Concentration Dependent Dynamics of Semidilute DNA Solutions
Prof. Ravi Prakash Jagadeeshan (Department of Chemical Engineering, Monash University, Melbourne, Australia)

There are a number of contexts involving polymer solutions, such as in the spinning of nanofibres or in ink jet printing, where in order to achieve the most optimal outcome, the concentration of polymers must not be too dilute or too concentrated, but somewhere in between. While a lot is known about dilute and concentrated solutions and melts, not much is currently known about the vast regime of concentrations that lie in between; namely, the so-called semidilute concentration regime. Significant progress has been made so far in the description of dilute and concentrated solutions because their behaviour can be understood by understanding the behaviour of single molecules. This approximation, however, is not valid in semidilute solutions because it is necessary to take into account all the many-body interactions that arise in this regime. Notably, the onset of the semidilute concentration regime occurs at fairly low monomer concentrations because the stringing together of many monomers to form polymer chains leads to their being extended objects in space that can interact with each other even at low concentrations. We have recently developed a scaling theory with detailed predictions for the behaviour of semidilute solutions in the entire temperature-concentration plane of the phase diagram. Additionally, we have developed both a simulation algorithm, and an experimental protocol to verify the predictions of the scaling theory. In this talk, I will outline the arguments that lead to a scaling theory for semidilute solutions in the double crossover regime of temperature and concentration. I will also describe the experimental protocol that involves the use of double-stranded DNA as a model polymer system. This enables a systematic characterisation of semidilute polymer solution behaviour across a range of molecular weights, temperatures and concentrations, and an experimental validation of the scaling theory predictions. Finally, I will discuss the development of a mesoscopic Brownian dynamics simulation algorithm that is capable of accurately describing semidilute solutions. The validity of the new simulation algorithm is demonstrated by comparing the predictions of simulations with experimental observations and scaling theory predictions.