Scientists recently found implanting softer subjects could stiffen solid composites.
There are many ways to stiffen composite materials; the most common way is to put stiffer subjects into the composites. However, scientists recently found that under a certain condition, implanting softer subjects could also stiffen the composites.
Dr. Eric Dufresne, a former Yale professor at mechanical engineering, currently serves as the Professor of Soft and Living Materials at ETHZ. His group designed experiments to increase elastic modulus of silicon gels by using ionic liquid droplets as inclusions [1]. They found the strength of the materials was increased by adding more liquid droplets. This did not always happen, since the materials became stiffer only when the drops were small enough, typically under 10 µm. To detect the strength, they stretched the composite materials and analyzed the deformation of droplets under the microscopy. To their surprise, the strain of drops depended on drops’ radius, which contradicted the classic theory that inclusions equally deform in a contentious matrix.
This might be new physics, and they developed a theory to explain this phenomenon [2]. When drops are small, the interfacial stress would not be negligible. Here, interfacial stress is the tendency of fluid surfaces to maintain a round shape and this stress increases as the size of drops decreases. When the loading stress is smaller than the interfacial stress, droplets prohibit the deformation and strengthen the composite materials. Considering interfacial stress and modulus of matrix, Dr. Dufresne’s group derived equations to describe the deformation of droplets and fill in the blank of the classic theory.
Actually, using soft subjects to stiffen the composites is not rare in the kitchens. Emulsifying air bubbles into egg white to make scream is an example, see the picture above. Interfacial stress of bubbles is too strong to resist deformation, so scream loses fluidity and stands like a solid. Currently, this discovery could be applied to study the mechanical properties of soft tissues and design new biomaterials, especially in soft connective tissues.
For more details, please refer these papers...
Ref.
[1] Wettlaufer, J. S. et al. Stiffening solids with liquid inclusions. Nature Physics 11, 672 (2015).