In the group, we have previously developed a novel technique (CoManD: Contactless Manipulation of Droplets) for polymer printing and alignment based on the contactless manipulation of binary droplets on pristine surfaces. While moving, the droplets can deposit dissolved molecules on a surface with a preferential alignment to the direction of motion. Binary droplets are exceptional candidates for deposition due to an increased compatibility with different chemical species. The use of pristine surfaces (i.e. with no surface modification) is an additional advantage of CoManD for applications in molecular deposition.

The schematics to the right (a) shows the deposition and alignment of polyethylene glycol (PEG) crystallites by a binary droplet (water/propylene glycol) moving on a pristine glass substrate. Droplet motion is triggered by a remote localised vapour source (e.g. a blunt needle) from where vapour of the more volatile component in the droplet (here, water) diffuses. For asymmetric vapour distributions around the droplet, tiny imbalances of surface tension (<0.1%) form at the droplet’s free interface due to gradients in evaporation rate (b). These gradients give rise to Marangoni flows at the interface, which drive droplet motion. PEG chains dissolved in the droplet align perpendicular to its moving front whose shape depends on the droplet’s speed (c): for slower droplets, the front is circular and leaves a scallop-shell pattern behind; for faster droplets, the front is triangular and leaves a herringbone structure behind.

More details can be found in our previous publication:

Nonmonotonic Contactless Manipulation of Binary Droplets via Sensing of Localized Vapor Sources on Pristine Surfaces
Robert Malinowski, Ivan P Parkin & Giorgio Volpe
Science Advances 6, eaba3636 (2020)