Tough and tunable adhesion of hydrogels: experiments and models

As polymer networks infiltrated with water, hydro-gels are major constituents of animal and plant bodies and have diverse engineering applications. While natural hydro-gels can robustly adhere to other biological materials, such as bonding of tendons and cartilage on bones and adhe-sive plaques of mussels, it is challenging to achieve such tough adhesions between synthetic hydrogels and engineer-ing materials. Recent experiments show that chemically anchoring long-chain polymer networks of tough synthetic hydrogels on solid surfaces create adhesions tougher than their natural counterparts, but the underlying mechanism has not been well understood. It is also challenging to tune sys-tematically the adhesion of hydrogels on solids. Here, we provide a quantitative understanding of the mechanism for tough adhesions of hydrogels on solid materials via a com-bination of experiments, theory, and numerical simulations. Using a coupled cohesive-zone and Mullins-effect model val-idated by experiments, we reveal the interplays of intrinsic work of adhesion, interfacial strength, and energy dissipation in bulk hydrogels in order to achieve tough adhesions. We fur-ther show that hydrogel adhesion can be systematically tuned by tailoring the hydrogel geometry and silanization time of solid substrates, corresponding to the control of energy dis-sipation zone and intrinsic work of adhesion, respectively. The current work further provides a theoretical foundation for rational design of future biocompatible and underwater adhesives.     

Mechanics of soft active materials with phase evolution

This paper studies the mechanics of soft active materials where the actuation is generated due to the formation of phases that are stress-free at the moment of their creation and therefore experience no deformation in the associated configuration. Phase formation is a continuous time-dependent process, which results in individual phases forming at different times and in different configurations of the material body, and thus it is coupled with mechanical deformation. Subsequent deformation of the material body results in individual phases experiencing different states of deformation and the overall material response results from the combined responses of the individual phases weighted by their respective volume fractions. Therefore, a great challenge in modeling the mechanics of soft active materials with evolving phases is to track the deformation and evolution of individual phases formed at different times and in different configurations. In this paper, a generalized one-dimensional model framework is presented to address this challenge. However, this model proves to be computationally inefficient. In response, an effective phase model is developed that tracks the combined deformation histories of new phases through a single, effective deformation. Both the general and effective phase models are evaluated with two fundamentally distinct phase evolution rules for three common mechanical problems: extension, stress relaxation, and creep. The first evolution rule represents a discrete transition from one phase to another while the second rule corresponds to a general transition from several phases into one phase. The effective phase model demonstrates excellent agreement with the generalized theory for all three mechanical problems considered under both types of evolution rules.     

Relationship between paddy soil adhesion to steel and to rubber

Forty-three pairs of samples concerning adhesion of paddy soil to steel and to rubber are used to identify the relationship between these two groups of variables. Results show that there is no significant difference between them, implying that they are roughly equal to each other in engineering practice.     

Effects of large deformation and material nonlinearity on spherical indentation of hyperelastic soft materials

Up to now, the indentation of hyperelastic soft materials has not been completely understood. In this paper, the spherical indentation on hyperelastic soft solids was systematically investigated through theoretical analysis and finite element method (FEM). The validation and application of the Hertzian load-displacement relation for indentation of hyperelastic soft materials were clarified, the effects of large deformation and material nonlinearity on spherical indentation of hyperelastic soft materials were analyzed and discussed. It was found that the complicated indentation behaviors of hyperelastic soft solids mainly depended on the coupling interactions of large deformation and material nonlinearity. Besides, we proposed two new nonlinear elastic contact models to separate the effects of large deformation and material nonlinearity on spherical indentation responses of hyperelastic soft solids. Our efforts might help to enhance the understanding of hyperelastic indentation problems and provided necessary instructions for the mechanical characterization of hyperelastic soft materials.     

Bio-inspired mechanics of reversible adhesion: Orientation-dependent adhesion strength for non-slipping adhesive contact with transversely isotropic elastic materials

Geckos and many insects have evolved elastically anisotropic adhesive tissues with hierarchical structures that allow these animals not only to adhere robustly to rough surfaces but also to detach easily upon movement. In order to improve our understanding of the role of elastic anisotropy in reversible adhesion, here we extend the classical JKR model of adhesive contact mechanics to anisotropic materials. In particular, we consider the plane strain problem of a rigid cylinder in non-slipping adhesive contact with a transversely isotropic elastic half space with the axis of symmetry oriented at an angle inclined to the surface. The cylinder is then subjected to an arbitrarily oriented pulling force. The critical force and contact width at pull-off are calculated as a function of the pulling angle. The analysis shows that elastic anisotropy leads to an orientation-dependent adhesion strength which can vary strongly with the direction of pulling. This study may suggest possible mechanisms by which reversible adhesion devices can be designed for engineering applications.     

