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Sweden: Biotechnology Harnessing Capillary Flow for New Pump Design

Editor: MA Alexander Stark

A team of researchers from Swedish KTH have found a way to fully control capillary action, and they’ve designed a device that harnesses it for possible use in biotech applications such as biomolecular analysis and body fluid handling.

Capillary flow is a common phenomenon inherent in everyday tasks, from wiping up spills to watering plants.
Capillary flow is a common phenomenon inherent in everyday tasks, from wiping up spills to watering plants.
(Source: David Callahan)

Stockholm/Sweden — The capillary effect is something you deal with every time you wipe up a spill or put flowers in water. The Swedish scientist Wouter van der Wijngaart has spent most of his life contemplating this phenomenon, which enables liquid to flow through narrow spaces like the fibres of a cloth, or upwards through the stems of flowers, without help from gravity or other forces.

Capillarity is a very common phenomenon that the professor of Micro and Nanosystems at KTH now has dissected into its details and turned into an engineering device, that is, a simple pump which can be fully controlled.

Capillary flow is independent of gravity. In fact, it actually acts in opposition to gravity. Wijngaart wondered why water flowing against gravity couldn’t be used to create a perpetuum mobile (a motion that continues infinitely), and today he asked his students each year to explain this effect.

The phenomenon is an interplay between two kinds of forces, cohesion and adhesion. Cohesion is the attraction between similar types of particles, such as water molecules. And adhesion is the attraction between different kinds of particles, such as water and the fibres of a towel. When adhesion is stronger than cohesion, capillary action occurs.

The rate of capillary flow is still affected by the viscosity of a fluid and the geometry and surface energy of the surfaces of the channels through which it flows. Yet, after five years of study, the researchers have managed to make these variations negligible. In a series of three publications, they first showed how to make the flow constant in time; then independent of viscosity; and, finally, independent of the surface energy.

Reporting in Microsystems & Nanoengineering, the researchers, which also include Weijin Guo and Jonas Hansson, tested pumps of their new design with a variety of sample liquids, including water, different samples of whole blood, different samples of urine, isopropanol, mineral oil and glycerol. The capillary filling speeds of these liquids vary by more than a factor 1000 when imbibed by a standard constant cross-section glass capillary, van der Wijngaart says.

By contrast, the new pump design resulted in flow rates in a virtually constant range with a variation less than 8 %, the researchers report.

The research was funded in part by the European Union through the ITN Marie Curie project ND4ID.