arXiv subjects

Tingtao Zhou

Publications and source records attributed to Tingtao Zhou.

23 records · Page 2Linked to original sources

arXiv2019

Multiscale Poromechanics of Wet Cement Paste

Capillary effects such as imbibition-drying cycles impact the mechanics of granular systems over time. A multiscale poromechanics framework was applied to cement paste, that is the most common building material, experiencing broad humidity variations over the lifetime of infrastructure. First, the liquid density distribution at intermediate to high relative humidities is obtained using a lattice gas density functional method together with a realistic nano-granular model of cement hydrates. The calculated adsorption/desorption isotherms and pore size distributions are discussed and compare well to nitrogen and water experiments. The standard method for pore size distribution determination from desorption data is evaluated. Then, the integration of the Korteweg liquid stress field around each cement hydrate particle provided the capillary forces at the nanoscale. The cement mesoscale structure was relaxed under the action of the capillary forces. Local irreversible deformations of the cement nano-grains assembly were identified due to liquid-solid interactions. The spatial correlations of the nonaffine displacements extend to a few tens of nm. Finally, the Love-Weber method provided the homogenized liquid stress at the micronscale. The homogenization length coincided with the spatial correlation length nonaffine displacements. Our results on the solid response to capillary stress field suggest that the micronscale texture is not affected by mild drying, while local irreversible deformations still occur. These results pave the way towards understanding capillary phenomena induced stresses in heterogeneous porous media ranging from construction materials, hydrogels to living systems.

cond-mat.soft↗

arXiv2018

Capillary stress and structural relaxation in moist granular materials

We propose a theoretical framework to calculate capillary stresses in complex mesoporous materials, such as moist sand, nanoporous hydrates, and drying colloidal films. Molecular simulations are mapped onto a phase-field model of the liquid-vapor mixture, whose inhomogeneous stress tensor is integrated over Voronoi polyhedra in order to calculate equal and opposite forces between each pair of neighboring grains. The method is illustrated by simulations of moisture-induced forces in small clusters and random packings of spherical grains using lattice-gas Density Functional Theory. For a nano-granular model of cement hydrates, this approach reproduces the hysteretic water sorption/desorption isotherms and predicts drying shrinkage strain isotherm in good agreement with experiments. We show that capillary stress is an effective mechanism for internal stress relaxation in colloidal random packings, which contributes to the extraordinary durability of cement paste.

cond-mat.soft↗

arXiv2018

Inferring Pore Connectivity from Sorption Hysteresis in Multiscale Porous Media

Vapor adsorption experiments are widely used to assess pore size distributions, but the large hysteresis sometimes observed between sorption and desorption isotherms remains difficult to interpret. Such hysteresis is influenced pore network connectivity, which has previously been modeled by percolation on infinite lattices. Our hypothesis is that percolation occurs instead through finite networks of micropores connecting accessible macropores, always exposed to the outside environment. We derive a general formula for sorption/desorption isotherms that introduces a simple measure of hierarchical pore connectivity -- the fraction $f$ of always exposed pores. The model thus accounts for "small world" connections in finite-size percolation, while also incorporating other hysteresis mechanisms, in single-pore filling, liquid insertion into the solid matrix, and cavitation. Our formula is able to fit and interpret both primary and scanning sorption/desorption isotherms for a variety of adsorbates (noble gases, water, and organics) and porous materials (cement pastes, dental enamels, porous glasses, carbon black and nanotubes), including cases with broad pore-size distributions and large hysteresis. It allows quantification of the prevalence of percolating macropores in the material, even though these pores are never filled during the sorption experiments. A distinct bump in sorption isotherms is explained as a lowering of the barrier to nucleation of the vapor phase with a universal temperature scaling.

cond-mat.mtrl-sci↗

arXiv2015

On the IMF in a Triggered Star Formation Context

The origin of the stellar initial mass function (IMF) is a fundamental issue in the theory of star formation. It is generally fit with a composite power law. Some clues on the progenitors can be found in dense starless cores that have a core mass function (CMF) with a similar shape. In the low-mass end, these mass functions increase with mass, albeit the sample may be somewhat incomplete; in the high-mass end, the mass functions decrease with mass. There is an offset in the turn-over mass between the two mass distributions. The stellar mass for the IMF peak is lower than the corresponding core mass for the CMF peak in the Pipe Nebula by about a factor of three. Smaller offsets are found between the IMF and the CMFs in other nebulae. We suggest that the offset is likely induced during a starburst episode of global star formation which is triggered by the formation of a few O/B stars in the multi-phase media, which naturally emerged through the onset of thermal instability in the cloud-core formation process. We consider the scenario that the ignition of a few massive stars photoionizes the warm medium between the cores, increases the external pressure, reduces their Bonnor?Ebert mass, and triggers the collapse of some previously stable cores. We quantitatively reproduce the IMF in the low-mass end with the assumption of additional rotational fragmentation.

astro-ph.GA↗

arXiv2013

On the Coagulation and Size Distribution of Pressure Confined Cores

Observations of the Pipe Nebula have led to the discovery of dense starless cores. The mass of most cores is too small for their self gravity to hold them together. Instead, they are thought to be pressure confined. The observed dense cores' mass function (CMF) matches well with the initial mass function (IMF) of stars in young clusters. Similar CMF's are observed in other star forming regions such as the Aquila Nebula, albeit with some dispersion. The shape of these CMF provides important clues to the competing physical processes which lead to star formation and its feedback on the interstellar media. In this paper, we investigate the dynamical origin of the the mass function of starless cores which are confined by a warm, less dense medium. We consider the coagulation between the cold cores and their ablation due to Kelvin-Helmholtz instability induced by their relative motion through the warm medium. We are able to reproduce the observed CMF among the starless cores in the Pipe nebula. Our results indicate that in environment similar to the Pipe nebula: 1) before the onset of their gravitational collapse, the mass distribution of the progenitor cores is similar to that of the young stars, 2) the observed CMF is a robust consequence of dynamical equilibrium between the coagulation and ablation of cores, and 3) a break in the slope of the CMF is due to the enhancement of collisional cross section and suppression of ablation for cores with masses larger than the cores' Bonnor-Ebert mass.

astro-ph.GA↗