Department of Earth & Environmental Science, University of Pennsylvania, Philadelphia, PA 19104.
Department of Earth, Atmospheric, & Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.
Proc Natl Acad Sci U S A. 2020 Feb 18;117(7):3375-3381. doi: 10.1073/pnas.1913855117. Epub 2020 Feb 4.
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 μm), 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. These results may help toward understanding the strength and erodibility of natural soils, and other polydisperse particulates that experience transient hydrodynamic forces.
当胶体悬浮液干燥时，毛细压力可能会超过排斥静电的力，从而组装出处于热平衡之外的聚集体。这个尚未被充分理解的过程为许多地质和工业材料赋予了内聚强度。在这里，我们观察了天然和合成颗粒的蒸发驱动聚集，探测了它们在重新润湿下的稳定性，并使用原子力显微镜测量了结合强度。在共同的长度尺度（约 5 μm）上出现内聚，此时颗粒间的吸引力超过颗粒的重量。在多分散混合物中，较小的颗粒在收缩的毛细桥内凝聚，在较大颗粒之间形成稳定的“固体桥”。这种动态在各个尺度上重复，形成了非常坚固的、分层的簇，其内聚强度源于颗粒大小而不是矿物学。这些结果可能有助于理解天然土壤以及其他经历瞬态水动力的多分散颗粒的强度和可侵蚀性。
