Materials scientists are looking to nature — at the discs in human spines and the skin of ocean-diving fish — for clues about how to design materials with both flexibility and stiffness. The solution to increasing stiffness (elastic modulus) of a soft, silicon-based polymer was to infuse it with pockets of liquid gallium. The solution contradicts the traditional belief that adding a harder substance increases modulus, and adding a softer one decreases modulus.
The discs between the vertebrae in human spines — that act like both shock absorbers and ligaments — are made of a tough outer layer of cartilage and a soft, jelly-like interior. The outer skin of deep-diving ocean fish and mammals contains myriad tiny oil-filled chambers — some no larger than a virus and others larger than entire cells — that allow the animals to withstand the intense pressures that exist thousands of feet below the ocean’s surface.
In choosing the basic materials to model these living systems, the researchers selected polydimethylsiloxane (PDMS) as the soft encapsulating layer because it is inexpensive, inert, nontoxic, and widely used in products such as caulk, sealants, cosmetics, and food additives. It also dries clear, which enabled the researchers to see the bubbles of liquid that were to be encapsulated. The liquid chosen was gallium, which, like mercury, is liquid at room temperature. Unlike mercury, it is non-toxic and relatively easy to work with.
Test samples about the diameter of a small coin and as much as ¼” thick were developed. By curing the PDMS slowly, a process was developed by which gallium droplets of various sizes could be added. Some samples contained one large inner chamber, and others contained up to a dozen discrete droplets.
Each sample was subjected to dozens of tests. A dynamic mechanical analysis instrument was used to measure how much the material deformed under load, and various measures like stiffness, toughness, and elasticity were measured under a variety of conditions. With a relatively small amount of cooling, gallium can be turned into a solid, which enabled the team to compare measurements taken when the gallium spheres were liquid with measures taken when the spheres were solid.
Finite element modeling and hydro-dynamic simulations were used to analyze how the materials behaved under mechanical stress. Based on this, the researchers determined that pockets of liquid gallium gave the composite higher energy absorption and dissipation characteristics than plain PDMS or PDMS with air-filled pockets.