New model explains unusual feature in high field spectra of supercooled liquids
New model explains unusual feature in high field spectra of supercooled liquids lead image
In most cases, the dielectric polarization of a material caused by an electric field is proportional to the electric field strength (E) for sufficiently low E, but for higher E this response becomes non-linear. Through a simple model, Thoms and Richert reproduced an unusual behavior of a non-linear feature.
The feature in question is a hump in the third harmonic electric susceptibility. It has formerly been associated with regions of cooperative molecular motion, so a change in the intensity of the hump implies a change in the length scale of such regions. In the supercooled liquid (S)-(-)-4-methoxymethyl-1,3-dioxolan-2-one (SMPC), the hump seemingly disappears from 195 K to 200 K. This would imply a rapid breakdown of cooperativity.
Thoms et al. found, however, that this hump is predominantly the result of accelerated dynamics caused by the energy that the sample absorbs from the electric field. In SMPC, the hump only seemingly disappears because it is superimposed by a different non-linear effect, polarization saturation.
“Notably, our model reproduces this feature, and its disappearance, based only on low field (linear response) dielectric relaxation data and the static limits from high dc-bias field (non-linear) experiments evaluated at the fundamental frequency,” author Erik Thoms said. “No length scale is required to describe the hump”, contrary to a cooperative regions interpretation.
“We employed the concept of a fictive electric field for non-equilibrium systems, equivalent to the field at which the equilibrium counterpart would show the same polarization state,” Thoms said.
The results from this study are of fundamental interest for unveiling the nature of the elusive glass transition.
Source: “A ‘hump’ in the third-order dielectric response of a highly polar liquid: Now you see it, now you don’t,” by Erik Thoms and Ranko Richert, Journal of Chemical Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0248963 .
