"Bullet-proof custard": Fluids that stop flowing when you push them too
hard. Professor Michael Cates, University of Cambridge

When small, hard particles are suspended in a fluid they make it more
resistant to flow: the viscosity increases with the particle
concentration. Ultimately the fluid stops flowing altogether and becomes
a jammed solid. Especially intriguing are suspensions that flow smoothly
if you push gently, but suddenly stop flowing if you push too hard. This
behavior can be observed by mixing corn-starch powder (custard powder)
with just enough water, so that the material flows smoothly when stirred
gently with a spoon... but then stir it hard, and it refuses to flow at
all! Such behaviour can cause catastrophic failure of industrial pumping
equipment, and is exploited in some designs of bullet-proof vest. For
many years the physics behind this type of response was unclear, but in
the past decade we have established a new and simple explanation in
terms of what happens at the contacts between particles as the force
between them is increased. I will explain this mechanism with some
equations, lots of diagrams, and (if things go to plan) a few
experiments as well.