Problems Of Adhesive Contact At Bio-and Nano-interfaces

Adhesive contact mechanics is an important branch of solid mechanics,which plays a significant role in understanding numerous problems from engineering machines,wheel-rail contact,climbing of animals and plants,functional coatings,etc.With the rapid development of science and technology,it has been noticed that adhesive contact becomes more important in mesoscopic and microscopic scales where surface-related phenomena play a dominant role over body forces,such as the scenarios in cell adhesion and drug delivery systems.In this dissertation,we focus on two reprentative problems of adhesive contact in bio-and nano-systems:nanoparticles adhering to compliant substrates and biointerfaces mediated by specific molecular bonds.The first chapter briefly reviews the classical models in describing particle-substrate adhesion,and introduces the research background of nanoparticle adhesion as well as bond-mediated interfaces.The Johnson-Kendall-Roberts(JKR)theory in adhesive contact mechanics has been successfully applied in the fields of gecko adhesion,colloid assembly and cell adhesion.In the second chapter,we consider nanoparticle-surface interactions as those in drug delivery and cellular uptake systems.A consequence of particle size reduction to nanometer scale is that the energy scale confining the state of system equilibrium becomes comparable to the unit of thermal energy,leading to statistical particle detachment even below the critical pull-off force.We describe the process by Kramers' theory as a thermally activated escape from an energy well and develop a Smoluchowski partial differential equation that governs the spatial-temporal evolution of the adhesion state in probabilistic terms.The results show that the forced or spontaneous separation of nanometer-sized particles from compliant substrates occurs diffusively and statistically rather than ballistically and deterministically as assumed in existing models.The adhesive state of multiple nanoparticles on a compliant substrate exhibits strong spatiotemporal coupling,as demonstrated by a two-nanoparticle system.Nanoparticle pairs on a compliant substrate show a form of communication through the elastic interaction.The adhesive state of one nanoparticle can be effectively influenced by the behavior of neighboring nanoparticle through the overlapping fields generated by individual adhesion sites.Moreover,the spatiotemporal coupling between nanoparticle pairs is more pronounced as the substrate stiffness is reduced.This principle on statistical and mutual interaction of nanoparticle pairs is important in understanding the targeting and docking processes of nano-sized particles on compliant substrates,and provides a promising strategy to control the detachment of a target nanoparticle by adding and manipulating another nanoparticle nearby.In the third chapter,we investigate the cohesive response of biointerfaces mediated by noncovalent receptor-ligand bonding under monotonic,cyclic or other types of loading.By examining the spatiotemporal evolution of the state probability distribution that describes the collective association and dissociation kinetics of interfacial bonds,we show that such interfaces resist the imposed surface separation in a strongly rate-dependent manner.Remarkable hysteresis is exhibited when the interfaces are exposed to single stretching and relaxation cycles at high loading rates,and this hysteretic response shifts in consecutive multiple cycles.There generally exists an optimal ramping velocity that gives rise to the maximum energy dissipation at the interfaces.These results should be useful in understanding the cell-matrix adhesion and de-adhesion phenomena under dynamic and repetitive forces,as well as the adhesion-mediated cellular behaviors such as migration and reorientation.A statistical model is proposed in the fourth chapter to describe the peeling of an elastic strip in adhesion with a flat substrate via an array of non-covalent molecular bonds.Under an imposed tensile peeling force,the interfacial bonds undergo diffusion-type transition in their bonding state,a process governed by a set of probabilistic equations coupled to the stretching,bending and shearing of the elastic strip.Because of the low characteristic energy scale associated with molecular bonding,thermal excitations are found to play an important role in assisting the escape of individual molecular bonds from their bonding energy well,leading to propagation of the peeling front well below the threshold peel-off force predicted by the classical theories.Our study establishes a link between the deformation of the strip and the spatiotemporal evolution of interfacial bonds,and delineates how factors like the peeling force,bending rigidity of the strip and binding energy of bonds influence the resultant peeling velocity and dimensions of the process zone.In terms of the apparent adhesion strength and dissipated energy,the bond-mediated interface is found to resist peeling in a strongly rate-dependent manner.In the fifth chapter,we study the mechanical response of composite interfaces consisting of integrin-based catch bonds and folded fibronectin domains.The kinetic dissociation/rebinding of catch bonds and unfolding/refolding of fibronectins simultaneously occur at the interfaces,which are coupled to the elastic deformation of surrounding media.We adopt Monte Carlo procedure to simulate this mechano-kinetic coupling process,with an emphasis on the strong rate dependence that is rooted in the competition among multiple stochastic and kinetic processes.We discuss the size effect,shape effect as well as stiffness effect of the integrin-fibronetin interfaces,with the results showing that the stress concentration and crack-like failure at the interfaces can be modulated by domain unfolding.In the following chapter,we theoretically model the combined dry and wet adhesion between a rigid sphere and an elastic substrate,where the dry contact area is surrounded by a liquid meniscus.The influence of the liquid on the interfacial adhesion is twofold:inducing the Laplace pressure around the dry contact area and altering the adhesion energy between solid surfaces.The behavior of such combined dry and wet adhesion shows a smooth transition between the JKR and DMT models for hydrophilic solids,governed by the prescribed liquid volume or environmental humidity.The JKR-DMT transition vanishes when the solids become hydrophobic.An inverse scaling law of adhesive strength indicates that size reduction helps to enhance the adhesive strength until a theoretical limit is reached.This study also demonstrates the jumping-on and jumping-off hysteresis between the combined dry-wet adhesion and pure liquid bridge in a complete separation and approach cycle.