Liquid reinforcement of soft polymer matrix, liquid gallium inside PDMS and its optical image; the stress-strain behavior of PDMS/Ga shows high modulus compared to pristine PDMS or air-filled PDMS and finite element distribution of stresses around the Ga sphere inside the PDMS matrix.Scientists from Rice University in the US investigating the self-stiffening behavior of materials have designed liquid–solid silicone and gallium composites using liquid reinforcement based on nature – such as the intervertebral discs in human spines, which comprise a hard outer layer of cartilage with a softer interior, and the outer skin of many deep-sea fish, which have numerous small chambers filled with oil to help them withstand huge pressures.
The composite materials, in combining both flexibility and stiffness, could find applications in high-energy absorption materials and automobile shock absorbers, as the liquid should distribute the impact force evenly, and also in biomimetic structures such as artificial intervertebral discs.
The study, reported in Advanced Materials Interfaces [Owuor et al. Adv. Mater. Interf. (2017) DOI: 10.1002/admi.201700240], found that, unlike conventional composites that need a hard or stiff component to be added to a soft matrix to attain high stiffness, that a soft reinforcement with certain properties can achieve the same effect of higher modulus using natural materials designed to take advantage of this approach. They opted for Polydimethylsiloxane (PDMS) for the soft layer as it is inexpensive, inert and non-toxic. It also dries clear, making it easy to observe the liquid bubbles they wanted to encapsulate, for which they picked gallium as it is liquid at room temperature.
“we hope our work showing an improvement of mechanical behavior by addition of a soft reinforcement should spur other researchers[…] We believe there are numerous natural materials which may offer more insights in how to design even better composite based on this mechanism”Chandra Sekhar Tiwary
By curing the PDMS slowly, the team evolved a process to which they could add gallium droplets of different sizes, with some having one large inner chamber and others up to a dozen discrete droplets. Each sample was tested, with a dynamic mechanical analysis instrument measuring the deformation under load and measures such as stiffness, toughness and elasticity were taken under different conditions. The liquid reinforcement in natural materials has the characteristics of high viscous and bulk modulus, and are arranged in a hierarchical manner similar to the skin membrane of ocean fish.
As researcher Chandra Sekhar Tiwary told Materials Today, “we hope our work showing an improvement of mechanical behavior by addition of a soft reinforcement should spur other researchers[…] We believe there are numerous natural materials which may offer more insights in how to design even better composite based on this mechanism.” As well as new systems waiting to be discovered, they hope to design pure hierarchical systems based on the approach, and to look at other liquids with similar or even better properties than liquid metals as reinforcement.