The structural force arising from magnetic interactions in polydisperse ferrofluids

Anrong Wang; Jian Li; Rongli Gao
May 2009
Applied Physics Letters;5/25/2009, Vol. 94 Issue 21, p212501
Academic Journal
Ionic ZnFe2O4 is a weak magnetic ferrofluid with different particle sizes (a polydisperse ferrofluid), in which the dipolar coupling constant λ is less than 2. By comparing the reduced magnetization curves and initial magnetic susceptibilities of ferrofluids with different particle volume fractions [lowercase_phi_synonym], it is found that there are field-induced attractive interactions between the colloidal particles, which make them aggregate; this interaction decreases with [lowercase_phi_synonym]. The dipolar coupling constant is so small that particle aggregation cannot be induced by magnetic interaction alone. Thus, it can be concluded from the experimental results that a nonmagnetic attractive interaction is stimulated during the magnetization process for ferrofluids. Using a model based on “large” and “small” particles (the bidisperse model), this nonmagnetic interaction can be explained as a field-induced structural force. This force is enhanced following magnetic interactions between the large particles as the applied magnetic field increases. The field-induced structural force is similar to a depletion force and can also arise in strong magnetic ferrofluids (λ>2) with polydisperse particle sizes under an applied external magnetic field.


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