Electrorheological Source of Nonlinear Dielectric Effects in Molecular Glass-Forming Liquids

Abstract

We have measured the dielectric relaxation spectra of eight glass-forming liquids in the presence of electric direct current (dc)-bias fields ranging from 100 to 500 kV/cm. For every sample, we observe two distinct field-induced effects: a reduction in the relaxation amplitude and an increase in the primary structural relaxation time that is associated with viscous flow. Whereas amplitude change is typical of the well-known dielectric saturation, the field-induced increase in viscosity is a source of nonlinear behavior that has been recognized only recently. We find that this electrorheological behavior occurs in all polar liquids of this study, and its magnitude is correlated with the field-induced change in thermodynamic entropy. It constitutes a significant source of nonlinear dielectric behavior, which occurs for both dc and alternating current fields.

Third and fifth harmonic responses in viscous liquids

“…Summarizing this section about entropy effects, we remind the two main assumptions made by Johari: (i) the field-induced entropy variation mainly goes into the configurational part of the entropy; (ii) its effects can be calculated by using the Adam-Gibbs relation. Once combined, these two assumptions give a contribution to χ (1) 2;1 reasonably well in agreement with the measured values in several liquids [65,66,67,68]. An extension to χ (3) 3 is even possible, at least in the region ωτ α ≤ 1 and fairly well accounts for the measured hump of |χ The "Box model" is historically the first model of nonlinear response in supercooled liquids, designed to account for the Nonresonant Hole Burning (NHB) experiments [73].…”

“…Motivated by several works [65,66,67,68] showing that Johari's reduction of entropy fairly well captures the measured χ…”

Investigation of nonlinear effects in glassy matter using dielectric methods

“…ii) Application of a high ac field and determination of the higher harmonics of the response, leading to higher-order permittivities [12,13,18,20,21,22,76,77]. iii) Application of a high dc bias voltage and measurement of the permittivity, usually with a smaller ac signal [78,79,80,81,82]. In addition, also hole-burning experiments can be considered as nonlinear measurements: iv) Brief application of a high ac field ("pumping"), followed by a measurement with low field [26,83].…”

Third and Fifth Harmonic Responses in Viscous Liquids

“…Johari's idea was positively tested in the corresponding χ (1) 2;1 experiments in Refs. [64][65][66][67]-see however Ref. [68] for a case where the agreement is not as good.…”