## 2 Materials and Methods
### Preparation Process and Characterization  
An HP-MRF is a colloidal suspension system in which soft magnetic particles are uniformly distributed within the carrier. This uniform dispersion is essential for achieving optimal performance in its applications. Consequently, the soft magnetic particles were evenly distributed within the carrier through the ball mill dispersion method to formulate HP-MRFs. Figure 1 illustrates the preparation process. Recent research findings have demonstrated that particle surfactants such as PEG-400 and oleic acid can significantly enhance the shear yield strength and stability of MRFs [35]. The procedure for surface active agent treatment in CIPs is illustrated in Figure 1a:  
1. CIPs (initial particle size: 3.5 \(\upmu\)m), surfactants such as PEG-400 and oleic acid, and an absolute ethyl alcohol dispersion medium were measured in proportion and placed in a beaker, followed by mixing with an electric stirrer for 15 min. 2. The amalgamation was transferred into a milling pot. Steel balls of varying sizes, although with identical diameters and numerical ratios, were subsequently introduced into the mill pots. 3. The mill pots were firmly positioned in the SHQM double planetary ball mill, which operated at a constant rotational speed. Subsequently, ball milling was conducted for durations of 6, 12, 24, 36, 48, 60, and 72 h for the different CIP samples with various particle sizes. 4. The ball mill pots containing the mixture post ball milling were placed in a vacuum oven at 85 \({}^{\circ}\)C for drying at a vacuum pressure of \(-\)0.1 MPa for a duration of 480 min to eliminate the absolute ethyl alcohol, resulting in the acquisition of the treated CIPs after drying and cooling. Secondly, the procedure of HP-MRF preparation is illustrated in Figure 1b:  
1. The measured quantities of gelatin and agaropectin were combined with purified water at 70 \({}^{\circ}\)C in a flask for 15 min to create a sol solution. 2. Methyl silicone oil was weighed and added to the sol solution, which was thoroughly mixed before being transferred to a vacuum oven for decompression at \(-\)0.1 MPa for 45 min. 3. The modified CIPs and glycerol were weighed and incorporated into the mixture following vacuum decompression and then mixed for 15 min. Figure 1: Process of HP-MRF preparation. (**a**) The procedure of treating CIP; (**b**) HP-MRF based on a composite carrier. The mixture was placed in the mill pots to achieve uniform dispersion for 60 min in the double planetary ball mill, resulting in the acquisition of HP-MRF samples. The phase analysis of untreated and surfactant-treated CIPs was conducted using a D8 Advance X-ray diffractometer (Bruker Co., Karlsruhe, Germany), and the particle size of the ball-milled CIPs was assessed. A testing apparatus utilizing a D8 Advance X-ray diffractometer was constructed to assess the particle size distribution of the CIPs post ball milling, as seen in Figure 2a. Figure 2b illustrates the particle size distribution of ball-milled CIPs across varying durations, indicating that as the ball milling period increases, the size of the ferromagnetic particles diminishes from an initial 3.5 \(\upmu\)m to 300 nm after 72 h of milling. Particle sizes of 900 nm, 600 nm, and 360 nm, corresponding to 12 h, 24 h, and 60 h of ball milling, respectively, were chosen for the CIPs. Furthermore, as illustrated in Figure 2c, pronounced diffraction peaks (2\(\theta\) = 40.4\({}^{\circ}\), 45\({}^{\circ}\), and 82.6\({}^{\circ}\)) are evident in both treated and untreated CIPs, aligning well with the standard spectrum of iron (Fe), albeit with variations in peak intensity. The diffraction peak intensity of the treated CIP is somewhat reduced, maybe due to the adsorption of a coating of active agent on the particle surface, but the crystal structure of Fe remains unchanged post treatment.