Liquid bridges for solid matter
The image that Erin Koos uses to explain the subject of her research is as fitting as it is simple: “Imagine a child building a sandcastle. If the child uses only dry sand, the result will always be a shapeless heap. However, as soon as a little water is added, all sorts of elaborate structures become possible.” Koos, who holds a PhD in mechanical engineering, has developed her own method of improving material properties. This method exploits the principle of capillary action, the same physical effect that holds the sand and water mixture together: water molecules penetrate between the grains of sand, forming liquid connections known as capillary bridges. Surface tension is created between the liquid and solid media, keeping the connection stable. “We transfer this concept to a scale of a few micrometres or nanometres. And when the effect is applied to suspensions, we obtain materials with very interesting properties,” explains Koos.
Suspensions are mixtures with a viscous to gel-like consistency, comprised of a liquid in which solid particles are evenly distributed, such as paint and molten chocolate. The method developed by Koos consists of adding a second, immiscible liquid to the mixture. This liquid forms bridges between the solid particles, resulting in a stable meshed structure. The advantages of this procedure over conventional suspensions go beyond stability, however. “The new method makes it possible to fine-tune the desired material properties in advance by combining different liquids,” says Koos. The method could be used to manufacture ultra-lightweight and resilient building materials, but also, for instance, filters made from glass which are both porous and robust.
Koos came across this potential application of capillary action by chance, shortly after leaving the California Institute of Technology in Pasadena, US, to join the Applied Mechanics working group at the Karlsruhe Institute of Technology (KIT) in 2009. “We were investigating the cause of some quality issues experienced by a manufacturer of PVC products, and discovered that the solid particles were adsorbing tiny amounts of water during processing due to humidity in the air.” It quickly became clear to Koos that the cause was capillary action. She recognised the potential implications of the process for materials research, and developed the concept of capillary suspensions. Since August 2013, her work has been funded by the European Research Council (ERC) with a Starting Grant for young researchers.
The 33-year-old’s arrival at the KIT was anything but a coincidence, however. Koos had set herself a clear objective: “I wanted to take the next step on the road to becoming a professor and conduct independent research with a group of my own. I also wanted to experience a different academic culture.” She found both things at the KIT: a research topic that interested her, and colleagues willing to support her in developing her career. “The KIT helped me find the right funding opportunity with the ERC grant.”
Her group, comprising two PhD students and a post-doc, is still coming together. She has already been able to buy a laser microscope that generates three-dimensional images. “We use it to examine the physical processes caused by capillary action on a scale of a few micrometres,” says Koos. The nature of the capillary bridges enables the researcher to determine the force exerted between the solid particles. “And if we can describe the microstructure, it helps us predict how the material will behave in larger amounts.”
By the end of the five-year funding period, Koos hopes to understand the underlying principles and effects of capillary action in suspensions. But that is not all: “It would be nice to develop some prototypes in collaboration with industry. Then I could say: my ideas contributed to the design of this product.”
Research field:Process Engineering, Material Science
Principal investigator:Dr Erin Koos
Host institution: Karlsruhe Institute of Technology
Funding period:1 Aug. 2013–31 July 2018