Microcapsule mechanics : deformation, failure, and time-dependent mechanical behaviors and mechanisms

Abstract

Microcapsules containing functional liquid core materials are of great interest for the past several decades. Developed encapsulation techniques have shown enormous potential in designing novel intelligent materials used for food and cosmetic industries, self-healing and self-cleaning composites, coatings, and construction. The organic or inorganic shell can provide important protection for the liquid core material during manufacturing, processing, transportation, and real applications. While the introduction of different types of core-shell structured microcapsules as functional fillers can bring various promising functions to the matrix material, it tends to decrease the strength and robustness of the matrix since majority of artificial microcapsules are susceptible to deformation and failure. Moreover, the release of the encapsulated liquid under certain loading conditions in its service life is a significant issue worth considering. In this thesis, the intrinsic mechanical parameters of the solid shell are successfully extracted from single-microcapsule compression experiments based on related theories and numerical simulations. The deformation behaviors of a microcapsule under compression have been investigated and influencing factors such as particle size, material properties, thickness ratio and liquid core effect are reported to affect the structural response. Meanwhile, it is obvious that the strength of a microcapsule is vital for its processing and applications. Different failure modes and underlying mechanisms are revealed from compression experiments for three categories of fabricated microcapsules (PS, PUF, ABS-shelled). Shell material properties, strain rate, shell thickness ratio and shell microstructure are reported to greatly affect the failure behaviors of microcapsules. Good agreements are found between numerical simulations and experiments. Furthermore, microcapsules tend to suffer various types of loads including static and dynamic during processing and applications. Since polymers usually utilized as shell materials have a viscoelastic-plastic constitutive, time-dependent response of core-shell microcapsules is studied through stress relaxation test and low cyclic fatigue test on PS microcapsules. Reissner-Maxwell model is introduced to obtain the relaxation modulus. Damage accumulation induced stiffness degradation and hysteresis resulted by viscoelasticity until final stable state of a cyclically loaded microcapsule are analyzed. The present research has endeavored to systematically investigate the mechanical behaviors of core-shell microcapsules subjected to diversified external loads so as to have a more comprehensive understanding of microcapsule mechanics and better guide the design, fabrication, and applications of microcapsules.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jinglei YANG (Supervisor)