A quartz-crystal-embedded split Hopkinson pressure bar for soft materials

A dynamic experimental technique that is three orders of magnitude as sensitive in stress measurement as a conventional split Hopkinson pressure bar (SHPB) has been developed. Experimental results show that this new method is effective and reliable for determining the dynamic compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths, such as elastomeric materials and foams at high strain rates. The technique is based on a conventional SHPB. Instead of a surface strain gage mounted on the transmission bar, a piezoelectric force transducer was embedded in the middle of the transmission bar of a high-strength aluminum alloy to directly measure the weakly transmitted force profile from a soft specimen. In addition, a pulse-shape technique was used for increasing the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation in the low-impedance and low-strength specimen.     

材料力学教学中的人物与事迹

通过讲述与引申几位在《材料力学》教材中出现的人物事迹,展示充分利用人物背景在《材料力学》教学上的意义,以及如何培养学生的意志品质.     

Snap transitions in adhesion

Equilibrium adhesion states are analyzed for nonlinear spherical caps adhered to a rigid substrate under the influence of adhesive tractions that depend on the local separation between the shell and substrate. Transitions between bistable snapped-in and snapped-out configurations are predicted as a function of four nondimensional parameters representing the adhesive energy, the undeformed shell curvature, the range of the adhesive interactions, and the magnitude of an externally applied load. Nonuniform energy and traction fields associated with free-edge boundary conditions are calculated to better understand localized phenomena such as the diffusion of impurities into a bonded interface and the diffusion of receptors in the cell membrane. The linear Griffith approximations commonly used in the literature are shown to be limited to shells with a small height to thickness ratio and short-range adhesive interactions. External loading is found to alter the adhered configurations and the spatial distributions of both adhesive and elastic energies. An important implication of the latter analysis is the theoretical prediction of the pull-off force, which is shown to depend not only on the interface properties, but also on the geometric and material parameters of the shell and on both the magnitude and type of external loading.     

Non-slipping adhesive contact between mismatched elastic spheres: A model of adhesion mediated deformation sensor

A generalized JKR model is established for non-slipping adhesive contact between two dissimilar elastic spheres subjected to a pair of pulling forces and a mismatch strain. We discuss the full elastic solution to the problem as well as the so-called non-oscillatory solution in which tension and shear tractions along the contact interface is decoupled from each other. The model indicates that the mismatch strain has significant effect on the contact area and the pull-off process. Under a finite pulling force, a pair of adhering spheres is predicted to break apart spontaneously at a critical mismatch strain. This study suggests an adhesion mediated deformation sensing mechanism by which cells and molecules can detect mechanical signals in the environment via adhesive interactions.     

Multi-scale cohesive laws in hierarchical materials

Motivated by the observations that natural materials such as bone, shell, tendon and the attachment system of gecko exhibit multi-scale hierarchical structures, this paper aims to develop a better understanding of the effects of structural hierarchy on flaw insensibility of materials from the viewpoint of multi-scale cohesive laws. We consider two idealized, self-similar models of hierarchical materials, one mimicking gecko’s attachment system and the other mimicking the mineral–protein composite structure of bone, to demonstrate that structural hierarchy leads to multi-scale cohesive laws which can be designed from bottom up to enable flaw tolerance from nanoscale to macroscopic length scales.     

Thermophoresis and Brownian motion effects on boundary layer flow of nanofluid in presence of thermal stratification due to solar energy

The problem of laminar fluid flow,which results from the stretching of a vertical surface with variable stream conditions in a nanofluid due to solar energy,is investigated numerically.The model used for the nanofluid incorporates the effects of the Brownian motion and thermophoresis in the presence of thermal stratification.The symmetry groups admitted by the corresponding boundary value problem are obtained by using a special form of Lie group transformations,namely,the scaling group of transformations.An exact solution is obtained for the translation symmetrys,and the numerical solutions are obtained for the scaling symmetry.This solution depends on the Lewis number,the Brownian motion parameter,the thermal stratification parameter,and the thermophoretic parameter.The conclusion is drawn that the flow field,the temperature,and the nanoparticle volume fraction profiles are significantly influenced by these parameters.Nanofluids have been shown to increase the thermal conductivity and convective heat transfer performance of base liquids.Nanoparticles in the base fluids also offer the potential in improving the radiative properties of the liquids,leading to an increase in the efficiency of direct absorption solar collectors.     

A mode III crack in functionally graded piezoelectric materials

This paper considers the mode III crack problem in functionally graded piezoelectric materials. The mechanical and the electrical properties of the medium are considered for a class of functional forms for which the equilibrium equations have an analytical solution. The problem is solved by means of singular integral equation technique. Both a single crack and a series of collinear cracks are investigated. The results are plotted to show the effect of the material inhomogeneity on the stress and the electric displacement intensity factors.     

