A Colloidal Gel with a Hair Trigger
The vast tidal flats of Mont-Saint-Michel Bay, spanning some 250 square kilometers, are a natural laboratory for the physics of non-Newtonian fluids. The phenomenon known as quicksand here is not a bottomless pit but a complex physical process called liquefaction. The bay experiences some of the highest tides in continental Europe, with a vertical range that can reach up to 15 meters during spring tides. This immense and rapid movement of water is the engine that primes the sand for its sudden transformation from a solid to a liquid state.
The sediment in the bay is a specific mixture of sand, silt, and clay, saturated with salt water from the English Channel. Research, by physicist Daniel Bonn, has shown that the clay and salt are ingredients. The clay particles form a fragile matrix that holds the sand grains and water together in a semi-solid state known as a colloidal gel. When an external pressure is applied—like a person's footstep—this delicate structure collapses. The sudden stress increases the water pressure between the sand grains, overcoming the friction that holds them together. The mixture's viscosity drops, and it behaves like a liquid, causing the object or person to sink. Bonn's research calculated that the force required to pull a foot out of this liquefied sand can be equivalent to that needed to lift a small car.