The friction of methyl-branched fatty acids was studied with a surface forces apparatus (SFA) with attachments for low-speed (μm/s) and high-speed (cm/s) sliding. The systems studied were isooleic acid, a mixture of unsaturated C-18 isomers with one methyl group located along the chain, and isostearic acid, which consisted of saturated C-18 isomers with one methyl branch. The methyl group was expected to disrupt the close packing of the molecules in the pure substance and when adsorbed on a surface. The friction forces and friction coefficients were compared with results for stearic and oleic acid. Isostearic acid showed low friction at low speeds. At high speeds, a peak was found in the friction coefficient of both isostearic and isooleic acid layers, whereas a gradual increase in friction coefficient with speed was seen for oleic acid, suggesting different relaxation times of the structures formed on the surfaces.

中文翻译：

---

低速和高滑动速度下甲基支化脂肪酸层的摩擦

使用带有低速 (μm/s) 和高速 (cm/s) 滑动附件的表面力装置 (SFA) 研究了甲基支化脂肪酸的摩擦。研究的系统是异油酸（一种沿链带有一个甲基的不饱和 C-18 异​​构体的混合物）和异硬脂酸（由带有一个甲基支链的饱和 C-18 异​​构体组成）。当甲基吸附在表面上时，预计会破坏纯物质中分子的紧密堆积。将摩擦力和摩擦系数与硬脂酸和油酸的结果进行比较。异硬脂酸在低速下表现出低摩擦。在高速下，等硬脂酸层和异油酸层的摩擦系数均出现峰值，而油酸层的摩擦系数随速度逐渐增加，这表明表面上形成的结构具有不同的弛豫时间。