In this work we present a wave equation for ferrofluids including fluid or rigid spherical aggregates and derive expressions for the ultrasound propagation velocity and absorption coefficient. These include separate terms making evident the viscous and thermal effects of oscillating and pulsating aggregates. We are only analysing the ultrasound propagation in ferrofluids with micronic aggregates and with variable structural parameters in the absence of a magnetic field. The obtained and discussed numerical data give qualitative information preceding the study of ultrasound propagation in ferrofluids with fluid aggregates in the presence of a magnetic field.
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