void assign(const FluidState& fs)
 {
     typedef typename FluidState::Scalar FsScalar;
     typedef Opm::MathToolbox<FsScalar> FsToolbox;
     for (unsigned phaseIdx = 0; phaseIdx < numPhases; ++phaseIdx) {
         viscosity_[phaseIdx] = FsToolbox::template toLhs<Scalar>(fs.viscosity(phaseIdx));
     }
 }