Zhe Zhang

In this article, we investigate the nanoscale soil-water retention mechanism of unsaturated clay through molecular dynamics and machine learning. Pyrophyllite was chosen due to its stable structure and as the precursor of other 2:1 clay minerals. A series of molecular dynamics simulations of clay at low degrees of saturation were conducted. Soil water was represented by a point cloud through the center-of-mass method. Water-air interface area was measured numerically by the alpha-shape method. The soil-water retention mechanism at the nanoscale was analyzed by distinguishing adsorptive pressure and capillary pressure at different mass water contents and considering the apparent capillary interface area (i.e., water-air interface area per unit water volume). The water number density profile was used to quantify the adsorption effect. A neural-network based machine learning technique was utilized to construct function relationships among matric suction, the mass water content, and the apparent water-air interface area. Our numerical results have demonstrated from a nanoscale perspective that the adsorption effect is dominated by the van der Waals force and hydroxyl hydration between the clay surface and water. As the mass water content increases, the adsorption pressure decreases, and capillarity plays a prominent role in the soil-water retention mechanism at the nanoscale.

The strength origin of C-S-H matrix is a key question in cement and concrete research, which remains largely unclear despite a century of study. Here we present a novel approach to endow strength integrity to C-S-H powders via mechanical compaction at room temperature (cold-fusion). The unprecedented 2000 MPa compaction load results in a bulk C-S-H with tensile strength (~24 MPa) one order of magnitude higher than C-S-H matrix from hydration. This practice enables observations on how the Ca/Si ratio (C/S) and initial moisture conditions affect their overall mechanical properties (hardness and strength), which are correlated with the micro-structures studied by 1 H NMR and electronic microscope. We conclude that the bond between closely-contacted C-S-H particle, rather than the total porosity, is key to the bulk strength. The bond is clearly weakened when excessive water exists between C-S-H particles. This work provides novel insight to the strength origin of C-S-H.

A strain gradient induced by mechanical bending on a SrTiO₃ substrate is demonstrated, and has a pronounced influence on the carrier density and mobility of the interfacial 2D electron gas at the LaAlO₃/SrTiO₃ heterointerface. Tensile and compressive strain gradients represent two states of upward and downward bending. Under the tensile strain gradient, the carrier density decreases and the mobility has about 200% increase. Conversely, under the compressive strain gradient, the mobility decreases and the carrier density increases by up to 107%. These results demonstrate a range of opportunities to modulate the carrier density and mobility at oxide heterointerface and open up a promising way for further research on application of oxide devices.