Growth of the instability as a function of time for an overdensity of χ = 10, color-coded for different values of viscosity. The value of the critical viscosity is equal to η_Crit = 515 ± 5, where the error is given by the average between the viscous run with a positive and a negative slope. The solid lines indicate the simulations in which the KHI is still unstable and the dash-dotted line where the instability is suppressed (η > η_Crit).