Date of Award
8-2026
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Mechanical Engineering
Committee Chair/Advisor
Xiangchun Xuan
Committee Member
Yuhao Xu
Committee Member
Joshua Bostwick
Committee Member
Zhen Li
Abstract
Electrokinetic instabilities (EKI) arise in electrically driven flows because the applied electric field interacts with spatial variations in electrical conductivity, generating electric body forces that can destabilize fluid interfaces. EKI can be beneficial by enhancing mixing in microchannels where diffusion is slow, but it can also be detrimental by disrupting stable sample transport, focusing, and separation. EKI has been widely studied in Newtonian fluids, and the electric Rayleigh number is known to govern the onset of this instability. However, very few studies have examined its behavior in non-Newtonian fluids, despite their prevalence in biological, chemical, and industrial applications.
This thesis investigates EKI in two major classes of non-Newtonian fluids: shear thinning and viscoelastic fluids. Scaling analyses are performed to formulate the electric Rayleigh number expressions for the power-law based shear thinning and Oldroyd-B based viscoelastic fluids by incorporating the rheological contributions of each constitutive model into the electroviscous momentum balance. Experiments were conducted in a T-shaped microchannel using phosphate buffer solutions with a tenfold conductivity contrast. Xanthan gum solutions were used to examine the fluid shear thinning effect, while hyaluronic acid solutions were used to study the fluid viscoelasticity effect. To assess the role of rheology gradients, the non-Newtonian fluid was placed in the high-conductivity stream, the low-conductivity stream, or both streams, allowing the direction of the rheology gradient to be varied relative to the conductivity gradient.
For shear thinning fluids, our proposed critical electric Rayleigh number based on the power-law model shows a decreasing trend with increasing shear thinning behavior. This behavior is consistent with the observed reduction in the critical electric field for instability onset as the xanthan gum concentration increases, highlighting the enhancing effects of fluid shear thinning on EKI. This enhancement is strongest when the shear thinning gradient aligns with the conductivity gradient, producing lower onset fields, larger interfacial wave amplitudes, and higher wave speeds than when the two gradients oppose each other. When shear thinning behavior is present in both streams, the critical electric field generally lies between the values observed in the two mixed-rheology cases.
For viscoelastic fluids, scaling analysis based on the Oldroyd-B model shows that viscoelasticity increases resistance to EKI through both viscous and elastic contributions. Experiments confirm that adding hyaluronic acid suppresses EKI by increasing the critical electric field and reducing the perturbation energy of interfacial motion. The strongest suppression occurs when the viscoelasticity gradient opposes the conductivity gradient. Power spectrum analysis further shows that viscoelasticity alters the nature of the instability dynamics: Newtonian fluids transition from periodic oscillations to broadband fluctuations as the electric field increases, whereas viscoelastic systems exhibit broadband fluctuations even near onset, likely because of elastic stresses.
Overall, this thesis demonstrates that fluid rheology is an effective control parameter for EKI. Shear thinning enhances EKI and can be used to trigger mixing at lower electric fields, while viscoelasticity suppresses EKI and can be used to maintain more stable interfaces. The findings also show that the direction of rheology gradients provides an additional means of controlling EKI, offering insights for improved electrokinetic microfluidic devices.
Recommended Citation
Islam, Md Mainul, "Electrokinetic Flow Instabilities in Non-Newtonian Fluids With Conductivity Gradients" (2026). All Theses. 4836.
https://open.clemson.edu/all_theses/4836
Author ORCID Identifier
https://orcid.org/0009-0001-7037-0176