Insights on the wall friction using squeeze flow of an electrorheological fluid

In this paper, the squeeze flow of an electrorheological (ER) fluid has been studied under a constant electric field and squeezing speed to understand the link between the extent of structural arrest and the wall slip. The flow behavior during squeezing has been captured in terms of normal force as a function of gap under a parametric variation of electric field intensity for four different compression speeds to understand the impact of both these parameters on wall slip in terms of wall friction coefficient. Based on the growth rate of normal force with respect to inter-plate gap, these squeeze flow curves have been divided into four zones: elastic response, relaxation zone, slip flow, and squeeze-strengthening regime. The first two zones are very short-termed, whereas the major portion of the flow regime is slip flow. The squeeze strengthening effect is only pronounced at very low compression speeds for moderate/higher electric field strengths in the low gap regime. The slip flow regime has been modeled using a squeeze flow model, which renders the wall friction coefficient as a model parameter. The major finding of our analysis is that the friction coefficient is an increasing function of electric field strength at a fixed compression speed, whereas an increase in compression speed leads to a decrease in the extent of wall friction. Based on these observations, it has been established that electric field-induced structural jamming and compression-led breakage of structure in the ER fluids play a crucial role in controlling the wall friction during its squeeze flow. We have also modeled the slow squeezing-led structural strengthening regime by considering a particle-scale model, which accounts for the local electric field contribution. Finally, a broader physical picture has been provided to underline how the degree of the structural arrest/jamming affects wall friction during the squeeze flow of such field-responsive soft materials.