Challenging the Autoinjector: Investigating Mechanical Degradation of High-Molecular-Weight Polymers in a Contraction Flow

The addition of high-molecular-weight polymer to solvent, even in dilute amounts, can significantly change the response of a fluid to an extensional flow. This behavior is important when formulating models and assessing the ability to print, spray, jet, and spin complex fluids. Generally, macromolecular solutions exhibit large resistance to stretching deformations known as extensional viscosity.

Molecular degradation can make additives useless in liquid solutions and change the characteristics of the fluid itself. Degradation can occur when non-Newtonian fluids flow through regions of high extension, such as a contraction. As a fluid flows through a radially decreasing geometry, the molecules in the solution can be mechanically broken.

A common contraction is drug delivery via an injector through a needle. The recipient will request that the injection would be done as quick as possible and with the smallest needle possible. This request translates to higher shear rates and greater changes in radii that are experienced by the solution, all of which favors molecular degradation. To better understand how pharmaceuticals should be delivered, specifically using an autoinjector, characterizing the viscoelastic behavior provides detailed information on how these fluids should be handled.

We perform a variety of molecular and rheological characterization on model polymer solutions before and after flowing them through a contraction flow. The pressure drop across the contraction is also measured for comparison to theoretical and empirical models.