Student Chapter Presentation – Friday 13th March 2020

The coffee ring effect can be observed in many particle suspension systems. This is where the constituents of a pinned sessile drop are deposited preferentially around the circumference of a stain left after the liquid has evaporated. The phenomena has been shown to be highly dependent on the initial particle concentration and contact angle of the suspension. The effect is also seen in the dried deposits of whole blood. As low red cell concentration is a symptom of some types of anaemia, drying blood samples on a range of surfaces and visually comparing the patterns formed is proposed to be a new rapid, low-cost diagnostic tool.

During blood drop drying, two phases are present: a central fluid region and a deformable porous outer region where the particles (red blood cells) have consolidated. As drying continues this inner region shrinks and the outer region grows. A numerical model was developed to predict the evolution and final height profile in thin droplets containing flexible particles. The model is based on coupling lubrication theory assuming infinite vertical diffusion in the centre region, with Darcy’s law continuum model in the outer region.