Abstract
Uniaxial extensional flow is a canonical flow typically used in rheological characterization to provide complementary information to that obtained by imposing simple shear flow. In spite of the importance of having a full rheological characterization of complex fluids, publications on the rheological characterization of mobile liquids under extensional flow have increased significantly only in the last 20 years. In the case of the rheological characterization of electrorheological fluids, the situation is even more dramatic, as the ERFs have been exclusively determined under simple shear flow, where an electrorheological cell is attached to the rotational rheometer generating an electric field perpendicular to the flow direction and that does not allow for inverting the polarity. The very recent work published by Sadek et al., who developed a new electrorheological cell to be used with the commercial Capillary Breakup Extensional Rheometer (CaBER), allows for the very first time performing electrorheometry under extensional flow. By means of the same experimental setup, this study investigates the influence of the polarity of the imposed electric field on the filament thinning process of a Newtonian and an electrorheological fluid. Results show that a polarity against the gravity results in filament thinning processes that live longer or reach a stable configuration at lower intensities of the applied electric field.
Highlights
IntroductionMore than eighty years ago, W.M. Winslow 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]
It can be observed that all the curves overlapped, which was clear evidence for the presence of the electric field, and its intensity did not affect the rheological behavior of the olive oil under shear flow
The electrorheological cell in the rotational rheometer did not allow changing the polarity in the electric field, which was perpendicularly oriented to the flow field
Summary
More than eighty years ago, W.M. Winslow 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]. Winslow 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 known as the electrorheological effect. 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 been
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