For steady state deformations a surface @S05642@ \(\eta ^{\text{s}}\), and an area viscosity or surface dilatational viscosity \(\zeta ^{\text{s}}\) can be defined. In a Cartesian system with the x-axis normal to the surface, they are defined by the equations: \[\eta ^{\text{s}} = \frac{\sigma _{xy}}{\frac{\partial \nu_{y}}{\partial \nu_{x}}}\] \[\zeta ^{\text{s}}=\frac{\Delta \gamma }{\frac{\mathrm{d}(\ln A)}{\mathrm{d}t}}\] where \(\sigma _{xy}\) is the shear component of the @S06191@ tensor, \(\nu_{x}\) and \(\nu_{y}\) are the \(x\) and \(y\) components of the surface velocity vector, respectively, \(A\) is the surface area, \(t\) is the time, and \(\Delta \gamma \) is the difference between the (steady state) @D01875@ and the equilibrium surface tension.