Shear Thinning of Nanometer-Thick Liquid Lubricant Films Measured at High Shear Rates

Viscosities of liquid films that were confined and sheared in a nanometer-scale gap were higher than those in the bulk state, and decreased with increasing shear rate, which is called shear thinning. However, the previous findings were based on the experimental results obtained at shear rates of 10^?1–10⁶ s^?1 by using a surface force apparatus (SFA). In this study, we succeeded in measuring shear rate dependence of viscous friction at the high shear rates of 10⁵–10⁸ s^?1. To measure the viscous friction at high shear rates, we used a fiber wobbling method that is a highly sensitive shear force measurement method we developed. The confined polymer lubricant (PFPE Z03) showed shear thinning behavior even at high shear rates up to 10⁸ s^?1, and the relationship between effective viscosity η and shear rate $\dot{\gamma }$ was well expressed by the equation that was $\log_{10} \eta = C - n\log_{10} \dot{\gamma }$ where C ≈ 4.6 ± 0.6 and n ≈ 1.1 ± 0.1. This equation agreed well with the universal curve of shear thinning determined based on the SFA measurements conducted at shear rates of 10^?1–10⁶ s^?1. This result indicates the measured shear thinning behavior was consistent at wide range of shear rates from 10^?1–10⁸ s^?1.