Understanding small-scale fluids using colloids

Lab-on-a-chip devices are the result of a quiet revolution that has been unfolding over the past years in chemistry and medicine, made possible thanks to our understanding of how fluids behave at small scales. In the future, shrinking devices to even smaller sizes, even comparable to the size of single liquid molecules, will be a huge challenge, and we will need to devise ways of understanding and controlling how fluids behave at these extreme scales. In a recent paper in Nature Communications, a team of researchers including experts from the University of Barcelona has come up with a new way of doing this using colloids as fat, oversized atoms.

Colloids are particles small enough that they are not too heavy – so they will not settle if you disperse them in a fluid, such as air or water – but not too small so that they dissolve in the fluid. Colloidal particles can range from 1 nanometer (a millionth of a millimeter) to 1 millimeter in size, and can be made of many different components.

In the laboratory, researchers have used an ingenious colloidal mix of spherical particles and polymer strands to understand how fluids behave in extremely small channels. The size of the particles in this mix is about 200 nanometers, so they fit nicely into the colloidal particle classification. The colloid is enormous compared to a water molecule, but it is tiny if you compare it to a microfluidic channel. At that scale, the colloid appears to be a small lump.

The clever thing about this colloidal mix is that the polymer strands are able to squeeze between the spherical particles, sort of elbowing them out. This effect eventually results in the creation of a two-phase mixture, very similar to having oil separated from water.

Using these blown-up liquids, researchers have studied a variety of phenomena in microchannels. By changing the size of the channels, they were able to reveal in detail how a fluid interacts with the boundaries encasing it. This understanding can be used to control the formation of drops, jets and fingers only a few hundred times larger than the size of a colloidal particle. Crucially, the size of the colloidal particles made it possible to observe the fluid dynamics under such an extreme confinement in all its glory using nothing but direct optical techniques, something that would have been impossible to do with a common liquid such as water.

Therefore, this simple technique has a lot of potential in advancing our understanding of how fluids behave at small scales. Because the colloidal particles play the role of oversized atoms, many of the phenomena revealed using colloids can be transferred to understanding other liquids, such as water, just as milk and a flashlight can be used to understand sunsets. Using knowledge from one system to understand another is not particular to our colloids, it is an underpinning principle of how physics works to make sense of the world around us, and unveiling such generality is perhaps one of the most beautiful aspects of it.

Contributors to this research project, led by Prof. RodrigoLedesma-Aguilar from the University of Northumbria (UK), included Prof. Ignacio Pagonabarraga from the Department of Fundamental Physics and Prof. Aurora Hernández-Manchado from the Department of Structure and Constituents of Matter, both from the University of Barcelona.

Referència a l'article:

S, A, Setu, R. P.A. Dullens, A. Hernández-Machado, I. Pagonabarraga, D. G.A.L. Arts i R.Ledesma-Aguilar. «Superconfinement tailors fluid flow at microscales». Nature Communications, Juny 2015. Doi: :10.1038/ncomms8297

Source: Universitat de Barcelona





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