The high-shear rheology of single-stranded DNA solutions: A model system for flexible polyelectrolytes and neutral polymer solutions

• Applicant: Sharadwata Pan, Ph.D.
• Subject Area: Experimental and Theoretical Physics of Polymers
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 557249284

Single-stranded (ss) DNA have profound importance in physiology and genetics. Their in vivo functions are governed by their mechanical properties, which also offers numerous in vitro applications towards bio- and nanotechnology. In solutions, ss DNA behave as a flexible polymers and polyelectrolytes. Characterizations of these solutions at high shear rates have significant fundamental and industrial applications. While for semiflexible polymer and polyelectrolyte solutions significant progress has been achieved recently, the corresponding data available on flexible chains is grossly insufficient despite the high necessity. The objective of the current project is to achieve insight on the rheological behavior of ss DNA solutions at high shear rates. ss DNA are suited as a model system for linear flexible polymers and polyelectrolytes. It shows high chain flexibility, high-level monodispersity, can be studied both as a neutral polymer and as a polyelectrolyte. Furthermore, different from synthetic polymers, they can be characterized in tiny amounts via gel electrophoresis after exposure to high shear rates. All these benefits significantly favor using ss DNA over other synthetic polymers. Our principal aim is to investigate the concentration dependent rheology of ss DNA flexible chains as both charge neutral polymers, as well as polyelectrolytes, in dilute and semidilute regimes, in terms of three suitable parameters, namely the infinite shear viscosity, the shear-thinning exponent, and the normal stress difference. Based on these studies, we aim to examine the transition states between flexible polyelectrolytes to neutral polymers, by progressively varying the salt induced charge-screening rate, and study the influence of solvent quality on the critical physical parameters at high shear rates. These will answer several open questions concerning flexible chain rheology, including concentration and solvent quality dependent scaling behaviors of the critical parameters, as well as universal shear thinning at high shear rates. We will also verify its mechanical stability at different concentrations after measurements at high shear rates. For high shear rheometry, we will use a customized narrow gap rotational rheometer, developed by Wierschem and coworkers at FAU. In our preliminary work with ss DNA, we have shown that the device can consistently access ultra-high steady shear rates beyond 100000/s under ambient conditions, and the concentration dependent flow behavior of flexible chains can be investigated via aforementioned identical critical parameters at high shear rates. Finally, we validated that the ss DNA sample remains mechanically stable during the rheological measurements, especially at the highest concentration up to a shear rate of 30000/s. The studies proposed here will provide insight into DNA rheology in particular and into that of true flexible polymer and polyelectrolyte solutions at large shear deformations in general.

• DFG Programme: Research Grants