The process of drying of a little drop of fluid can tell scientists a whole story. For about two decades now, the ‘coffee ring effect’ has been known as when a drop of spilt coffee dries up, the outermost edge of the dried drop is a little darker than the centre, forming a darker ‘ring’. This is caused by the outward drift of suspended coffee particles from the centre, causing a denser, darkened rim. Now, researchers from Indian Institute of Technology, Madras, have shown that after reaching the rim, as the drop dries, some of the particles undergo an inward drift too. This research has applications in agriculture, forensic science and even disease diagnosis. The present work has been published in the journal, Soft Matter.
The work consists of experimental and theoretical parts. Taking polystyrene microbeads of diameters 3 microns, 5 microns and 10 microns, the researchers suspend them in clean water at concentration of 30,000 beads per microlitre for the first two sizes and 10,000 beads per microlitre for the last, and place a drop of the suspension on a glass plate. They are able to observe the inward drift. “The migration of particles to the edge has been known for more than 20 years now. But, after the particles reach the rim, they experience an inward drift and this is what we have newly discovered,” says Madivla G. Basavaraj, Chemical Engineering Department and an author of the paper, in an email to The Hindu. He clarifies that while the inward drift would persist for sub-micron-sized particles, it would decrease with particle size.
Gapped coffee ring
The inward movement takes place because the particles are ‘squished’ between the solid plate and the evaporating liquid interface. Hence, the ‘coffee’ ring is not formed at the point where the liquid touches the solid, but there is a small gap between the outermost edge and the ring. Using the theory of evaporating drops, the researchers could calculate the rate at which the liquid interface gets flattened. “We could convert the rate of flattening of the interface to the rate of inward movement of the particles. Hence, we were able to predict the extent up to which the particles recede, namely the stain recedes,” says Sumesh P. Thampi, also from the Chemical Engineering Department, and an author of the paper. Perhaps measuring the gap can help in forensics.
Prof. Basavaraj explains that some qualitative studies have shown that understanding the drying of biologically relevant fluids like blood can help diagnose anaemic and hyperlipidaemic conditions.
“One of the long-term goals of this team is to work with simulated [fluids] as well as blood drawn from different individuals with different conditions to come up with early diagnostic strategies,” he says. The researchers hypothesise that size-based separation of different molecules in biological fluids can provide useful information about disease conditions.