Manipulation of ferrofluids encapsulated in sandwich structures using alternating magnetic field for high contrast in transmittance

In this study, we synthesized Fe3O4 magnetic nanoparticles (FMNPs, ca. 13.6 nm) that we dispersed in aqueous solutions at various concentrations to generate ferrofluids. Alternating magnetic field (AMF), generated by alternating currents with various frequencies, was exploited to manipulate the particle-chaining behavior in the ferrofluids. After encapsulating these ferrofluids in a sandwich-structured display (SSD), we evaluated the particle-chaining behavior of the FMNPs through measurements of their transmittance. The transmittance of the SSD encapsulating the 8.26 wt% ferrofluid changed significantly from 1.3 to 83 % under an AMF having a field strength of 87.5 G at a frequency of 700 Hz within a response time of <1 s. The high contrast could be observed by naked eyes due to 81.7 % of sufficient change in transmittance and 1:64 of the contrast ratio. Moreover, an iron plate featuring hollowed-out letters “NTUST” was exploited as a magnetic shielding material to verify the high contrast. When we placed the iron plate between SSD and AMF, a patterned AMF was generated with open and closed zones, respectively. The FMNPs in the SSD assembled as particle chains behind the open zone, resulting in a transparent state, while the FMNPs remained well dispersed behind the closed zone, resulting in an opaque state. The letters “NTUST” were readily recognizable because of the high contrast between the transparent and opaque states. The FMNPs in this ferrofluid could be manipulated into particle chain assemblies reversibly for magnetic display applications.