# Characterization of Magnetic-Silica Magnetic Fluids by Laser Scattering
## 1 Introduction  
Magnetic fluids have been the subject of scientific research for a long time [1] and are used in various fields of engineering, including mechanical engineering, mining, and electronics [2, 3], as well as in medical diagnostics [4] and sensing devices [5, 6, 7]. The main qualities that a magnetic fluid must possess are aggregate and sedimentation stability, saturation magnetization, and biocompatibility (when used in medicine). An important difference between technical requirements to magnetic fluids used in these areas is the concentration of magnetic particles in their compositions. Typical concentrations used in alternating magnetic field sensors should be up to 370 mg/mL and higher [8], in the preparation of polymer composites for electromagnetic shielding they should be 20-80 mg/mL [9], to use acoustic properties of magnetic fluids the concentration should be 10-40 mg/mL [10], for the separation of nonmagnetic particles the concentration should be up to 10 mg/mL [11], as the fuel additive concentrations less than 2 mg/mL are used [12], and for the use as a contrast agent for magnetic resonance imaging the concentration should be less than 1 mg/mL [13]. For such a variety of applications [14], the original magnetic fluids are usually diluted 200 or more times, which can cause violation of the colloidal solution stability.  
In the case of high aggregate stability of a magnetic fluid in the ultrawide range of concentrations of magnetic particles, the sol-gel method [15] may be used. As shown in [16],the study of diluted magnetic fluids prior to their use requires the analysis of particle size distribution using optical methods, including dynamic light scattering (DLS) [17].  
The conventional DLS [18] method does not allow analyzing the shape of magnetic particles, which necessitates the use of additional research methods, such as scanning and transmission electron microscopy. To study the sizes of nonspherical nanoparticles in diluted magnetic fluid it is possible to use the promising depolarized dynamic light scattering (DDLS) method.  
Despite the fact that the DDLS method has been known for at least 30 years, it remains relevant in the study of colloidal particles. In [19], the method was applied to distinguish nearly monodisperse hematite spindles with an average length of 280 nm and a minor axis of 57 nm, coated with a layer of silica of variable thickness, which revealed the particle aspect ratio between 5 and 2. Characterization of optically anisotropic round Au nanoparticles and determination of the number-averaged size distribution and polydispersity was presented in [20]. The authors of [21] confirmed the colloidal stability of Au nanoparticles coated with polyvinylpyrrolidone via steric repulsion even under high salt conditions by the DDLS method. The size and shape of the magnetic nanoparticles were studied using DDLS in [22]. One of the main problems of studying the parameters of single magnetic nanoparticles by optical methods is the aggregation of nanoparticles in dilute magnetic fluids.  
In this article, we propose the use of an original modification of the DDLS method to obtain data not only on the size and shape distribution of aggregates in a magnetic fluid, but also on individual magnetic particles directly. It is expected that this method will be promising for studying the structural and morphological properties of nanoparticles in dilute liquids.