Miniature magnetic rod interfacial stress rheometer for general-purpose microscopes

Despite the great demand for the characterization of interfacial rheology in academic and industrial research, the study of interfacial rheology is still scarce compared to its counterpart of bulk rheometry and limited only to specialized laboratories. One of biggest hurdles impeding the broad application of interfacial rheometry is the delicate design and the high cost of interfacial rheometers. Here, we propose a new design of a miniature magnetic rod interfacial stress rheometer (mini-ISR), which uses a pair of small permanent magnets as a magnetic trap and a single magnetic coil for perturbation. The simple design of our ISR substantially reduces its dimension, allowing us to directly couple the rheometer to a conventional commercial optical microscope. Such unprecedented adaptability makes the new ISR highly portable and cost-effective. Moreover, the use of a commercial microscope improves the imaging quality and lowers the difficulty of synchronized imaging of interfacial rheometry. We show the calibration of the mini-ISR and demonstrate its functionality by measuring the interfacial rheology and imaging the microscopic dynamics of particle monolayers at a water-oil interface. Our design can be used by any laboratories that have access to optical microscopes for a wide range of interfacial rheology studies.