The investigation of fluid instabilities and surface tension in a double-layer magnetic tape coater

The instabilities of a double-layer channel Couette flow for application in a magnetic tape coater were investigated experimentally and compared with the theoretical predictions of a linear instability analysis. The linear instability analysis shows that the interfacial instability regime depends strongly on the depth ratio between the more viscous fluid and the thinner fluids. Our experimental results agree well with the theoretical predictions of the linear instability analysis in the short wave instability regime. However, we can find short waves even in the range of the depth ratios when the linear instability predicts only long waves are unstable. The reasons are believed to be the dimensional limitation of the experimental apparatus. In addition, we studied the dependence of the onset of the short waves on the viscosity and density of the fluids.;Surface tension is a very important parameter in the two-layer fluid flow. Higher surface tension can significantly stabilize the short waves. In our study, the surface tension of charge-stabilized non-magnetic suspensions was first investigated. Our results show that the surface tension of the dispersions strongly depends on the particle concentration. Thermodynamic arguments are used to explain the initial decrease in the surface tension for lower particle concentrations. For higher concentrations, it is hypothesized that the capillary forces acting between the immersed particles at the interface cause the increase in the surface tension.;The surface tension of acicular magnetic dispersions is believed to depend on the particle orientation. Because external shear and magnetic fields can change the particle orientation in the dispersions, these will also affect the surface tension of the dispersions. The interfacial stress balance of a flat interface between a homogeneous acicular particle dispersion phase and an isotropic phase shows that the bulk stresses can balance each other and would not change the surface tension directly. Therefore, we can study the effect of external shear and magnetic fields on the surface tension by investigating the bulk particle orientation and structure parameter. Further experimental measurements of the surface tension of a magnetic dispersion under external shear flow or magnetic field are necessary to verify this interesting prediction.