Viscoelastic instability in electro-osmotically pumped elongational microflows

• Author / Creator

  Bryce, Robert M

• The focus of this thesis is on electro-osmotically pumped flow of viscoelastic fluids through microchannels.

  Fluid transport in microscaled structures is typically laminar due to the low Reynolds numbers involved. However, it is known that viscoelastic polymeric liquids can display striking instabilities in low Reynolds number flows.

  The motion of polymer doped solutions electrically pumped through microchannels is studied at low Reynolds number. It is found that extensional instabilities can be excited in such microflows with standard electro-osmotic pumping (approximately mm/s flow rate regime), occurring at the viscoelastic instability threshold. The existence of these instabilities must inform design as microfluidic applications move beyond simple fluids towards using biological materials and other complex suspensions, many of which display elasticity. It is further found that discrete and persistent microgels are formed at sufficiently high current densities.

  Prior work has found up to orders of magnitude increase in mixing rates, however additional fluid deformation effects (notably shear) exist in other studies and high viscosity solvents are used. The flows here exclude shear, a ubiquitous feature in mechanically driven cavity flows, and low viscosity solvents typical in microfluidic applications are used. The device is also highly symmetric minimizing Lagrangian chaos deformation and mixing of fluids. It is demonstrated that viscoelastic instabilities reduce mixing relative to low viscosity polymer-free solutions. The decrease in mixing found is consistent with the understanding that viscoelastic flows progress towards Batchelor turbulence, and demonstrates that, in contrast to common expectations, viscoelastic flows are effectively diffusion limited. Electro-osmotic pumped devices are the ideal platform to study isolated viscoelasticity and elastic turbulence, where additional effects (such as shear, or Lagrangian deformation manipulations) can be introduced in a controlled manner allowing fundamental studies of viscoelasticity and mixing.

  Besides the viscoelastic experimental observations it is shown that (1) a recently discovered instability due to density fluctuation has an analogue in polymeric fluids corresponding to the viscoelastic instability threshold, (2) inspection of correlations in microparticle image velocimetry (micro-PIV) data in unstable polymer flows reveals the relaxation time of polymer solutions, and (3) poly(ether sulfone) polymer films can act as negative electron beam resist.

• Subjects / Keywords

  • Microfluidics
  • Instability
  • Viscoelastic

• Graduation date

  Spring 2010

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3KH09

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.