For a Newtonian fluid, 粘度 is a function of temperature and 压力 but not shear rate. In liquids, viscosity decreases significantly as temperature increases because higher thermal energy allows molecules to overcome cohesive intermolecular forces more easily. Conversely, in gases, viscosity increases with temperature as more frequent molecular collisions at higher speeds lead to greater momentum transfer.
The relationship between viscosity and temperature is fundamentally different for liquids and gases, stemming from their distinct molecular mechanisms for momentum transfer. In liquids, molecules are closely packed and held together by strong intermolecular cohesive forces. Viscous forces arise from the resistance of these molecules to sliding past one another. As temperature rises, the kinetic energy of the molecules increases, allowing them to overcome these cohesive forces more readily. This results in a decrease in the liquid’s resistance to flow, and thus, a lower viscosity. This effect is pronounced; for example, the viscosity of water decreases by a factor of about 6 between 0°C and 100°C.
在气体中，分子间距较大，主要通过碰撞相互作用。气体粘度是衡量以不同速度运动的层间动量传递的指标。这种动量是通过分子在层间移动和碰撞而传递的。随着温度升高，气体分子的随机热速度也会增大。这会导致碰撞更频繁、更剧烈，从而提高层间动量传递的效率，最终导致粘度增加。这一现象是气体动力学理论早期的一大成就，因为它是一个反直觉的预测，后来被实验证实。
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