Sedimentation Characterization of Gelatin Added Bidisperse Magnetorheological Fluids Containing Nanoparticles

In this paper, the bidisperse magnetorheological (BMR) fluids containing micron carbonyl iron (CI) particles, nanoscale magnetite (Fe3O4) and gelatin were prepared. Gelatin was used as a coating layer to improve stability of bidisperse particles and restrict the oxidation of Fe3O4. Under the effect of magnetic field, Fe3O4 particles were attached at the end of CI chains and filled into the interspace of micron CI particles, which influences the interactions among BMR fluids. Some groups with different concentrations and mass fractions of gelatin and Fe3O4 were considered to reveal the sedimentation characterization of BMR fluids. The BMR fluids showed enhanced dispersion stability.


Introduction
Magnetorheological (MR) fluids, composed of micron soft magnetic particles dispersed in nonmagnetic carrier liquid, have rapidly and reversibly transformed between fluid-like and solid-like state under the effect of magnetic field [1,2]. Therefore, MR fluids have been widely used in many engineering applications, such as dampers, brakes, torque transducers, and shock absorbers [3][4][5]. However, the urgent problem is the aggregation and sedimentation of micron CI particles, which can seriously reduce the magnetism and rheological properties of MR fluids.
To mitigate sedimentation of CI particles, many surface-modification CI particles were synthesized to prevent large particles from aggregation, such as lipophilic coating, organic coating and nanoparticles [6][7][8]. Nanoparticles Fe 3 O 4 , due to their tiny size, will be attached at the end of CI chains and filled into the interspace of micron CI particles to influence the interactions among dissimilar particles. However, Fe 3 O 4 particles are susceptible to oxidation at elevated temperatures and lose their magnetism.
In this study, magnetic micron CI and nanoscale Fe 3 O 4 particles were coated with gelatin layer to restrict the oxidation of Fe 3 O 4 and improve stability of bidisperse particles. Furthermore, the sedimentation characterization of various gelatin-based BMR fluids is measured to improve the dispersion stability of conventional MR fluids. Soft micron magnetic CI particle (average diameter 3.5 μm) was purchased from Zhixing Science and Technology Nantong Co. Ltd. And nanoscale magnetic Fe 3 O 4 (average diameter 20 nm) were prepared by the co-precipitation method.

BMR fluids materials and synthesis
The preparation process of gelatin-coated BMR fluids is as follows: first, the mixture of solution containing soft CI, Fe 3 O 4 and gelatin was dissolved in deionized water and heated to 55℃ for 30 min, in which the CI and Fe 3 O 4 surfaces were coated with gelatin layer. And then magnetic particles were separated by a magnet and washed by distilled water. Finally, appropriate amounts of gelatin coated magnetic particles were poured into mixture of polyolefins synthetic oil, sodium chloride and some additives, which was stirred for 40 min at 60℃. After ultrasonic oscillation, the BMR fluids were poured into graduated cylinders for investigation of sedimentation characterization.

