Paper Number
FI1                         My Program 

Session
Flow-Induced Instabilities and Non-Newtonian Fluids

Title
Effects of polymer concentration and polydispersity on elastocapillary thinning of dilute solutions

Presentation Date and Time
October 14, 2024 (Monday) 9:50

Track / Room
Track 5 / Room 405

Authors (Click on name to view author profile)

  1. Calabrese, Vincenzo (Okinawa Institute of Science and Technology)
  2. Shen, Amy Q. (Okinawa Institute of Science and Technology Graduate Univers, Micro,Bio,Nanofluidics Unit)
  3. Haward, Simon J. (Okinawa Institute of Science and Technology Graduate Univers, Micro,Bio,Nanofluidics Unit)

Author and Affiliation Lines (in printed abstract book)
Vincenzo Calabrese, Amy Q. Shen and Simon J. Haward
Okinawa Institute of Science and Technology, Onna-son, Okinawa 904-0495, Japan

Speaker / Presenter 
Haward, Simon J.

Keywords
experimental methods; non-Newtonian fluids; polymer solutions

Text of Abstract
The thinning of a liquid bridge under the action of capillarity gives rise to an extensional flow capable of stretching dissolved polymers. Polymer stretching results in elastic stresses that, if sufficient to overcome the viscous stress, lead to an “elastocapillary” (EC) thinning regime in which the radius of the liquid bridge decays exponentially in time. The characteristic time of the decay τEC is commonly considered equivalent to the longest relaxation time λ of the polymer. However, although it is well known that λ depends on the polymer molecular weight M, it remains unclear how τEC is affected by the polydispersity (i.e., the molecular weight distribution, MWD) inherent in real polymer samples. Furthermore, the elastic stress generated by the stretching polymer depends on the concentration of polymer in solution, so an interplay between MWD and polymer concentration might be expected. We demonstrate this interplay by blending low-M and high-M polymer samples with narrow MWDs at dilute concentrations and in different ratios, and by measuring the resulting τEC of each blend in a capillary thinning experiment. For a fixed concentration of the low-M polymer, we find a systematic variation of τEC with the concentration of the high M polymer species. We explain the results by considering the progressive stretching of decreasing molecular weight modes in the MWD, as the extension rate in the thinning liquid bridge increases with time prior to the onset of the EC regime. On this basis, our experimental results are qualitatively captured by a backwards summation of the elastic stress contributions arising from each fully stretched mode, and by extracting the Zimm relaxation time of the first unstretched mode once the accumulated elastic stress has exceeded the viscous stress. Our results have significant implications for the application of capillary thinning to extensional rheometry and for the interpretation of such measurements.