Interfacial crack problem in layered soft ferromagnetic materials in uniform magnetic field

The magnetoelastic plane strain problem of an interfacial Griffith crack between two dissimilar soft ferromagnetic elastic materials subjected to a uniform magnetostatic field is considered within the framework of linear magnetoelasticity. By making use of the Fourier integral transform technique, the mixed boundary problem is then reduced to a pair of singular integral equations of the second kind. Solutions of the singular integral equations are obtained numerically by means of a Jacobi polynomial expansion method. Effects of the magnetic field, the combinations of the magnetic properties of materials and the geometric parameters on the magnetoelastic stress intensity factors in the vicinity of crack tip are shown graphically.     

Kinematics and elasticity framework for materials with two fiber families

A kinematics framework is developed for materials with two fiber families that are not necessarily orthogonal or mechanically equivalent. These two latter conditions represent important subclasses that are analyzed. To succinctly define the strain, six scalar strain attributes are developed that have direct physical interpretation. In the hyperelastic limit, this approach allows the Cauchy stress t to be expressed as the sum of six response terms, almost all of which are mutually orthogonal (i.e. 14 of the 15 inner products vanish). For small deformations, the response terms are entirely orthogonal (i.e. all 15 inner products vanish). Experimental advantage is demonstrated for finite strain hyperelastic materials by showing that common tests, for the first time, can directly determine terms in the strain energy function of two fiber composites.Received: 18 September 2002, Accepted: 5 May 2003, Published online: 29 July 2003     

Engineering mechanics and materials research in the information technology age

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A new type of Maxwell stress in soft materials due to quantum mechanical-elasticity coupling

All dielectrics deform when subjected to an electric field. This behavior is attributed to the so-called Maxwell stress and the origins of this phenomenon can be traced to geometric deformation nonlinearities. In particular, the deformation is large when the dielectric is elastically soft (e.g. elastomer) and negligible for most “hard” materials. In this work, we develop a theoretical framework which shows that a striking analog of the electrostatic Maxwell stress also exists in the context of quantum mechanical-elasticity coupling. The newly derived quantum-elastic Maxwell stress is found to be significant for soft nanoscale structures (such as the DNA) and underscores a fresh perspective on the mechanics and physics of polarons. We discuss potential applications of the concept for soft nano-actuators and sensors and the relevance for the interpretation of opto-electronic properties.     

The relation between a microscopic threshold-force model and macroscopic models of adhesion

This paper continues our recent work on the relationship between discrete contact interactions at the microscopic scale and continuum contact interactions at the macroscopic scale(Hulikal et al., J. Mech. Phys. Solids 76,144–161, 2015). The focus of this work is on adhesion. We show that a collection of a large number of discrete elements governed by a threshold-force based model at the microscopic scale collectively gives rise to continuum fracture mechanics at the macroscopic scale. A key step is the introduction of an efficient numerical method that enables the computation of a large number of discrete contacts. Finally,while this work focuses on scaling laws, the methodology introduced in this paper can also be used to study roughsurface adhesion.     

Flow of soft solids squeezed between planar and spherical surfaces

The force to squeeze a Herschel–Bulkley material without slip between two approaching surfaces of various curvature is calculated. The Herschel–Bulkley yield stress requires an infinite force to make plane–plane and plane–concave surfaces touch. However, for plane–convex surfaces this force is finite, which suggests experiments to access the mesoscopic thickness region (1–100 m) of non-Newtonian materials using squeeze flow between a plate and a convex lens. Compared to the plane–parallel surfaces that are used most often for squeeze flow, the dependence of the separation h and approach speed V on the squeezing-time is more complicated. However, when the surfaces become close, a simplification occurs and the near-contact approach speed is found to vary as V h⁰ if the Herschel–Bulkley index is n<1/3, and V h^(3n-1)/(2n) if n 1/3. Using both plane–plane and plane–convex surfaces, concordant measurements are made of the Herschel–Bulkley index n and yield stress ₀ for two soft solids. Good agreement is also found between ₀ measured by the vane and by each squeeze-flow method. However, one of the materials shows a limiting separation and a V(h) behaviour not predicted by theory for h<10 m, possibly owing to an interparticle structure of similar lengthscale.     

Prediction of the softening and damage effects with permanent set in fibrous biological materials

Deformation induced softening is an inelastic phenomenon frequently accompanying mechanical response of soft biological tissues. Inelastic phenomena which occur in mechanical testing of biological tissues are very likely to be associated with alterations in the internal structure of these materials.In this study, a novel structural constitutive model is formulated to describe the inelastic effects in soft biological tissues such as Mullins type behavior, damage and permanent set as a result of residual strains after unloading. Anisotropic softening is considered by evolution of internal variables governing the anisotropic properties of the material. We consider two weight factors w_i (softening) and s_k (discontinuous damage) as internal variables characterizing the structural state of the material. Numerical simulations of several soft tissues are used to demonstrate the performance of the model in reproducing the inelastic behavior of soft biological tissues.