Two mica plates, smooth as mirrors, slowly approach each other in a calcium solution. At a distance of fifty nanometers, an invisible barrier suddenly appears between them: as if someone had stretched a web of molecules between the walls. This web prevents the plates from touching, even though by all calculations they should glide freely toward each other.

Chemists in Oxford measured this repulsive force using a surface force balance, an instrument that senses pressure with precision down to a tenth of a nanometer. In a calcium chloride solution at a concentration of 0.28 mol per liter, repulsion appeared at gaps from ten to fifty nanometers. Classical DLVO theory, which describes the interaction of charged surfaces in a liquid, is powerless here: it predicts only weak electrostatic repulsion that should have long since vanished at such distances.

To understand the nature of this viscosity, the scientists compared solutions with different ions. In a magnesium chloride solution, nothing of the sort was observed. But when potassium chloride was added to the calcium solution, the repulsion disappeared, as if the web had been washed away. Potassium ions displaced calcium from the mica surface and destroyed the network, which apparently consisted of hydrated calcium ions linked to carbonate ions, always present in water because of dissolved carbon dioxide.

A model in which viscosity decays exponentially with distance from the surface fit the data perfectly. The excess viscosity was 7.7 ± 2.7 millipascal-seconds, only seven times that of pure water, but that was enough to increase the repulsive force by tens of times. The characteristic decay length was 34.3 ± 8.7 nanometers. And at gaps smaller than ten nanometers, the force became elastic: the network stopped flowing and began to spring back, like a thin gel layer about five nanometers thick on each surface.

Until now, it was unclear whether such clusters could exist in an unsaturated solution, where the salt has not yet reached its solubility limit. It turns out they can, provided a mineral surface is nearby. The surface stabilizes the network, preventing it from falling apart, and thereby prepares the ground for a future crystal. This explains why calcium carbonate crystallization so often begins on vessel walls, in bone pores, or on mollusk shells, even when the solution itself is far from saturation.

Understanding how surfaces stabilize ion networks could help control calcium carbonate crystallization in carbon capture technologies, where this mineral deposits on reactor walls. It could also aid in creating biomimetic materials that mimic natural composites like nacre. The invisible web at the mineral wall turned out to be not just a curiosity but a key link in the chain from solution to crystal.