Wikipedia.org/ Viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal concept of "thickness". For example, honey has a much higher viscosity than water.
Wall shear stress during plug propagates and ruptures in channel. Numerical computation. Solid curves: wall shear stress behind rear meniscus tip before rupture or along whole channel wall after rupture. Dashed curves: wall shear stress in front of rear meniscus tip before rupture. ASME SBC 2011.