Structure Evolution of Electrorheological Fluids under Flow Conditions

Abstract

ABSRACT Transparent electrode with a conducting film on a glass surface provides us a useful tool to observe the fired-induced structure formation in ER fluids directly under both quiescent and dynamic conditions. In this paper the flow and field-induced structure evolution in ER fluids will be studied in three flow conditions, i.e., (a) ER fluids flowing through a slit channel between two fixed parallel transparent electrodes, which construct a model ER valve, (b) ER fluids being sheared between two concentric transparent electrode tubes, which correspond to a conventional rotary rheometer and (c) ER fluids being sheared between two parallel plate disks, which correspond to an ER clutch. Shear-induced layer structure was observed even when the shear rate reaches 600 s" 1 . A tentative phase transition diagram between the layer structure and the homogeneous flow state will be given.

Yield Hardening of Electrorheological Fluids in Channel Flow

“…We note that the structure formed under dynamic flow conditions may be trapped at a maximum packing volume fraction ϕ M that is lower than the maximum possible packing fraction reached under static conditions ϕ m . These conditions are in agreement with the experimental observations of Tang et al [8] and Nam et al [15] that show that cluster size observed in channel flow decreases with the imposed flow rate. The existence of several time scales in the evolution of the structure of ER fluids has been shown in previous reports [8,15,17,40,41]: a short time scale related to the aggregation of particles into chains and a longer time scale associated with cluster formation.…”

“…[25] is shown to ensure that a reproducible value of the dynamic yield stress is reached at steady state for a similar class of materials. The maximum shear rate applied is chosen as _ γ ≤ 4 s −1 to minimize formation of shear-induced lamellar structures during the steady-shear flow tests that tend to be associated with a nonmonotonic flow curve [7,8]. This protocol ensures that the ER fluid remains homogeneous during the steady-shear flow tests and that modeling using the Bingham model [Eq.…”

“…Optical transmissivity measurements performed by Qian et al [17] show cluster formation in pressure-driven flow and the formation of a compaction front at the entrance of the microchannel that is associated with an increase in volume fraction. In addition, images taken by Tang et al [8] show that after densification, at sufficiently high pressures, fingers appear near the inlet as the material yields. This behavior is reminiscent of fluidization in jammed granular media [30] where compaction fronts and finger formation at sufficiently high pressures are known to occur.…”

“…A similar model was used by Mueller et al [38] and Heymann et al [39] to fit the yield stress of suspensions of solid spheres with Kϕ m ¼ 2. We expect that this divergence from the case of solid suspensions is due to the fact that ER fluids are active materials that do not exhibit a yield stress in the absence of an electric field but rather develop this property through the aggregation of particles into chains and columns [4,15] and can form ordered lamellar structures upon the application of the electric field [7,8].…”

“…Thus, an understanding of the microstructures that form is crucial to predicting the mechanical performance of devices utilizing ER fluids. In shear mode, shear-induced lamellar structures are known to form, while in flow mode, the suspension microstructure tends to contain clusters and aggregates [7][8][9]. In addition, an enhancement in the shear yield strength has been shown in magnetorheological fluids as the fluid is compressed in the direction orthogonal to shear [10] and a strengthening of the microstructure of ER fluids has also been shown in squeeze mode by Tian et al [11].…”