Polymer Scission in Contraction Flows*

Polymer scission has been studied for many years! It has numerous practical implications in areas such as drug injection, spraying and printing, and oilfield harvesting. When injected through a contraction flow, high molecular weight polymer solutions exhibit a sharp increase of apparent viscosity due to chain stretching during fluid extension. This stretching can induce polymer scission, which then decreases the extensional viscosity. We revisit this old problem using specially-designed microfluidic hyperbolic contraction flows. We study the pressure-flux relation for high molecular weight polymer solutions passing through the contraction, and find that the ratio of the pressure drop to that of the (Newtonian) solvent has a maximum due to the competition between polymer extension and degradation (scission). From the dependence of the pressure maximum on flow rate and molecular weights we can quantify the decrease in equivalent molecular weight due scission in the contraction
. We find a geometry-dependent quasi-universal relation between the flow rates at which the maximum occurs for successive passage in a given contraction, which appears to be independent of molecular weight, concentration, solvent quality and viscosity, and can be used to predict degradation under successive passes. I will discuss what we can learn about the the details of the scission kinetics in flow from these observations.