The Cure for the Common Cold (Spot in Your Microwave)

Microwave ovens may get the job of heating food done quickly, but they do not do an especially good job of it. They are notorious for keeping the center of your leftovers colder than a penguin's toes while turning the edges into molten lava. With all that spinning and buzzing, you would think that the microwave would be able to distribute the heat evenly, but no — time after time they fail us. That leaves us suckers taking one bite that is frozen and another that burns like the fiery door of the oven that we probably should have used instead.

But when you have a craving for pizza rolls that just cannot wait for a traditional oven to be preheated, before taking its jolly old time to cook the food, a microwave might be your only real option. If only they could heat our food more evenly. Fortunately, a team led by researchers at Carnegie Mellon University was also annoyed by the poor heat distribution of microwave ovens, and they decided to do something about it. They have developed a low-cost, passive metasurface, called MicroSurf, that alters the heat distribution of a microwave to more evenly warm food.

Traditional methods, such as turntables and stirrers, are intended to solve these problems, but do not do enough in practice. More modern approaches, like advanced software-defined cooking techniques, have limitations in achieving a consistent 3D heat distribution and are often impractical to implement.

To overcome these problems, the researchers first modeled microwave ovens to better understand the nature of the problem. They did this by creating detailed 3D CAD models that captured the geometry and key components of the ovens, including the main chamber, waveguide, and antenna. Using the Ansys HFSS simulator, they modeled the electromagnetic field distribution and used Ansys Icepak to estimate the resulting heat distribution. To align simulations with real-world behavior, they fine-tuned the models based on measurements from five microwave brands.

Using insights gained from the models, MicroSurf was developed. It utilizes passive metasurfaces to control electromagnetic (EM) energy distribution within the microwave cavity. These metasurfaces are carefully engineered to resonate with the microwave's operating frequency while meeting safety and performance criteria, such as smooth edges to prevent sparks, substrate-free construction to avoid overheating or discharge, and sufficient phase control of traveling EM waves. The metasurfaces are optimized in terms of shape, material, and placement using advanced optimization frameworks.

MicroSurf was evaluated using multiple microwave ovens and a variety of food items to assess its performance in enhancing heating efficiency and uniformity. Tests with water, bread, meat slices, and milk demonstrated significant improvements over standard microwave heating with a rotating plate. For example, MicroSurf increased average temperatures by 2 to 4 degrees Celsius while reducing temperature variation by up to 80 percent in some cases. Additionally, MicroSurf optimized heating for specific food shapes and regions by, for example, reducing the standard deviation in tall objects like milk or evenly heating edges and centers in bread and meat slices. Tests across different foods further validated its generalizability, showing consistent gains in uniformity and efficiency.

Looking down the road, the team is planning to explore how their technology might be used for applications beyond food heating, including in material processing and the actuation of soft robots. Until then, we will just have to be content with pizza rolls that do not have a core of ice.