mechanical movement). Would that water have to be topped of with cold water for the gradient?
I demand further research to clarify my points. I recommend the researchers try different colors of the silver strip, real tea in the cup but also to step into the microwave themselves for closer inspection (while the experiment is ongoing).
(Score: 1) by jurov on Wednesday August 05 2020, @08:41AM
Convection happens only if denser (cooler) liquid is on the top, so it tends to displace less dense liquid under it.
(Score: 0) by Anonymous Coward on Wednesday August 05 2020, @09:50AM
I think the premise is wrong. It is not uniformly heated. A good trick if you're heating a plate of food is shape it as a torus or else the middle will end up colder.
As far as I remember microwave heating creates microexplosions in water molecules and it intuitively makes sense that most reactions would happen as soon as possible near the outer edges.