Conference Agenda

Hydrogels are three-dimensional networks of polymer chains that are linked together by chemical and physical crosslinks. They are highly swellable, capable of changing chemical energy to mechanical energy and vice versa. They have unique properties such as low elastic moduli and large deformability. The main constituent of hydrogels are the polymer chains that are highly hydrophilic. When immersed in water they absorb water molecules increasing the volume, resulting in swelling. This generally takes place in three steps: one, diffusion of water into the polymer network, two, relaxation of network chains and three, expansion of the polymer network. Normally hydrogels in the fully swollen state are viscoelastic and rubbery, similar to the biological fluids. These properties make them biocompatible. Thus, hydrogels have found applications in biomedical fields, such as making contact lenses, wound dressings, and tissue engineering. They are also used in fluid control and drug delivery systems. In this work, we focus on free swelling of a hydrogel from dry state to fully swollen state. We take the polyacrylamide (PAAm) hydrogel with degree of swelling Q = 42.5. Further, we use this state as the reference state and apply uniaxial load in tension. We assume that swelling is homogeneous. We focus on the non-linear theory of swelling. We plot the stress versus stretch diagram under uniaxial loading conditions. The model is validated with the available experimental results.

Electroactive polymers are a class of smart materials which change shape when stimulated by an electric field. Typical applications are in the areas of, e.g., robotics, artificial muscles and sensors. For such applications a reliable prediction of properties and performance, including loading and performance limits, is important. The occurrence of damage and fracture has a strong influence on the material behaviour. In this context, this work combines a material model for electroactive polymers with a fracture model.

The behaviour of electroactive polymers is modelled as a quasi-static large strain electro-mechanical material. The model is derived from a potential. The mechanical part of the model is a Neo-Hooke material and the electro-mechanical coupling is described by the relative permittivity. The material parameters are chosen such that the model mimics the behaviour of a soft electroactive polymer. The model is analysed with respect to the physically reasonable response and numerical stability. A phase field model for crack propagation is applied as fracture model. This method describes the crack propagation by means of an additional scalar field, the phase field. This phase field takes values between zero and one, whereby value zero represents undamaged material and value one corresponds to a fully damage state, respectively crack at the particular location. Since the model is used for polymers, the phase field model is adapted to large strains. The electro-mechanically coupled problem is solved within a monolithic scheme. The phase field problem, however, is solved within a staggered algorithm. The crack-propagation turns out to be different for the purely mechanical case as compared to the electro-mechanically coupled case.

The proposed model is implemented in a nonlinear finite element framework. Representative numerical examples are discussed in order to show the applicability of the model.

Soft solids such as silicone gels, with bulk shear modulus ranging from ∼10 to 1000 kPa, often exhibit strongly strain-dependent surface stresses. Moreover, unlike conventional stiffer materials, the effects of surface stress in these materials manifest at length scales of tens of micrometers rather than nanometers. The theoretical framework for modelling such problems envisages a soft hyperelastic bulk on which the infinitesimally thin surface that acts as a `wrapper’, with its own constitutive equation. We will recall the essential features of this theoretical framework and its FE implementation in the first part of this talk.

In the second, we will highlight simple force-twist, torque-twist, and force-extension (force-compression) responses of a soft cylinder held between two inert, rigid plates to demonstrate the role that the parameters in the surface constitutive model play in modulating the overall response of the bulk-surface system.

Finally, we will, through Finite Element simulations, demonstrate the effect of surface elasticity on two problems. The first is a variation of the well-known problem of an axisymmetric liquid capillary bridge between two rigid surfaces, with the liquid replaced by a soft solid. When the associated length scales are small, the shapes of the meniscus of a soft solid capillary with significant surface elasticity exhibits a much richer variety of shapes than a liquid. However, with stretch, the meniscus tends behave like a liquid bridge.

In the second problem, we explore the recent rather counter-intuitive experimental observation that, soft solids, when reinforced with small liquid inclusions, can become stiffer than the matrix material. We perform computational homogenisation on liquid inclusion reinforced soft solids with a view to understand the effect of surface stresses on their overall stiffness and manner in which cracks propagate in them.