A novel approach for visualising the liquid droplet transfer mechanism from a surface to a finger pad using optical coherence tomography

Viruses and microbes can be transferred from one surface to another via liquid droplets that result from, for example, sneezing. The exact mechanism of transfer from a surface to a finger-pad, however, has not been well studied. This novel work investigated how droplets are picked-up and how much liquid remains on each surface after contact is broken during a ‘touching event’ using optical coherence tomography (OCT) images. These showed that the droplet was pulled towards the finger-pad as it approached and formed an ‘hour-glass’ shape as the finger-pad moved away from the surface before splitting between surfaces as contact was broken. Despite the spread of liquid during the contact, after the touch, droplets reformed. Three-dimensional images of the droplets before and after the touch were used to determine the change in volume. For the droplets tested (0.40–1.85 mm3), the results consistently showed a transfer of 77%–94% of the volume to the finger pad, suggesting that within this volume range, the transfer mechanism remains stable for this specific liquid-surfaces interaction. This study demonstrates the potential of OCT for studying liquid droplet transfer. This is important in designing surfaces for reduced liquid transfer and for improving models of droplet behaviour.