Consideration of electromagnetic wave interference on microorganism inactivation on reflective surfaces

This paper explores how wave interference, a well-established physical phenomenon, significantly difficult the radiation inactivation of microorganisms on a reflective surface. Incident waves on a surface interact with the reflected ones generating an interference pattern very close to it, that is, spatially stable regions of exposure maxima and minima. Particularly during surface sterilization, pathogens may find refuge within these areas of minimal, or even null exposure. Regardless of the radiation source or its wavelength, these regions of minimal or null exposure due to wave interference will always persist close to the surface. We study this phenomenon within two distinct contexts: ultraviolet (UV-C) light exposure, which primarily induces photochemical damage to microorganisms, and microwave exposure on microorganisms deposited on the internal wall of a domestic oven, where the inactivation mechanism is predominantly thermal. In the first case, we provide estimations of this effect for different UV-C sources (Hg and LED like) and for COVID-19 viruses on metallic surfaces. The inactivation curves with irradiated dose present relevant saturation tails. For the case of microwave oven exposure, we present both analytical and experimental results that link irradiation fluence and thermal rise of area sensors at different locations within the oven. A thermal imaging camera corroborates that an interference minimum is always present at the cavity wall metallic surfaces. Typically, the thermal rise ΔT is of 1-2 orders of magnitude lower near the wall (< 1 mm) than in the interior (e.g. ΔT ~ 1 to 3 °C near the wall for ΔT ~ 30 to 100 °C in the interior).