Liquids are softer than solids, so incorporating droplets of a liquid into a solid will always make it weaker ... right? Actually, no. Scientists at Yale University have discovered that if the drops are just the right size, they can actually make the solid stiffer. Their findings could pave the way for composite materials that use liquids for added optical or electrical functionality, yet that don't compromise strength.

The secret lies in the liquid's surface tension, which remains the same regardless of the size of the droplet.

If too large of a droplet is used in the solid, its surface tension isn't sufficient to hold all that liquid in place when the solid is mechanically stressed. As a result, the drop deforms and the solid is weakened. If the droplet is small enough, however, the surface tension holds it intact. This not only keeps it from deforming, but also adds strength to the surrounding material.

Led by mechanical engineering and materials science Eric Dufresne, the Yale team experimented by embedding multiple micron-diameter droplets of ionic liquid (liquid salt) in a piece of silicone. When that material was subsequently stretched, it was found to be up to 30 percent stronger than pure silicone.

"It turns out that the importance of surface tension is inversely proportional to the size," said Dufresne. "So what’s just a negligible force for big things becomes a strong force for very small things — which in turn can strongly affect the material as a whole."

A paper on the research was published today in the journal Nature Physics.