Abstract
The flow instabilities of solutions of high molecular weight DNA in the entangled semi-dilute concentration regime were investigated using optical coherence tomography velocimetry, a technique that provides high spatial (probe volumes of 3.4 pL) and temporal resolution (sub μs) information on the flow behaviour of complex fluids in a rheometer. The velocity profiles of the opaque DNA solutions (high and low salt) were measured as a function of the distance across the gap of a parallel plate rheometer, and their evolution over time was measured. At lower DNA concentrations and low shear rates, the velocity fluctuations were well described by Gaussian functions and the velocity gradient was uniform across the rheometer gap, which is expected for Newtonian flows. As the DNA concentration and shear rate were increased there was a stable wall slip regime followed by an evolving wall slip regime, which is finally followed by the onset of elastic turbulence. Strain localization (shear banding) is observed on the boundaries of the flows at intermediate shear rates, but decreases in the high shear elastic turbulence regime, where bulk strain localization occurs. A dynamic phase diagram for non-linear flow was created to describe the different behaviours.
Highlights
Conventional turbulence in simple fluids, as envisaged by Osbourne Reynolds, occurs at high Reynolds numbers
As the DNA concentration is increased to 15 mg/mL, wall slip behaviour continues to be observed with some strain localization on the boundaries, irregularities in the time averaged velocity profiles are observed above a critical shear rate, (Fig. 2a) i.e. bulk strain localisation occurs
As the DNA concentration was varied in high salt conditions the dynamic phase diagram (Fig. 3) has a series of regimes
Summary
Conventional turbulence in simple fluids, as envisaged by Osbourne Reynolds, occurs at high Reynolds numbers Elastic turbulence in complex fluids has been explored much less than conventional turbulence in simple fluids and there are large gaps in our understanding, such as the nature of the chaotic intermittency of the velocity fluctuations that are characteristic of these turbulent flows and the nature of the boundary layers. The linear and non-linear rheology of DNA has been the subject of a number of previous studies with bulk rheometers[10, 18,19,20,21,22]. In terms of their structure and dynamics, DNA solutions can be satisfactorily modelled as flexible polyelectrolytes[23, 24]. The velocimetry of concentrated DNA solutions has been little studied in the literature
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