Drops on the networks

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.

Floriane 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.”

The device tested by the researcher, which works every time, is as follows: the network is made up of a horizontal fibre and several vertical ones with different diameters. A drop of water is released on every vertical fibre, thus creating several residues and, several water droplets, which can differ because the fibres are different. It is therefore possible to go from one component to another without any contamination or mixing. The device is then turned 90° and a drop of oil is released onto the large horizontal fibre which is now vertical (see animation). The latter will collect all the residues and encapsulate them.

The goal of the study was to manage to create such drops simply and in a very flexible way: if you want internal components for instance, you just have to add as many fibres as you want extra components. This may seem easy but, in fact, it was a question of trial and error and a great many experiments to determine the optimum device. If you don’t use fibres with different diameters, and if you don’t turn the device to invert the symmetry, then encapsulation is impossible. The drop of oil passes the node without taking any of the trapped water with it. That was the trick we had to find.

Applications

The most interesting point is the fact that there are several different droplets within the same drop. “This is what the pharmaceutical industry is looking for”, Floriane Weyer explains. “There are often several active ingredients in a capsule and for the time being, drug manufacturers are obliged to mix them. They are interested in being able to put them in separate compartments so that they can act one after the other, for instance. This is what happens in our system: if you use a component that polymerises (hardens) instead of oil, the capsule is solid and the droplets are separated from each other. And they will remain so whereas if you use oil, they will eventually merge owing to gravity.” A property that has attracted the attention of the chemical industry, which is interested in putting different reagents in the droplets; when they merge, a chemical reaction can occur, making these drops true microreactors.

The production of such drops also offers opportunities in terms of research, for instance in the field of biology. In GRASP, they are being studied in order to examine how they pass through cell membranes. The cells are represented by the water droplets while the membrane is the film of oil separating them. This enables the researchers to study membrane transport by examining how a component passes from one drop to another through the film of oil. We undoubtedly haven’t heard the last of these droplets.

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