Long-range attraction of Marangoni-contracted sessile drops

The work in this thesis investigates the behaviour of binary mixture ethylene glycol and water (EG/W) sessile drops, which when placed on a hydrophilic substrate, can attract at up to a distance of 5x their diameter. An optical rig was built to observe the interactions between two 100 pL drops from the side and underneath perspective. Drops were jet into a humidity cell and videos of their interactions were recorded with high-speed cameras. Videos were analysed with MATLAB algorithms able to extract drop velocities, volumes, radii, heights and contact angles. Initial EG/W content was adjusted for evaporation from the nozzle, and the subsequent composition changes were predicted.

This study found that different sizes of contracted drops do not appear to share the same contact angle when their compositions are the same, despite the experiments being performed far below the capillary length. This gives rise to an aging effect, whereby drops that started at different initial mass fractions of EG, will show different contact angles after evaporation when measured at the same composition.

A mechanism for drop attraction was proposed where the driving force for movement is the vapor gradient imposed by the neighbouring drop. In accordance with this model experiments found that drop speeds appear to scale with the vapor gradient, 1/r2, where r is the centre-centre distance between the drops. Drop speeds also scale with the radius R of the opposing drop. In 3D, a scaling analysis mirrored experimental results suggesting drop speeds are independent of their own size. Drops appear to reach a ‘fastest composition’ between 2 and 6 wt% of ethylene glycol, despite contact angles reaching a maximum between 30 and 40 wt%. This was shown to be due to a lower viscosity and increased vapour pressure favouring higher speeds at low fractions of EG.