Formation of erosion-resistant aggregates by evaporation-driven solid bridges

Fluid-driven colloidal aggregates form the building blocks of many natural and industrial particulate systems from soils to soil-hosted microbiomes and contaminants. These systems are composed of particles of various size, shape and surface charge, and are subject to intermittent cycles of wetting and drying. When suspended particles are dried, transient hydrodynamic forces may overcome interparticle repulsion, forming aggregates that are out of thermal equilibrium. In the absence of internal cohesive forces, however, the fluid-driven assembly should be unstable when subject to rewetting or fluid shear. The particle-scale mechanisms and hydromechanical stability of aggregates under these conditions are poorly understood, and cannot be predicted from interfacial electrostatic forces. Here we present results from unprecedented experiments to probe the particle-scale formation and hydromechanical stability of evaporation driven aggregates. This includes (i) a microfluidic channel to visualize the formation and to determine the fluid erosion threshold of aggregate deposits; and (ii) novel pull-off experiments with an Atomic Force Microscope to directly measure the strength of particle-particle bonds. Aggregates are created from a wide range of synthetized and natural particles: polydisperse silica microspheres to serve as an idealized model system, and naturally occurring particles including various clay minerals, silt, and elongated chrysotile particles. Despite a significant variation of surface charge, shape and material properties, we find that particle size principally controls the cohesive strength of aggregates assembled by wetting and drying. In polydisperse mixtures, smaller particles condense within shrinking capillary bridges to form stabilizing 'solid bridges' among larger grains, giving rise to an effective cohesion. This process repeats across scales to form remarkably strong, hierarchical clusters. Results provide a new physical basis, and a new set of tools, for understanding the rewetting behavior of soil aggregates. More broadly, our results open the path to studying the effect of particle size polydispersity and transient hydrodynamic forcing on the multiscale hydromechanical behavior of particulate assemblies ranging from industrial to complex living systems.