The viscosity characteristics and rheological behavior of fluorosilicone oil are the core indicators of its application performance, which are mainly affected by molecular structure (fluorine group substitution), molar mass, temperature and shear force, and are specifically manifested as follows:

1. Viscosity characteristics
‌Viscosity-temperature stability‌
Compared with ordinary silicone oil, fluorosilicone oil has slightly weaker viscosity-temperature stability, but is significantly better than mineral oil. The sensitivity of its viscosity to temperature change is ranked as follows: ‌Dimethyl silicone oil < trifluoropropyl methyl silicone oil (fluorosilicone oil) < high phenyl silicone oil‌8. For example, the fluorine-containing group causes the slope of its viscosity-temperature curve to be slightly higher than that of dimethyl silicone oil, but it still maintains fluidity within a wider temperature range (-60~250℃).

‌Viscosity-pressure characteristics‌
The viscosity-pressure coefficient (the degree of viscosity change with pressure) of fluorosilicone oil is lower than that of ordinary mineral oil. When the pressure rises to 400MPa, it still maintains a liquid state, and the viscosity rises slowly, which is suitable for high-pressure sealing environments.

‌Viscosity and molecular structure‌
‌Effect of fluorine content‌: The introduction of fluorine groups (such as trifluoropropyl) enhances the intermolecular forces, resulting in a higher viscosity than dimethyl silicone oil at the same molar mass.
‌Molar mass control‌: The viscosity is adjusted by the degree of polymerization (usually 5~50). For example, the industrial viscosity of hydroxy fluorosilicone oil ranges from 40~150 mm²/s (25℃).

2. Rheological behavior
‌Shear thinning effect‌
High viscosity fluorosilicone oil (>1000 mPa·s) exhibits pseudoplastic fluid behavior under shear force:
The molecules are oriented at high shear rates, and the viscosity is significantly reduced;
The viscosity recovery has a hysteresis, and it takes time to rebuild the disordered state of the molecules.
Example: Vinyl fluorosilicone oil (viscosity ≥20,000 mPa·s) needs to optimize the shear rate in the coating process to control the leveling.
‌Long-term shear stability‌
Under continuous shear stress (such as hydraulic systems), the viscosity decay rate of fluorosilicone oil is lower than that of mineral oil. After long-term use, the viscosity loss is <10%, ensuring the life of the equipment.

‌Low-temperature rheology‌
The freezing point of fluorosilicone oil is as low as -50℃, and it still maintains good fluidity at low temperatures, which is suitable for aerospace lubrication scenarios.

3. Comparison of key parameters
‌Properties‌ ‌Fluorosilicone oil‌ ‌Dimethyl silicone oil‌
Viscosity-temperature sensitivity Medium↑ Low
Viscosity-pressure coefficient 0.005~0.01 GPa⁻¹ 0.003~0.008 GPa⁻¹
Surface tension (25℃) 20~24 mN/m 21~22 mN/m
Typical application temperature range -60~250℃ -50~200℃
Conclusion
The viscosity and rheological behavior of fluorosilicone oil are dominated by the intermolecular forces enhanced by the fluorine group:

The temperature resistance and chemical stability are better than those of ordinary silicone oil, but the viscosity-temperature stability is slightly reduced;
The shear-thinning effect requires precise control of the shear rate in the process;
The low viscosity-pressure coefficient and low-temperature fluidity make it irreplaceable in extreme environments (high pressure, deep cold).