  DNA is an astonishing material that can be used as a molecular building block
to construct periodic arrays and devices with nanoscale accuracy and precision.
Here, we present simple bead-spring model of DNA nanostars having three, four
and five arms and study their self-assembly using molecular dynamics
simulations. Our simulations show that the DNA nanostars form thermodynamically
stable fully bonded gel phase from an unstructured liquid phase with the
lowering of temperature. We characterize the phase transition by calculating
several structural features such as radial distribution function and structure
factor. The thermodynamics of gelation is quantified by the potential energy
and translational pair-entropy of the system. The phase transition from the
arrested gel phase to an unstructured liquid phase has been modelled using
two-state theoretical model. We find that this transition is enthalpic driven
and loss of configuration and translational entropy is counterpoised by
enthalpic interaction of the DNA sticky-ends which is giving rise to gel phase
at low temperature. The absolute rotational and translational entropy of the
systems, measured using two-phase thermodynamic model, also substantiate the
gel transition. The slowing down of the dynamics upon approaching the
transition temperature from a high temperature, demonstrating the phase
transition to the gel phase. The detailed numerical simulation study of the
morphology, dynamics and thermodynamics of DNA gelation can provide guidance
for future experiments, easily extensible to other polymeric systems, and has
remarkable implications in the DNA nanotechnology field.
