Mechanical Engineering for Society and Industry

Accurate characterization of viscosity behavior is essential for the design of advanced heat-transfer fluids, particularly hybrid emulsions involving immiscible polar and non-polar phases. However, the rheological behavior and molecular interactions of silicone oil/ethylene glycol (EG)–water emulsions stabilized with surfactants remain insufficiently understood. Therefore, this study experimentally investigates the temperature-dependent viscosity and molecular characteristics of low-viscosity Silicone Oil PMX-200 dispersed in an EG–water (10:90 v/v) base fluid stabilized with Tween 80 over a temperature range of 30–70 °C. Fourier Transform Infrared (FTIR) spectroscopy was employed to evaluate molecular interactions and emulsion stability, while viscosity measurements were performed using a calibrated rotational rheometer under controlled thermal conditions. FTIR analysis confirmed characteristic polydimethylsiloxane (PDMS) absorption bands associated with C–H, Si–CH₃, and Si–O–Si vibrations. The attenuation of Si–O–Si intensity and the appearance of broad O–H stretching bands indicated enhanced interfacial interactions between the hydrophobic silicone-oil phase and the polar EG–water medium, confirming successful emulsification by Tween 80. Rheological measurements showed that viscosity decreased significantly with increasing temperature, from 6.2 mPa·s at 30 °C to 4.2 mPa·s at 70 °C. The emulsions exhibited nearly Newtonian behavior with flow behavior indices close to unity (n ≈ 1), indicating minimal shear-rate dependence within the investigated range. The viscosity–temperature relationship followed Arrhenius-type behavior, with activation energies decreasing from 8.93 kJ·mol⁻¹ for SO‒50 to 4.86 kJ·mol⁻¹ for SO‒10, indicating enhanced molecular mobility at lower silicone-oil concentrations. Experimental deviations remained below 2% compared with literature data, confirming measurement reliability. The novelty of this work lies in integrating FTIR molecular characterization, rheological validation, and Arrhenius-based viscosity modeling for silicone oil/EG–water emulsions. The proposed correlation provides reliable viscosity prediction for thermal-fluid design and CFD-based heat-transfer simulations.