A magnetic rod interfacial shear rheometer driven by a mobile magnetic trap

Abstract

The magnetic rod interfacial shear rheometer has been successfully used over the past years to measure mechanical properties of fluid-fluid interfaces. However, the resolution is limited due to the finite system compliance and the probe mass and size. Here, we propose a new design that modifies the way in which the magnetic force is imposed on the probe by substituting the usual coils by a pair of small permanent magnets forming a magnetic trap. Suitably displacing the magnet pair imposes a longitudinal force on the probe that depends on the probe-trap displacement. We show that the relative importance of the surface drag to the rest of the forces acting on the probe is increased, allowing more precise measurements. We have checked our dynamical model on a clean air-water interface, showing that the device response depends only on the elastic constant of the magnetic trap. We have made a full characterization of the performance of several trap-probe combinations showing that the new configuration yields a null system compliance and a stable probe positioning. We have checked the rheometer performance on silicone oil thin films and pentadecanoic acid Langmuir monolayers, showing that surface viscosities as low as 10−9 N · s/m are measurable.