Debye relaxation in water, aka “how microwaves work” explained at last

My new paper,The origin of the Debye relaxation in liquid water and fitting the high frequency excess response” has been published in The Royal Society of Chemistry’s journal Physical Chemistry Chemical Physics.

Back in July 2013 I wrote a blog post about how microwave ovens work. Prior to starting my Ph.D. research, I thought that microwaves were tuned to one of the molecular vibrations of water, which are at 1500 1/cm and 3200 1/cm. Actually, they are tuned to the Debye peak, which is a collective relaxation at ~0.45 1/cm and the lowest frequency peak in the dielectric spectrum. During the past 3-4 years I have been working off an on to understand Debye relaxation. It started at the beginning of my Ph.D. research, when my adviser suggested an analogy between the dielectric response of relaxor ferroelectric materials and water, which was discussed in our first paper together.  While we found many remarkable similarities between the dielectric spectra of water and relaxors, (especially the temperature dependance), I largely view the idea of polar nanoregions as a dead end in terms of teasing apart Debye relaxation – for instance  polar nanoregions in relaxors lead to stretched exponential relaxation, which is not observed in bulk water relaxation. Our work did find that Debye relaxation is very collective in nature, involving hundreds of molecules. This fact did not jive well with previous mechanisms that had been proposed.

I continued to study Debye relaxation and the excess response on the high frequency side of the Debye peak in the intervening years. There were three challenges. The first was reconciling the simple nature of pure Debye relaxation found with water with what we know about H-bond network dynamics, which are complex, stretched, and fractal. The second challenge was explaining the collective nature of the relaxation. The third challenge was explaining the excess response found on the high frequency side of the Debye peak – several different fits and interpretations have been proposed, none of which were entirely satisfactory. It was only recently that I have finally come across a satisfactory theory that answers all these questions in one unified approach, with the 2016 publication of the defect diffusion model of Popov, Ben Ishai, Khamzin, and Feldmin. My results support Popov’s et al.’s theory.

Water has been an interesting subject to study. As a complex liquid, there always seems to be another layer of detail to unravel. I am sure people will continue to explore more details about the dielectric response of water. In my mind though, the physical mechanism behind Debye relaxation has been sufficiently explained with the theory of Popov et al. and my supporting studies.



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