Abstract
The electrical charges naturally present at liquid/solid interfaces or which develop in liquids in a controlled or undesired way are at the origin both of numerous applications (micro-pumps, lab-on-a-chip, super-capacitors) and of industrial risks (inflammations, explosions). This requires the most precise possible quantification of the values and distributions of these charges in liquid dielectric media. The present measurement techniques are either non-resolutive, as the responses measured on the entire specimen allow to deduce information about the charges and the associated processes indirectly (through models), or limited in terms of sensitivity and resolution. For example, the electrical double layer model, proposed by Stern in 1924 and fully accepted by the scientific community since then, has not been confirmed experimentally yet, in particular due to the lack of resolution and sensitivity of the existing methods.Several techniques have been developed since 1980 to measure directly and non-destructively the electrical charges in solid insulators. Among these methods, those based on the application of low-amplitude thermal stimuli have shown high sensitivity and performance for the measurement of loads near interfaces. The application of these techniques to liquids is a research path to be explored in order to answer the above scientific and applicative questions. This work studies, both from theoretical and experimental points of view, the application of the thermal step principle to the measurement of electrical charges in insulating liquids. The electrical double layer, which sets up at the level of the liquid/solid walls, is used as object of the study. Several dielectric liquids are concerned: cyclohexane in liquid and solid state, pure and additive mineral oil and silicone oil.Through numerical simulations, electrical responses expected from the application of thermal stimuli of several degrees to insulating liquids are calculated, considering the diffuse layer present at the interface with the walls. The influence of different parameters of the electrical double layer on the simulated signals is studied. In particular, the effects of thermo-convection are quantified and criteria to identify them in the electrical responses are established.Experimental results, obtained with a measuring installation designed and built specifically for liquids, prove that the responses resulting from the application of low-amplitude thermal steps to insulating liquids are well measurable. Their detailed analysis leads to the conclusion that these responses are indeed due to charges from the electrical double layer. The probable contribution to the measured signals not only of the diffuse layer, but also of the compact layer, is highlighted. The results allow to conclude that thermal stimuli methods are applicable to dielectric liquids. Their further development should lead to experimental implementations with resolutions and sensitivities adapted to the study of electric charges and fields at the interfaces and in the volume of these materials.