Drops on the networks
11/5/15

Her drops made the headlines of the journal Soft Matter, published on 28 September 2015 (1). But what’s so special about them? They’re the first of their kind to have been created using a simple and flexible method. And pharmacists and chemists already have big plans for these microscopic droplets.

Soft Matter GouttesFloriane Weyer, an FNRS (Belgian National Fund for Scientific Research) research fellow in the GRASP research group led by Nicolas Vandewalle, began a PhD two years ago on multicomponent drops and, more generally, on microfluidics on fibre networks. Microfluidics is the manipulation of very small volumes of fluids, in this case, anything from microdroplets to picoliter microdroplets (10-12 litre). And a multicomponent drop simply refers to a drop containing several of these droplets without them mixing together.

There are two types of microfluidics, the most common kind being the one involving channels. This type involves creating small channels, which are a hundredth or tenth of a micrometer, in which fluids are circulated. This is referred to as a closed microfluidic system because the liquid doesn’t come into contact with the air. The disadvantage is having to use pumps, syringes and other relatively voluminous and restrictive devices to introduce the fluids into the channels. "Our goal", Floriane Weyer explains, "was to find a simpler device: microfluidics on fibres. The fibres guide the droplets which divide or recombine at the intersections, etc. Like dew drops on a spider’s web! It’s an open system because it’s in contact with the air. Therefore, it’s easier to manipulate than channel systems".

A system that nevertheless has two weak points. The droplets are generally water; therefore, they evaporate very quickly when they come into contact with the air considering their low volume, and they can’t be manipulated for long. Furthermore, while they’re in contact with the ambient air, they can be contaminated by dust. To avoid these pitfalls, the researcher encapsulated the water droplets in an oil shell (oil and water aren’t miscible).

But that’s not all: it was also necessary to find an efficient, simple and reproducible method. This is what the researcher from Liège managed to achieve and the results were published in the journal Soft Matter.

A fishing line network

The different water droplets have to be created first. To achieve this, the researcher used a fibre network made from nylon fishing line. In principle, placing a drop on a fibre isn’t very difficult, you just need a syringe. But this method can’t be used for the very small volumes sought after in microfluidics. “In this case, you have to resort to fibre networks”, explains Floriane Weyer. “When a drop reaches the point where two threads cross, if its volume is sufficiently great, it passes through the node, which doesn’t retain it, and it continues on its way. But as it does so, it deposits a small residue around the node, which is far smaller than the initial drop. We studied the formation of these water droplets around nodes, as well as their geometry, volume, etc.”

(1) F.Weyer, M.Lismont, L.Dreesen and N.Vandewalle, Compound droplet manipulations on fiber arrays, Soft Matter, 2015, 11, 7086.

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