  When a colloidal suspension is dried, capillary pressure may overwhelm
repulsive electrostatic forces, assembling aggregates that are out of thermal
equilibrium. This poorly understood process confers cohesive strength to many
geological and industrial materials. Here we observe evaporation-driven
aggregation of natural and synthesized particulates, probe their stability
under rewetting, and measure bonding strength using an atomic force microscope.
Cohesion arises at a common length scale (~ 5 um), where interparticle
attractive forces exceed particle weight. In polydisperse mixtures, smaller
particles condense within shrinking capillary bridges to build stabilizing
'solid bridges' among larger grains. This dynamic repeats across scales forming
remarkably strong, hierarchical clusters, whose cohesion derives from grain
size rather than mineralogy. Results may help to understand and control the
stability of natural soils and synthetic materials.

Soft Condensed Matter;Geophysics