"This paves the way for creating very sensitive microfluidic devices which respond readily to small physical changes like variations in temperature, gravity, acceleration or orientation" explains Carlos Rascón, one of the authors of this study. Rascón, from the UC3M Department of Mathematics, has just published a study in the latest issue of the journal Proceedings of the National Academy of Sciences (PNAS) along with Andrew O. Parry from Imperial College London and Dirk G.A.L. Aarts from Oxford University.
Their study focuses on the behavior of fluids in capillaries: containers with a constant cross-section, like cylindrical glasses, whose cross-section is a circle all along. "In addition to circles, we have also studied other cross-sections: ellipses and regular polygons (triangles, squares, pentagons, etc.). These non-circular sections seem to have more technological applications" explains Rascón, a researcher from the UC3M Interdisciplinary Group of Complex Systems (GISC, in the Spanish acronym). For example, if the cross-section of the capillary is a sufficiently flattened ellipse, the liquid can flow even if the capillary is extremely narrow, contrary to the standard assumption.
These results could have applications in nanotechnology. In narrow circular tubes, the liquid always stays inside (by capillary action), and emptying them is very difficult without pumping. By choosing the tube cross-section, one can control when and how they empty. "This would allows us to make very sensitive devices in which the liquid flows for specific values of the physical parameters," adds the researcher. In those devices, very small changes in the physical conditions could trigger the emptying of the capillary. This could be in itself the purpose of the device, or just a mere indicator of the change in the physical conditions, which opens the door to creating new diagnostic and measuring devices.
The technological implications of this research have been protected through patent applications in Europe and North America, and are commercialized by the UC3M Science Park. The control of micro and nanofluids is a multidisciplinary field that combines biotechnology, physics, engineering, nanotechnology and chemistry, and has practical applications in the design of systems used to control and manipulate small volumes of fluids. In this regard, the patent is very general and could be applied to the fabrication of measuring devices or, for example, to the design of high-resolution printers. It could also have applications in the food, aerospace and medical industries.
The study originated from a discussion of some experiments with colloidal fluids carried out at Oxford University. "Dirk asked Andy and me what the expected shape of the meniscus (the curvature of the surface of a liquid) was when a bi-dimensional capillary is in the horizontal position. In answering this question, we pondered what the shape of a 3D meniscus in a horizontal capillary would be. We needed more than five years of research to answer that second question," says Rascón. "We obtained numerical results that we couldn't interpret, and thought were programming errors but, after a very thorough checking, the results didn't change and we realized we needed to reinterpret them. Putting all the pieces of the puzzle together has been a long and tortuous journey."
Bibliographical reference: Carlos Rascón, Andrew O. Parry, and Dirk G.A.L. Aarts. Geometry-Induced Capillary Emptying. PNAS. http://www.
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