Sedimentation characterization
The stabilities of the MR fluids and BMR fluids were checked by qualitative observations of the sedimentation which occurs when placing them into graduated cylinders without magnetic field. Furthermore, this experiment was performed at room temperature. The sedimentation characterization is a significant fact for the properties of BMR fluids and can be qualitative estimated from sedimentation ratio β obtained by equation (1): where, H and h represent the initial height and residual height in the measuring cylinder after sedimentation of the BMR fluids, respectively. Figure 1 shows the schematic diagram of observation method.  Figure 2 shows the schematic diagram of additive added BMR fluids, in which some Fe 3 O 4 particles are attached to soft CI particles surfaces, others are suspended into polyolefins synthetic oil-additives solvent. Under the effect of magnetic field, due to dipolar interactions, Fe 3 O 4 particles are attached at the end of CI chains and filled into the interspace of micron CI particles, which influences the interactions among BMR fluids. The field induced linear structure based gelatin-coated BMR fluids is shown in figure 3. For BMR fluids, Interactions can be mainly classified as van der Waals force, Brownian force, repulsive force and magnetic force [9]. The magnitude of the van der Waals force generally decreases with the increase of intermolecular distance h. In this work, the dispersity of gelatin coated BMR fluids is superior to the previous reports [10,11]. The gelatin coated layer was selected herein for its better viscosity and stability to restrict the oxidation of Fe 3 O 4 . In addition, as we coated outside surface with gelatin, it was found that the aggregation and sedimentation of large magnetic particles were significantly prevented due to the decrease of particles interaction. The pair interaction for two dissimilar particles of radius R 1 and R 2 , respectively, can be calculated by Equation (2) [12]:
Brownian motion force has an important effect on preventing the aggregation of micron particles, which can be obtained by Equation (4): where, ζ is the random number of Gaussian distribution, R is the average radius of two dissimilar particles, k B is Boltzmann constant, μ is dynamic viscosity, T is thermodynamic temperature and t Δ is time step.  Figure 4 shows schematic diagram for the magnetic force induced by dissimilar dipoles. Dipole moment of a magnetic particle is given by: χ is the magnetic permeability of BMR particles, H is the applied magnetic strength. The magnetic potential energy between dipoles i and j can be expressed: where, r ij is a relative positive vector between the dissimilar dipoles, and 0 r is a unit vector of r ij .
And hence the magnetic force between dipoles i and j is: Therefore, sum of magnetic forces exerted by the surrounding magnetic particles can be given by: For BMR dispersions, the stability effect can be improved as a consequence of repulsion. The repulsion for per magnetic particles induced by other particles is given by Equation (9): Combined with Equations (2), (4), (8) and (9), the total interaction force for BMR fluids can be written as: According to Equation (10), the magnetic and repulsion force would be a major factor in describing dispersity behavior for BMR fluids. And nanoscale Fe3O4 increases the dipolar interaction between CI particles, which prevents the aggregation and sedimentation of larger particles.

Sedimentation results
To study the effect of gelatin coating and nanoscale Fe3O4 on the properties of MR fluids, some samples were prepared in the experiments. First, the details of the index and concentrations (wt%) of gelatin in CI-based MR fluids are showed in table 1. Figures 5 and 6 show the sedimentation images in various concentrations of gelation and sedimentation ratio as a function of time for samples with different conditions in CI-based MR fluids, respectively. It should be noted that, sedimentation ratio increases rapidly during the initial 25 days, then gradually tends to become steady. 40 days later, the sedimentation ratio of MR-1, MR-2, MR-3, MR-4 and MR-5 is 42.5%, 35.1%, 24.3%, 17.2% and 28.0%. It is obvious that, the concentration of gelatin significantly affects sedimentation ratio of CIbased MR fluids, and the sedimentation ratio for MR-4 is lower than 20%, which is superior to many MR fluids included in the literature [13].  It can be seen that the status of each sample will no longer change after 34 days and the sedimentation ratios are 14.2%, 11.5%, 10.3% for BMR-2, BMR-3 and BMR-4, which has better stability than those without nanoscale Fe3O4. In particular, the number of nanoparticles attached to the CI chains and filled into the interspace of CI particles increases with the increase in the mass fraction of Fe3O4 particles, in such BMR system. Thus, the sedimentation characterization has been further improved.

Conclusion
The BMR fluids containing CI particles, nanoscale Fe 3 O 4 , gelatin coating and polyolefins synthetic oil are synthesized. Gelatin is used as a coating layer to improve the stability of bidisperse particles and restrict the oxidation of Fe 3 O 4 . Fe 3 O 4 particles attached at the end of CI chains and filled into the interspace of micron CI particles influence the interactions among BMR fluids and reduce the aggregation of larger particles. As a result, the sedimentation characterization of the gelatin-based BMR fluids is measured, which shows that adding a certain concentration of gelatin (15.0wt%) and mass fraction of Fe 3 O 4 (5.0%) significantly improves the dispersion stability of BMR fluids.