Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 117, Issue 7, Pages 3375-3381Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1913855117
Keywords
aggregate stability; evaporation; solid bridges; cohesion
Categories
Funding
- Singh Center for Nanotechnologies at the University of Pennsylvania
- US National Institute of Environmental Health Sciences [P42ES02372]
- US Army Research Office [569074]
- Materials Research Science and Engineering Center [DMR-1720530]
- Swiss National Cooperative
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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 (similar to 5 mu m), where interparticle attractive forces exceed particle weight. In poly-disperse 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. These results may help toward understanding the strength and erodibility of natural soils, and other polydisperse particulates that experience transient hydrodynamic forces.
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