Convection currents don’t need the heat source to be directly at the bottom to stir the liquid, it just needs cold water to be on-top of hot, because cold is more dense.
Microwaves don’t really heat top to bottom either, it’s shooting waves through the body of the water and even the cup, directly exciting a bunch of individual H2O atoms in hot spots where the microwaves peak at, (e.g. the actual microwaves not the name of the machine) heating the liquid very unevenly. The wave could very much be heating a fraction of the top, middle, and bottom at different points in 3d space. it just depends on the peak of the micro-waves.
I mean, it’s not really a matter of debate TBH. There are a number of peer reviewed journal articles documenting the temperature stratification. Here is one source, where the authors attempt to create a special cup to heat the water more evenly.
I’m well aware of temperature stratification. It doesn’t happen in a microwave in the same way.
Micro waves don’t heat purely the top surface, they penetrate the entire waters body creating super-heated localized hotpots that shift the water around from Convection currents because the hotter more excited water atoms are less dense than the colder less excited water atoms above them spreading temperature out from those hotspots.
Temperature stratification only comes into play if there’s no nucleation point, in which you get this.
Also, your link is dead.
I’m well aware of temperature stratification. It doesn’t happen in a microwave.
It empirically does. We can argue about the theory all day but the research says microwaves produce stratified temperature gradients when heating liquids. However, I’d point out that, in atmosphere, when we have localized hot spots the warm air can effectively travel in bubbles without significant mixing for quite some distance. There seems to be a similar phenomena at work when microwaving liquids.
See the screenshot below.
I pulled this from “Multiphysics analysis for unusual heat convection in microwave heating liquid” published in 2020 in AIP Advances.
Relevant excerpts:
“ Usually, the fluidity of liquids is considered to make the temperature field uniform, when it is heated, because of the heat convection, but there is something different when microwave heating. The temperature of the top is always the highest in the liquid when heated by microwaves.”
“ The experimental results show that when the modified glass cup with 7 cm metal coating is used to heat water in a microwave oven, the temperature difference between the upper and lower parts of the water is reduced from 7.8 °C to 0.5 °C.”
“According to the feedback from Midea (microwave appliance makers), when users use the microwave oven to heat liquids such as milk or water, the temperature at the top of the liquid will be significantly higher than the temperature at the bottom.”
