(399g) Pure Elongation Flow Behavior of an Electrorheological Fluid As a Model Soft-Jammed System: Understanding Wall-Slip from Electrorheology

This study delves into the behavior of an electrorheological (ER) fluid as a model soft-jammed system under pure elongation flow conditions wherein the degree of jamming is regulated by an externally applied electric field. Specifically, we induced pure elongation flow by facilitating significant slip at the interface between the material and rheometer plate, while steadily pulling it at a constant velocity under a consistent external electric field. The shape evolution of ER fluid during pure elongation flow is shown in Fig.1. During the flow, we measured the normal force exerted by the top plate on the material as a function of gap for various combinations of pulling velocity and electric-field strength. The force-gap relationship followed a distinct pattern: initially, the force rises to a maximum (region-I), then decreases with increasing gap (region-II), as shown in Fig.2. Notably, in region-II, the force-gap curve shifts towards higher gaps with greater electric field strength for any given pulling velocity. Intriguingly, these curves in region-II display superposition, reflecting the self-similar nature of the flow across gap, electric field, and velocity. Furthermore, we modeled the flow curves using a slip-layer model, rendering remarkable predictions and enabling estimation of slip-layer thickness. Fig.3 shows slip-layer thickness as a function of electric field strength. We observed a decrease in slip-layer thickness with increasing electric field magnitude for a given pulling velocity, indicating the significant influence of jamming on slip dynamics. Hence, investigating pure elongation flow provides valuable insights into wall-slip dynamics and its correlation with material properties and surface characteristics.