# Influence of the Polarity of the Electric Field on Electrorheometry
## 1 Introduction  
More than eighty years ago, [PERSON] reported that some suspensions of non-conducting, but electrically active particles dispersed in an electrically insulating fluid exhibit a reversible sudden increase of the viscosity under the application of an external electric field [1, 2]. Still, currently, this phenomenon remains known as the Winslow effect, but it is also known as the electrorheological effect. Electrorheology is, therefore, a branch of rheology dealing with the study and characterization of the rheological properties of electro-responsive materials [3]. In order to model electrorheological fluids (ERFs) accurately, detect subtle dissimilarities in their composition, and describe or predict the processing conditions that will optimize the fluid flow or characteristics of the final product, a thorough rheological characterization of these materials in both shear and extensional flow conditions is recommended [4]. Among the plethora of techniques to perform a rheological characterization with commercial rheometers [5], the ERFs have beentraditionally studied by means of rotational rheometers, which are devices that impose either a torque or a velocity of rotation and measure the response in terms of angular deformation or torque, respectively. An electrorheological cell imposes the external electric field to the fluid sample, which is perpendicular to the flow field generated in the fluid by the rotational rheometer. It is widely accepted that, after a few milliseconds of the application of an external electric field, the suspended particles form chains along the direction of the electric field [6; 7; 8]. Based on this model, it can be concluded that under shear flow with an electric field perpendicular to the flow direction, the particle chains undergo a shearing force. In an attempt to characterize the effect of a force aligned with the particle chain, ERFs have also been characterized in squeeze flow by means of a rotational rheometry [9]; however, there are important issues with these kinds of experiments, such as the lack of accuracy in determining the real slip condition, the compliance of the instrument, and the fact that the shear wall effect makes it impossible to analyze the pure extensional behavior of the ERFs [10; 11]. To the best of the authors knowledge, the electrorheological cells designed for rotational rheometers do not allow changing the polarity in the electric field, and therefore, the influence of the polarity in the shear characterization of the ERFs has not been reported yet. In order to characterize the extensional properties of complex fluids properly, it is recommended to impose a uniaxial extensional flow. Nevertheless, due to technical challenges, there is no special add-on for rotational rheometers allowing the extensional characterization of liquid-like fluids. For the extensional characterization of fairly mobile liquids there are currently two commercial devices available, i.e., FiSER and CaBER, but none of them are equipped with an electrorheological cell allowing the application of an external electric field [12]. Very recently, [PERSON] et al. [13] developed an add-on, named the Extensional Electrorheological Fixture (ExERF) [13], that overcomes this limitation for the CaBER device and allows performing Capillary Breakup Extensional Electrorheometry (CaBEER) [14]. This pioneering work allows the characterization of ERFs under the simultaneous application of an electrical field and an extensional flow both aligned with gravity; however, the influence of the polarity of the electric field in the measurements has not been reported yet. In this study, we focus our attention on the influence of the polarity on the rheological behavior of ERFs and its suspending fluid under shear and extensional flow conditions. It has been found that the polarity of the electric field indeed affects the filament thinning process of both the carrier fluid (olive oil) and the electrorheological fluid.