For 10 points, name this measure of a fluid's internal resistance to flow.
A so-called bulk or second variety of this property exists, but is unimportant in the incompressible case. This property can decrease over time in response to applied shear, a phenomenon known as thixotropy. This quantity appears multiplying the axial length and volumentric flow rate in the usual form of Poiseuille's equation. The effects of this property dominate at low Reynolds' numbers as this quantity is the denominator of the Reynolds number. This quantity appears as the constant of proportionality between the sheer stress and perpendicular velocity gradient for a Newtonian fluid. This property increases with temperature for gases but decreases for liquids; relatedly, for a superfluid below the lambda point, this property has a value of zero. For 10 points, name this measure of a fluid's internal resistance to flow.
A so-called bulk or second variety of this property exists, but is unimportant in the incompressible case.
{viscosity}