The Coffee-ring Effect

The photo, “Just a Drop” (page 26, Art of Analysis, August 2014 issue: tas.txp.to/1114/drop) showed four aqueous drops containing ink. Each drop had been deposited in the center of an area – a light blue circle – on a paper substrate. The blue areas had been treated to make them very hydrophobic. For this reason, the drops were spherical and showed no tendency to spread out on the surface.

I wondered about that circle. After the aqueous solvent had all evaporated, had the ink dye molecules or pigment particles been deposited in a “coffee ring” with most of the deposition along the ring rather than evenly distributed like a disk – or would they have a gradual increase in concentration towards the center?

The coffee-ring effect has interested me since the early nineties when I read the paper, “Liquid sample concentration technique using perfluorated polymer FILM for picogram analysis by FT-IR” (1). They defeated the coffee ring effect by depositing a drop containing their sample onto a flat, highly-polished stainless steel surface that had been treated so that it contained a very thin fluorocarbon layer. As the solvent in the drop evaporated, the contact angle of the drop with the surface caused any solute to be pulled along rather than being deposited along an outer ring. Eventually, you ended up with a small spot of concentrated solute rather than having it distributed around an outer ring. Since samples in forensic science may be quite limited, this appeared to be an excellent way of concentrating a limited amount of sample.

However, although both of these examples feature a treatment with a perfluorated polymer, one sample was deposited on a hard smooth surface and the other on a fibrous matrix. I wondered, although treated to make it hydrophobic, how could a fibrous matrix be able to defeat the coffee-ring effect?

A Google search led me to a video, “Hydrophobic Coating Technology Inspired by Butterfly Wings” from Richard Hammond’s “Miracles of Nature” TV show (watch video: tas.txp.to/1114/butterfly). Hammonds explains that heavy raindrops do not harm the Morpho butterfly’s wings because the microstructure of the material makes minimal contact with the water drops. He uses a balloon and frame model to illustrate that even if the balloon is in contact with the frame, there is actually very little contact between them.

In much the same way, the paper’s fibrous cellulose matrix has minimal contact with the ink drop and the hydrophobic treatment prevents any wicking effect. As the solvent evaporates, the drop radius gets smaller. Presumably, as the drop becomes supersaturated with dye molecules, they are deposited onto the paper. In fact, there may even be a partial “inverse-coffee-ring effect” with a slight increase in deposited solute towards the center.

Although Xuan Mu envisions this effect as useful in paper microfluidics, it may also have application in mass spectrometry (MS). Something akin to the coffee-ring effect may happen in MS as a sample is deposited either on a desorption matrix or a substrate such as a silicon crystal. Sampling can be hit and miss, because you end up with hot spots (lots of sample) and dead spots (almost no sample).

It would be great to have my above reasoning tested. What would time-lapse environmental SEM show? What about MS imaging? Never stop reading the general scientific literature and keep an open mind; your next great inspiration may be sparked by something seemingly unrelated – like a beautiful photograph of ink drops.