Study on the influence of surface roughness on wall slip effect (in rheological measurement) of magnetic fluid

Abstract

Magnetic fluid is a new type of smart material, and its rheological properties have always been a hot research topic, and how to measure quickly and accurately is also a research difficulty, among which wall slip is one of the important influencing factors. In this work, we studied the effect of wall slip on the flow curve and yield stress of the magnetic fluid. To this end, we used multiple rheometer rotors with different surface roughness. The flow curves of two magnetic fluids with different viscosities were drawn, and the Herschel-Bulkley model was used to fit them to determine the yield stress. In addition, a forced shear oscillation test was carried out on the magnetic fluid. We found when using a rotor with a smooth measuring surface to measure the flow curve of the magnetic fluid, the shear stress of magnetic fluid is underestimated at low shear rates. At the same time, the fitted values of yield stress of various roughness measurement surfaces were similar and greater than those of smooth measurement surfaces. From an engineering point of view, the fitting values of non-smooth measurement surfaces were more reliable. Affected by the wall slip, the storage modulus curve of the magnetic fluid often dropped suddenly during the frequency sweeping oscillation process. By changing the surface roughness of the measuring rotor, the wall slip during the oscillation sweep process was effectively restrained. The research results will have certain guiding significance for the study of the state of the solid–liquid interface junction in applications such as magnetic fluids sealing and lubrication.