Radio-Frequency Heating of Colloidal Suspensions: An Electrokinetic Study

Radio-frequency (RF) heating of colloidal suspensions, as a promising noninvasive and volume heating technology, has garnered significant attention in hyperthermia therapy and food processing. Nonetheless, while particle-mediated RF heating enhancement has been reported and used in vivo, the underlying heating mechanism remains unclear and contentious, largely due to reliance on phenomenological models. In this work, we analyze RF absorption and heating in colloidal suspensions using a semianalytical electrokinetic model, which enables quantitative separation of different microscopic contributions and systematic parameter sweeps. The model is applicable to nonmagnetic dielectric colloids in the regime of κa ≥ 1, where κ^-1 is the Debye length and a is the particle radius. Our results reveal that electrophoresis and ionic conduction, previously regarded as the two primary heating mechanisms, are not as dominant as once believed. In particular, for large particles (e.g., a ≈ 100 nm corresponding to κa ≈ 10), polarization induced by ionic relaxation emerges as the dominant mechanism driving RF heating. By contrast, for smaller particles approaching the lower bound of the model validity (κa ≈ 1), the contributions from ionic relaxation, electrophoresis, and ionic conduction become comparable, with none being negligible. We also demonstrate that while colloidal particles enhance heating at low electrolyte molarities, they can suppress it at high molarities. Our findings may provide intriguing insights into the thermal behaviors of colloidal systems under RF exposure.