Fig. 1

Meaning of viscosity. a Suppose that a thin plate (zero thickness) having an area A [m2] is sandwiched between the material of interest and large plates. They are aligned vertically, and a paper weight having a weight W [kg] is suspended from the thin plate. The paper weight falls at a speed v [m/s] when the distance between plates is 2d [m]. Viscosity is defined as follows: Viscosity = (Wg/2A)/(v/d) ∝ W/v (e.g., water has viscosity of 0.001 Pa·s). This equation indicates that the falling speed of the paper weight is proportional to the weight under constant viscosity. A heavier paper weight falls at a higher speed, and a lighter one falls more slowly. b If the viscosity is decreased with increasing shear stress, as expressed by the red curve, how does the paper weight falls? If the paper weight is sufficiently light, the falling speed is roughly same as that in the case of constant viscosity, blue line, because they have almost the same viscosity in the small shear stress region. In contrast, when a heavy paper weight is suspended, the paper weight must fall down quickly in the case of non-constant viscosity, because the falling speed is inversely proportional to viscosity. Therefore, a higher viscosity at a larger shear stress means that an obstacle stuck to the material moves more slowly. In other words, the obstacle feels a larger resistance in a more viscous material, and is difficult to move, which is characterized as being “more sticky”