Origin and universality of the maximum drag reduction asymptote
In plain words
Polymers can cut pipe friction a great deal, but adding more stops helping beyond a limiting friction curve that is the same for all polymers. Why this limit exists and is universal is not understood.
Precise statement
In turbulent pipe flow of polymer solutions, the Fanning friction factor f approaches Virk's maximum drag reduction asymptote $1/\sqrt{f} = 19.0\operatorname{log10}(\mathrm{Re}\sqrt{f}) - 32.4$, independent of polymer species. Identify the flow state that realizes it (elasto-inertial turbulence driven by polymer stress, or marginally sustained Newtonian-like turbulence), derive the asymptote, and explain why friction can fall below it toward laminar values.
What would settle it
A derivation of the asymptote from the dynamics of the identified flow state, confirmed by DNS of a constitutive model reproducing the measured friction curve.
Status in the literature
Elasto-inertial turbulence (Samanta et al. 2013) and friction below the asymptote after relaminarization (Choueiri, Lopez, Hof 2018) favour an elasto-inertial origin; a predictive derivation is lacking.