  We investigate the effects of crowding on the conformations and assembly of
confined, highly charged, and thick polyelectrolyte brushes in the osmotic
regime. Particle tracking experiments on increasingly dense suspensions of
colloids coated with ultra-long double stranded DNA (dsDNA) fragments reveal
non-monotonic particle shrinking, aggregation and re-entrant ordering. Theory
and simulations show that shrinking is induced by the osmotic pressure exerted
by the counterions absorbed in neighbor brushes, while aggregation and
re-entrant ordering are the effect of a short-range attraction competing with
the electrostatic repulsion. Blunt-end interactions between dsDNA fragments of
neighboring brushes are responsible for the attraction and can be tuned by
inducing free-end backfolding through the addition of monovalent salt. Our
results show that base stacking is a mode parallel to hybridization to steer
colloidal assembly, in which attractions can be fine-tuned through salinity
and, potentially, grafting density and temperature.
