title: Comment on "Locomotion of a microorganism in weakly viscoelastic liquids".
abstract: We point out, and show the implications of resolving, an apparent conceptual difficulty in a recent article by De Corato et al. [Phys. Rev. E 92, 053008 (2015)PLEEE81539-375510.1103/PhysRevE.92.053008] on the locomotion of certain microorganisms in a second-grade fluid. The difficulty arises due to the assumption that α_{1}<0, where α_{1} is the first normal stress modulus of the (non-Newtonian) liquid, was chosen for this study. In particular, this choice of sign for α_{1} is inconsistent with thermodynamics, and as such casts considerable doubt on De Corato et al.'s assumption regarding the existence of a steady-state solution of the equations of motion of the fluid.
authors.affiliation: School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: Ivan C
authors.initials: IC
authors.lastname: Christov
authors.affiliation: Acoustics Division, U.S. Naval Research Laboratory, Stennis Space Center, Mississippi 39529, USA.
authors.forename: P M
authors.initials: PM
authors.lastname: Jordan