Fast-moving cornstarch acts like a liquid before hardening

A new study reveals how cornstarch behaves like a liquid before solidifying, with implications for 3D printing and soft robotics.

If you mix cornstarch and water, you will have a substance that defies common sense. If you punch it with your first, it will feel like you hit a solid wall. But if you move your finger through it slowly, the substance will flow like honey. And scientists have long struggled to understand how this transition happens.

A new study in Physical Review Letters has figured it out using high-speed photography and direct force measurements. The authors, from the University of Minnesota, reported that at an extreme speed, the cornstarch mixture behaves like a liquid first before becoming tougher.

For these studies, scientists have usually watched how a drop spreads when it falls on a surface using high-speed cameras. For the new study, the researchers built a specialised rig. They dropped mixtures of cornstarch and water with concentrations ranging from 30% to 43%, on a sensitive force sensor at speeds of up to 7.2 m/s, and recorded it with high-speed cameras.

Thus they could ‘see’ and ‘feel’ the impact simultaneously.

The team thus identified three ways in which the drops responded to the surface. The low-concentration mixture behaved like water: the impact force peaked early and then faded as the liquid spread. At a higher concentration and low impact speed, however, the fluid jammed” almost instantly, behaving like a solid sphere, exerting a symmetric force as it struck the surface and abruptly stopped spreading.

Then the team made its surprising discovery. At high concentrations and high impact speed, the mixture behaved like a re-entrant liquid. The drop acted like a liquid for the first fraction of a millisecond, then quickly transitioned to becoming a jammed solid.