  In this paper we investigate the effects of varying cation valency and
concentration on the rheology of entangled lambda DNA solutions. We show that
monovalent cations moderately increase the viscoelasticty of the solutions
mainly by stabilising linear condensation of lambda DNA ``monomers'' via
hybridisation of their sticky ends. On the contrary, divalent cations have a
far more complex and dramatic effect on the rheology of the solution and we
observe evidence of inter-molecular DNA-DNA bridging by Mg2+. We argue that
these results may be interesting in the context of dense solutions of single
and double stranded DNA, e.g. in vivo or in biotechnology applications such as
DNA origami and DNA hydrogels.
