As a novel functional nanomaterial, Fe₃O₄ magnetic nanoparticles (MNPs) modified by different surfactants have attracted and are attracting worldwide interest. In this research, we introduced two different silane coupling agents to modify Fe₃O₄ MNPs instead of a single surfactant to achieve complete coating and functionalization. The modification mechanism was also explained. Techniques such as TEM, XRD, FT-IR, TG-DSC, and VSM were applied to characterize the obtained modified Fe₃O₄ sample. From these techniques, the following information is obtained: The characteristic bands of TEOS and KH-792 were present in the FT-IR spectra and in the XPS plots of modified Fe₃O₄ MNPs, demonstrating that the silane coupling agents were present in the sample obtained after the modification. The TG analysis of the modified sample showed complete decomposition at 228 °C. The mass ratio of the sample obtained before and after the modification was close to 29:65. The XRD patterns show that the modified Fe₃O₄ MNPs possessed an identical reverse spinel crystal structure as an unmodified Fe₃O₄ sample. The modification decreased the saturation magnetization of Fe₃O₄ MNPs from 70.04 emu/g to 57.41 emu/g and the coating did not change the superparamagnetism of Fe₃O₄ MNPs.

4D printing technology refers to the manufacturing of products using 3D printing techniques that are capable of changing shape or structure in response to external stimuli. Compared with traditional 3D printing, the additional dimension is manifested in the time dimension. Facilitated by the advancement of magnetic smart materials and 3D printing technology, magnetically controlled 4D printing technology has a wide range of application prospects in many fields such as medical treatment, electronic flexible devices, and industrial manufacturing. Magnetically controlled 4D printing technology is a new scientific research field in the 21st century, which includes but is not limited to the following disciplines: mechanics, materials, dynamics, physics, thermodynamics, and electromagnetism. It involves many fields and needs to be summarized systematically. First, this article introduces various magnetic intelligent materials, which are suitable for magnetically controlled 4D printing, and discusses their programmability. Second, regarding the printing process, the article introduces how to preset the material distribution as well as the research progress about the optimization of magnetically controlled 4D printing platforms and the distribution of magnetic field profiles. Third, the article also makes a brief introduction to the applications of magnetically controlled 4D printing technology in medical, electronic flexible devices, and industrial manufacturing fields.

In this study, nanoparticles were suspended in L-AN32 total loss system oil. The thixotropic yield behavior and viscoelastic behavior of ferrofluid were analyzed by steady-state and dynamic methods and explained according to the microscopic mechanism of magneto-rheology. The Herschel–Bulkley (H–B) model was used to fit the ferrofluid flow curves, and the observed static yield stress was greater than the dynamic yield stress. Both the static and dynamic yield stress values increased as the magnetic field increased, and the corresponding shear thinning viscosity curve increased more significantly as the magnetic field strength increased. The amplitude scanning results show that the linear viscoelastic region (LVE) is reached when the shear stress is 10%. The frequency scanning results showed that the storage modulus increased with the increase of the frequency at first. The storage modulus increased steadily at a higher frequency range, while the loss modulus increased slowly at the initial stage and rapidly at the later stage. In the amplitude sweep and frequency sweep experiments, the energy storage modulus and loss modulus are enhanced with the decrease of temperature. These findings are helpful to better understand the microscopic mechanism of magneto-rheology of ferrofluids, and also provide guidance for many practical applications.

In this research, a new low refractive index ferrofluid is proposed by coating magnetic nanoparticles with a layer of silver, applying the method of modified chemical co-precipitation. This preparation method is green and environmentally friendly without toxic gases being released. Coated nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and vibration sample magnetometery (VSM). These characterizations show that the silver nanoparticles grow on the surface of magnetic nanoparticles in this new ferrofluid. The hysteresis loop of this new ferrofluid demonstrates that it maintains superparamagnetic properties. A new method of refractive index measurement is applied in this research by employing a long-period grating (LPG) optical fiber sensor. The change value in the refractive index per unit concentration reduces by 16.46% compared to the conventional ferrofluid.