Experimental and theoretical study of capillary force in adhesive contact between microspheres

This thesis describes investigations into the capillary forces between microparticles (ca. 200-1000 µm in diameter). Its original contributions to knowledge are the validations and developments of the existing theories both experimentally and theoretically. A novel technique for force-displacement measuring, in combination with an image acquisition system, has been developed to directly measure the capillary forces between two microparticles. The instrument is also cable of recording the side-view profile of the contacting process optically, which is necessary to correctly identify the geometric shape of the capillary, the pole-to-pole alignment, as well as the deformation behavior of these particles. The experiments cover samples with different mechanical properties, i.e. soft hydrogel and rigid polymer particles. The theoretical studies undertaken include both analytical and numerical analyses, which are developed for interpreting the experimental data. Classical contact mechanics theory such as Johnson-Kendall-Roberts (JKR) theory is used for the capillary formed spontaneously between two soft sponge-like hydrogel microparticles, while Kelvin equation and Young-Laplace equation are applied for the capillary formed between two PMMA microparticles due to liquid condensation. In general, the experimental data and the theoretical predictions show good agreement in both soft and rigid microparticles. For soft microparticles, the current research findings show that the work of adhesion is independent of the separation speed and, by contrast, Young's modulus exhibits a linear increase with the separation speed. A viscoelastic model is used to further quantitatively characterize the deformation behavior of the soft hydrogel. It also demonstrates that the JKR theory reconciles with the generalized Hertz theory, which takes capillary force into account for soft microspheres in a relationship that the work of adhesion is equal to twice the surface tension of water. For rigid microparticles, both analytical and numerical models have been used to interpret the force-displacement curves measured during separation. The results demonstrate both the adhesion force and the capillary volume increase monotonically with the rise of the relative humidity. Also, subtle differences in the calculated capillary profile and adhesion forces between the analytical solution and the numerical simulation have been revealed. A surface roughness model is used to quantify the adhesion under the effect of relative humidity. These new findings are essential for developing techniques to quantitatively characterize the capillary force of colloids and granular materials and potentially to improve the material performance in their applications.