Boleslaw Mielniczuk

Micro-scale study of rupture in desiccating granular media

Capillary bridges between two, three, and multiple fixed glass spheres are examined experimentally during their natural evaporation. The key variables of the process of evolution are measured using a calibrated balance recording and digital image processing with still and high-speed cameras. The calculations of Laplace pressure, as well as suction and surface tension resultant components of the interparticle force are made for two-grain systems. Evolution, properties and failure of evaporating liquid bridge are controlled and induced by decreasing liquid volume. For the two grain configuration, tests show a gradual decrease of suction down to zero and into a positive pressure range before a two step failure occurs, including a formation of a water wire according to a Rayleigh instability pattern followed by a simultaneous rupture at two points of the lowest (negative) total (Gauss) curvature of the bridge surface. For more complex systems, a thin-film pinching instability is shown to result from two-dimensional cavitation of water, leading to a re- configuration of the water body into separate bridges between individual pairs of grains, which then rupture as described above. Water body instability generated dynamic penetration of air may also provide an imperfection for the granular system potentially leading to cracking.

A three-scale cracking criterion for drying soils

Cracking is a most unwanted development in soil structures undergoing periodic drying and wetting. Desiccation cracks arise in an apparent absence of external forces. Hence, either an internal, self-equilibrated stress pattern resulting from kinematic incompatibilities, or a stress resulting from reaction forces at the constraints appear as a cracking cause, when reaching tensile strength. At a meso-scale, tubular drying pores are considered in the vicinity of a random imperfection, inducing a stress concentration in the presence of significant pore suction. This approach allows one to use the effective stress analysis, which otherwise, away from the stress concentration, usually yields compressive effective stress and hence a physically incompatible criterion for a tensile crack. Recent experiments on idealized configurations of clusters of grains provide geometrical data suggesting that an imperfection as a result of air entry deep into the granular medium penetrates over 4 to 8 internal radii of a typical pore could yield a tensile effective stress sufficient for crack propagation.

Rupture of an evaporating liquid bridge between two grains

The study examines rupture of evaporating liquid bridges between two glass spheres. Evolution of the bridge profile has been recorded with the use of high-speed camera. Geometrical characteristics of the bridge were then used to calculate evolution of the variables during the process: Laplace pressure, capillary force, and surface tension force. For the purpose of reference, the bridge evolution is followed also during kinematic extension. During both processes the diameter of the neck decreases, with an acceleration of about 1–2 ms before the rupture. Two distinct rupture modes are observed, depending on the bridge aspect ratio. After the rupture, the mass of liquid splits, forming two separate oscillating drops attached to the spheres, and a suspended satellite droplet. Just before the rupture, an increasing repulsive Laplace pressure, and decreasing negative surface tension force develop. Capillary force follows the trend of the surface tension force, with an accelerating decline. Duration of the whole process and liquid mass stabilization is from 10 to 60 ms.

Modeling Physico-Chemical Degradation of Mechanical Properties to Assess Resilience of Geomaterials

It is widely accepted that critical properties of geo-materials that play a key role in failure of earth-structures undergo often a substantial evolution induced by non-mechanical processes and variables. That includes: hydro-thermal fracture, thermal collapse, chemical mass removal or accretion (dissolution or precipitation), chemical shrinkage/swelling, drying shrinkage, capillary force evolution during pore water phase change. The properties affected are: strength in all its manifestation, compressibility, permeability, thermal conductivity, to mention just a few. The physical processes involved are either natural or engineered. Their phenomenology is per se a conundrum, as often they constitute a series of parallel or sequential processes. A review of several phenomena leading to geomaterial degradation, and methodology is presented to deal with multi-physical couplings in constitutive modeling. In plasticity, the central constitutive function is a hardening rule. Also in this case, phenomenological observations indicate a chemo-mechanical, two-way coupling. Other degradation phenomena discussed include drying—cracking, and or the role of suction induced hardening in unsaturated materials.

Micro-scale Testing of Capillary Bridge Evolution due to Evaporation

Capillary bridge evolution between two fixed glass spheres during its natural convective evaporation is examined experimentally. For comparison extension tests were also carried out. The calibrated balance recording and digital image processing allow monitoring of a number of key variables of the process: the resultant capillary force, the water mass loss, radii of the bridge curvature. On that basis evaporating surface area, suction and surface tension force, interparticle force, axial stress vs (relative) volumetric mass loss are calculated. Testing shows a gradual decrease of suction within bridges down to zero and into a positive pressure range before a two step failure including a formation of a water thread according to a traditional Rayleigh instability pattern followed by a simultaneous rupture at two points of the lowest (negative) total (Gauss) curvatures of the bridge surface.

Multi-Scale Approach to Cracking Criteria for Drying Silty Soils

Cracking is a most unwanted development in soil structures undergoing periodic drying. Desiccation cracking arises in the apparent absence of external forces. Hence, either an internal, self equilibrated stress pattern resulting from kinematic incompatibilities, or stress resulting from reaction forces at the constraints should be contemplated to arrive at cracking criteria. Three circumstances are considered for drying cracks: drying shrinkage, kinematic constraints impeding the shrinkage inducing reaction forces, and consequent tensile effective stress reaching tensile strength. An earlier tubular micro-scale model of porous drying medium is considered with constrained at restrictive inter - pore solid contacts. At the meso-scale tubular drying pores are considered in the vicinity of an imperfection, inducing a stress concentration near its tip, in the presence of significant pore suction. This approach allows one to use the effective stress analysis, which otherwise, away from the stress concentration usually yields compressive effective stress and hence a physically incompatible criterion. Recent experimental results from an idealized configuration of a cluster of grains provide geometrical data suggesting that an imperfection as a result of air entry deep into the granular medium penetrates over 8 internal radii of the typical pore

Multi-scale Approach for Modeling Desiccation Shrinkage in Granular Soils

This paper aims to identify and quantitatively evaluate various critical mechanisms associated with the processes of desiccation shrinkage and cracking in drying silty soils. A previously developed 1D bundle-of-tubes model is refined to simulate the various stages of drying shrinkage in 2D, using the actual pore size distribution based on Mercury Intrusion Porosimetry (MIP) data. Experimental evidence at a meso-scale has shown that the air entry phenomenon may occur in two possible scenarios: air incursion at the external surface and formation of vapor nucleus in the interior. Further transition of solid-water structural configuration into funicular and pendular states from initially capillary state is simulated.