Experimental method for measuring binodal curves of magnetic fluids exhibiting field-induced magnetic condensation of nanoparticles

• Published in: Journal of molecular liquids 2024-11, Vol.414, p.126020, Article 126020
• Main Authors: Ivanov, Aleksey S., Somov, Sergey A.
• Format: Article
• Language: English
• Subjects: Aggregation; Bi-disperse model; Binodal curve; Magnetic fluid; Phase transition
• ISSN: 0167-7322
• DOI: 10.1016/j.molliq.2024.126020

The paper describes theoretical foundations and an experimental method for measuring binodal curves of magnetic fluids undergoing field-induced magnetic condensation of colloidal nanoparticles (a phase transition of the “colloidal gas - colloidal liquid” type). The laboratory technique allows for the measurement of the volume fractions of the gaseous kI and condensed kII phases at fixed temperature T and applied field H using optical observations of a light-transparent Hele-Shaw cell filled with a magnetic fluid. A step-by-step algorithm details how to obtain a binodal curve of a magnetic fluid through numerical processing of experimental data. First, the bi-disperse model is used to simplify the continuous particle size distribution function of the test sample (which is known from magneto-granulometric analysis of its magnetization curve). Second, the hydrodynamic concentration of nanoparticles in both phases at a given set of parameters (H,T,kII) is calculated using a system of two linear and three non-linear transcendent equations that describe volume conservation of particles and thermodynamic equilibrium of phases within the framework of first-order theory of perturbations. For demonstration purposes, four magnetic fluids with significantly different dispersion compositions were studied. The results showed that binodal curves reflect the individual properties of ferrofluids, thus allowing for the description and prediction of their colloidal stability at any pair of parameters (H,T). •Binodal curve of a ferrofluid undergoing field-induced phase transition is measurable.•Binodal curve reflects individual disperse composition of any magnetic fluid.•Binodal curve predicts ferrofluid phase stratification at any field and temperature.