<pressure_penalty>1.0</pressure_penalty>

When either contact surface is biphasic, the surface outside the contact area(s) is automatically set to free-draining conditions (equivalent to setting the fluid pressure to zero).

As detailed in [76] and the FEBio Theory Manual, the sliding-biphasic contact algorithm can include frictional contact. The effective friction coefficient $\mu_{\text{eff}}$ in this frictional contact algorithm depends on the temporally evolving local fluid load support (the ratio of fluid pressure $p$ to the normal component of the mixture contact traction $t_{n}$ ), according to $$\mu_{\text{eff}}=\mu_{\text{eq}}\left(1+\left(1-\varphi\right)\frac{p}{t_{n}}\right)\,.$$ Recall that $t_{n}$ is negative whereas $p$ is positive in compression. Here, $\mu_{\text{eq}}$ (fric_coeff) is the friction coefficient in the limit when the fluid load support has reduced to zero, and $\varphi$ (contact_frac) is the fraction of the apparent contact area over which the solid matrix of the primary surface contacts that of the secondary surface. Thus, $1-\varphi$ is the fraction of the apparent contact area where fluid contacts fluid or solid. For perfectly smooth contact surfaces one may assume that $\varphi=\varphi_{1}^{s}\varphi_{2}^{s}$ , where $\varphi_{1}^{s}$ and $\varphi_{2}^{s}$ are the solid area (or volume) fractions of the primary and secondary contact surfaces, respectively. For example, when both contact surfaces are non-porous ($\varphi_{1}^{s}=\varphi_{2}^{s}=1$ ), the effective friction coefficient reduces to $\mu_{\text{eff}}=\mu_{\text{eq}}$ .

When performing biphasic-on-rigid contact a two-pass analysis should not be used; the rigid surface should be the secondary surface.