Application of falling-needle rheometry to highly concentrated DNA solutions.

High-concentration DNA solutions are common both in vitro and in vivo, and understanding the rheological properties is a critical area of bioscience. Our previous measurements on high-concentration DNA solutions (2-6 mg/ml) interestingly provided evidence for a viscosity maximum with temperature. Under the influence of temperature, the measured viscosities indicated distinct differences in the interactions of highly polymerized DNA in unbuffered and buffered aqueous solutions. Under the same conditions, the buffered solutions were always less viscous, and in addition the viscosity maximum was not observed. In this research we have utilized a falling-needle rheometer in order to gain more insight into the nature of the previously observed viscosity maxima. The shape of the flow curves for all the DNA solutions indicated that the solutions are shear-thinning and has allowed us to confirm the existence of the viscosity maximum in unbuffered DNA solutions. Also we have been able to measure flow curves at very low shear rates, <10 s⁻¹. These results showed that the flow curves intersect and that the lower the concentration of DNA in solution, the lower is the temperature where the flow curves will intersect. Thus, the viscosity-temperature dependence is also a function of the shear rates experienced by the solution. Finally, as expected, the flow behavior of the DNA solutions becomes more Newtonian with increasing temperature, and there appears to be a small yield stress that decreases with increasing temperature.