Analysis of changes in optical properties with time as a way to assess the instability of the structure of diluted aqueous-based ferromagnetic fluids

•Laser optoacoustic method was used.•Diluted ferromagnetic fluids were studied.•Light extinction coefficient and its depth distribution in ferrofluids were measured.•Light extinction coefficient decreases several days after dilution.•Optoacoustic method allows one to identify the violation of fluid stability. The work demonstrates the use of the laser optoacoustic method for observing the changes of the light extinction coefficient in unstable ferromagnetic fluids over time. The initial stabilized fluids were further diluted with a carrier liquid to violate the stability of the colloidal system. The form of optoacoustic signals excited in a ferromagnetic fluid carries information about its optical properties, which allows one to measure the light extinction coefficient of this fluid and obtain its distribution over the depth of fluid. The effect of dilution of ferromagnetic fluid on its optical properties was demonstrated. It was shown that the light extinction coefficient decreases by about 20% after 7 days after dilution, and after 30 days, its value decreases by half. The observed decrease in the extinction coefficient over time is a result of structural changes occurring in diluted ferromagnetic fluids. The proposed optoacoustic method can be useful for studying the optical properties of ferromagnetic fluids for developing optical devices based on them and also may be used to evaluate the stability of ferromagnetic fluids obtained by new technologies and fluids with a new type of stabilizer or carrier liquid.