  Liquid droplets of biomolecules serve as organizers of the cellular interior
and are of interest in biosensing and biomaterials applications. Here, we
investigate means to tune the interfacial properties of a model biomolecular
liquid consisting of multi-armed DNA 'nanostar' particles. We find that long
DNA molecules that have binding affinity for the nanostars are preferentially
enriched on the interface of nanostar droplets, thus acting as surfactants.
Fluorescent measurements indicate that, in certain conditions, the interfacial
density of the surfactant is around 20 per square micron, indicative of a
sparse brush-like structure of the long, polymeric DNA. Increasing surfactant
concentration leads to decreased droplet size, down to the sub-micron scale,
consistent with arrest of droplet coalescence by the disjoining pressure
created by the brush-like surfactant layer. Added DNA surfactant also keeps
droplets from adhering to both hydrophobic and hydrophilic solid surfaces,
apparently due to this same disjoining effect of the surfactant layer. We thus
demonstrate control of the size and adhesive properties of droplets of a
biomolecular liquid, with implications for basic biophysical understanding of
such droplets, as well as for their applied use.
