Unusual electrorheological response of low concentration suspensions subjected to simultaneous influence of electric and mechanical fields

The electrorheological (ER) effect describes the formation of particle structures within a suspension of polarizable, semiconducting particles dispersed in an insulating medium upon application of an electric field. This phenomenon creates columnar structures that span the electrodes, facilitating a transition from liquid-like to solid-like behavior and markedly improving the fluid's rheological properties. This study investigates the phenomenon referred to as electromechanical strengthening. We found that simultaneously applying an electric field and large-amplitude oscillatory shear deformations to ER suspensions substantially enhanced their linear rheological properties, compared to the response observed under small or moderate amplitude oscillations. While at small and moderate oscillation amplitudes, the width of the columnar structures remained largely unchanged, resulting in minimal dependence of the storage and loss moduli on strain amplitude, the column width began to increase with strain beyond a certain threshold of large strain amplitudes, accompanied by a more than tenfold rise in the dynamic moduli. At sufficiently large strain amplitudes, the disruptive mechanical forces surpassed the structuring effect induced by the electric field, leading to the erosion and thinning of columnar assemblies and a subsequent decline in both storage and loss moduli. The coupled electromechanical strengthening effect is demonstrated to be a generic feature of ER suspensions, manifesting independently of the suspended particles' chemical composition, morphological characteristics, or dimensional scale.