關於黏度問題

粘性（又稱黏度、黏滯係數，英文為 viscosity 或 dynamic viscosity），是度量流體力學性質的一個物理量，也是流體力學的專有名詞。
黏度 μ 定義為流體承受剪應力時，剪應力與剪應變梯度（剪應變隨位置的變化率）的比值，數學表述為：

圖片參考：http://upload.wikimedia.org/math/3/7/c/37cb5d8ff739d3a55e821c41bc094d53.png
式中：τ為剪應力，u為速度場在x方向的分量，y為與x垂直的方向坐標。
在國際單位制中，黏度的單位為Pa·s。

圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Laminar shear of fluid between two plates. Friction between the fluid and the moving boundaries causes the fluid to shear. The force required for this action is a measure of the fluid's viscosity. This type of flow is known as a Couette flow.

圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Laminar shear, the non-linear gradient, is a result of the geometry the fluid is flowing through (e.g. a pipe).

圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Pitch has a viscosity approximately 100 billion times that of water.

圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Example of the viscosity of milk and water. Liquids with higher viscosities will not make such a splash when poured at the same velocity.