Unexpected behaviour of microdroplets

Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw discovered a new phenomenon related to the fluid dynamics. It occurs when minute droplets translate through microfluidic channels. "The effect observed by our group is related to changes in swirls inside microdroplets and as yet has not been predicted by existing theoretical models," says Dr Sławomir Jakieła from the IPC PAS. The results of the research pursued thanks to a TEAM grant from the Foundation for Polish Science, have just been published in the journal Physical Review Letters.

Microfluidic systems are miniature chemical reactors of a credit card in size, or even less. Inside these systems, microchannels with diameters of tenths or hundredths of a milimeter provide a path for laminar flow of a carrier fluid (commonly oil) with floating microdroplets of appropriate chemical compounds.

"Using a single microfluidic system, even today one can carry out as much as a few tens of thousands of different chemical reactions a day. In future, these systems will become for chemistry what integrated circuits turned out to be for electronics. Yet before we build chemical devices as revolutionary as silicon microprocessors, we have to reach a comprehensive understanding of all physical phenomena occurring in flows of microdroplets," continues Dr Jakieła.

The flows that we experience at the macroscale are often dominated by inertia and turbulences. With small volumes that are typical for microfluidic systems, the flow of a liquid is laminar and subject to viscosity-related effects.

The speed of oil flowing in microchannels is not uniform. The layers close to the walls move with the lowest speed, whereas those near the middle of a channel -- with the highest speed. "If a microdroplet is distinctly smaller than the channel diameter, it can find a place in the middle part of the flow, reaching the speed even twice as high as the average oil speed. This is nothing surprising. Similar effect can be observed for instance in rivers: the current near the banks is much slower than in the middle of the river," explains Sylwia Makulska, a PhD student at the IPC PAS.

If a sufficiently large droplet flows in a circular channel, it occupies almost the entire lumen of the channel. The droplet speed is then almost identical as that of the oil flow. The situation gets much more interesting when the droplet translates in rectangular channels that are typical to microfluidic systems. Due to interfacial tension the cross-section of a microdroplet remains rounded leaving the corners of the channel free for the flow of oil.

The team from the IPC PAS produced microdroplets from aqueous solutions of glycerine of different concentrations, and therefore of different viscosities. They translated in oil (hexadecane) through a 10 cm long rectangular channel. The researchers measured the speed of microdroplets relative to the oil as a function of their volume (length in a microchannel), droplet and oil viscosities and the flow speed of the carrier liquid.

When the viscosity of microdroplets was less than or comparable to that of the carrier liquid, their speed relative to the oil turned out to decrease with increasing droplet length, but in a certain range only. The droplets were translating with the lowest speed when their length was two, three times greater than the channel width. "Every time we observed the minimum speed relative to oil. Everything seemed to be in line with what the theoreticians would expect," says Jakieła.

But what was really interesting were things that happened when the researchers started to change the rate of oil flow. It turned out that the minimum of the droplet speed relative to oil was disappearing with increasing flow rate. Further increase in the oil flow rate resulted, however, in reappearance of the minimum -- but this time deeper and wider. "To make the long story short: we discovered that, depending on the oil flow rate, a droplet of specific length can translate under some conditions faster and under other conditions slower relative to oil," concludes Jakieła.

To find out what is the reason for the surprising behaviour of the droplets, the researchers from the IPC PAS introduced to microdroplets fluorescent markers of a few micrometers in size. When the droplets were moving along the microchannel, they were irradiated with laser light to excite fluorescence of the markers, which allowed for observation of fluid movements inside the droplets.

The measurements revealed that the distribution of swirls inside a droplet changes with increasing flow rate of the carrier liquid. "We expected changes, but the existing theories suggested that the number of swirls in microdroplets decreases with increasing oil flow rate. We observed, meanwhile, an opposite phenomenon: the faster was the oil flow, the more swirls were in a droplet. The Nature played again a trick on theoreticians," sums up Prof. Piotr Garstecki (IPC PAS).

At the Institute of Physical Chemistry PAS a work has started to make use of the new phenomenon in processes related to mixing the contents of microdroplets in microfluidic systems.