Static and Dynamic Behaviour of Inter-granular Liquid Bridges: Hysteresis of Contact Angle and Capillary Forces.

Abstract

The wet granular material, a mixture of solid grains, liquid and air, has been studied in many industrial applications. Understanding the mechanical properties of wet granular materials is extremely important in geotechnical engineering. The cohesion in these materials arising from a liquid bridge between a pair of grains is produced by the capillary force and the viscous force. Thus, investigating the behaviour of liquid bridges between grains helps people obtain a better understanding of the factors influencing the cohesion, including the geometry and topological distribution of liquid bridges, the size of grains, or the liquid flow rate. This thesis focuses on the static and dynamic behaviour of liquid bridges connecting two solid substrates. The aims of our study are to (1) explore the quasi-static and dynamic force arising from liquid bridges when two substrates move at various speeds and (2) established an analytical model to describe and predict the behaviour of liquid bridge when it moves at different speeds. Our work can be divided into three parts: parallel plates experiments, analytical modelling and extended experiments. The working content and the main results are presented in the below. 1. A series of parallel plates experiments are conducted to systematically investigate the capillary force and the contact angle hysteresis of a liquid bridge which forms between two moveable parallel plates under the quasi-static and dynamic conditions. The experimental results show that the contact angle hysteresis can be divided into pinning and slipping stages in both of the extension and compression processes. With the increase of plate moving speeds, it is observed that in the slipping stage, the receding angle becomes smaller and the advancing angle becomes larger. The capillary force hysteresis also changes correspondently. The liquid bridge hysteresis can be used to predict the cohesion behaviour of wet granular material during the wetting and drying processes. Moreover, in some geotechnical events such as the seepage in soil or the shear movement of the grains, the rate effect on the contact angle and the force between grains can provide a potential direction to investigate the failure criterion of soils and their dynamic behaviour. 2. In the quasi-static regime, an analytical model has been first established based on the geometrical shape of liquid bridges. According to the pinning and slipping stages of liquid bridge hysteresis, the proposed model describes the geometrical characteristics of a liquid bridge being extended and compressed under the quasi-static condition. The models have been compared to the experimental results and the analytical model can particularly predict the quasi-static contact angle and contact radius hysteresis. In the dynamic regime, an empirical rate-dependent contact angle fitting function has been established to capture the change of dynamic contact angle with respect to the loading rate. Furthermore, the dynamic analytical model has been established to predict the variation of viscous force and dynamic contact angle hysteresis with the increase of plate moving speeds. The model predictions are in a good agreement with the experiments. 3. Moreover, to consider the effects of surface curvature and roughness, we extended the experiments forming liquid bridges between various substrates: (1) two plates with different surface roughness; (2) two spheres; (3) a sphere and a plate. The influence of surface properties and curvatures on the behaviour of liquid bridges has been discussed and possible future research directions are provided. As the cohesion between gains is influence by many factors such as the volume, size, contact angle of a liquid bridge, as well as the flow speeds and the grain movement, this study provides a good start to investigate the capillary force and the contact angle hysteresis under the quasi-static and dynamic conditions. In this work, the experimental observation of liquid bridge hysteresis is presented and the analytical models are established to predict the force and contact angle hysteresis at a given moving speed. This study demonstrates potential to use this microscopic information towards the macro scale properties of wet granular materials, such as the soil-water retention hysteresis under different wetting and drainage rates.