Study of the Flow Field in the Magnetic Rod Interfacial Stress Rheometer

Abstract

Several technological applications, consumer products, and biological systems derive their functioning from the presence of a complex fluid interface with viscoelastic interfacial rheological properties. Measurements of the "excess" rheological properties of such an interface are complicated by the intimate coupling of the bulk and interfacial flows. In the present work, analytical, numerical, and experimental results of the interfacial flow fields in a magnetic rod interfacial stress rheometer (ISR) are presented. Mathematical solutions are required to correct the experimentally determined apparent interfacial shear moduli and phase angles for the drag exerted by the surrounding phases, especially at low Boussinesq numbers. Starting from the Navier-Stokes equations and using the generalized Boussinesq-Scriven equation as a suitable boundary condition, the problem is solved both analytically and numerically. In addition, experimental data of the interfacial flow field are reported, obtained by following the trajectories of tracer particles at the interface with time. Good agreement is found between the three methods, indicating that both the analytical solution and the numerical simulations give an adequate description of the flow field and the resulting local interfacial shear rate at the rod. Based on these results, an algorithm to correct the experimental data of the ISR is proposed and evaluated, which can be extended to different types of interfacial shear rheometers and geometries. An increased accuracy is obtained and the measurement range of the ISR is expanded toward viscosities and elastic moduli of smaller magnitude.

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Interfacial rheology: An overview of measuring techniques and its role in dispersions and electrospinning

“…This viscometer is one of the most sensitive instruments for measuring interfacial shear rheological parameters (21) and is capable of measuring surface shear moduli in a wide range of values (22). The prime feature of this rheometer is its long magnetic needle floating at the interface, which is moved by an external magnetic field (15,22,23). Its primary advantage over torsion pendulum surface viscometers is that it can be combined with a Langmuir trough, which enables control of the thermodynamic state of the interfacial monolayer (13).…”

Bulk flow driven by a viscous monolayer

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