Smart polymers for advanced applications: a mechanical perspective review

Responsive materials, as well as active structural systems, are nowadays widely used to develop unprecedented smart devices, sensors or actuators; their functionalities come from the ability of responding to environmental stimuli with a detectable reaction. Depending on the responsive material under study, the triggering stimuli can have a different nature, ranging from physical (temperature, light, electric or magnetic field, mechanical stress, ...), chemical (pH, ligands, …), or biological (enzymes, …) type. Such a responsiveness can be obtained by properly designing the meso- or macroscopic arrangement of the constitutive elements, as occurs in metamaterials, or can be obtained by using responsive materials per se, whose responsiveness comes from the chemistry underneath their microstructure. In fact, when the responsiveness at the molecular level is properly organized, the nanoscale response can be collectively detected at the macroscale, leading to a responsive material. In the present paper, we review the huge world of responsive polymers, by outlining the main features, characteristics and responsive mechanisms of smart polymers and by providing a mechanical modeling perspective, both at the molecular as well as at the continuum scale level. We aim at providing a comprehensive overview of the main features and modeling aspects of the most diffused smart polymers. The quantitative mechanical description of active materials plays a key role in their development and use, enabling the design of advanced devices as well as to engineer the materials’ microstructure according to the desired functionality

The mechanical response of fire ant rafts

Fire ants (Solenopsis invicta) cohesively aggregate via the formation of voluntary ant-to-ant attachments when under confinement or exposed to water. Once formed, these aggregations act as viscoelastic solids due to dynamic bond exchange between neighboring ants as demonstrated by rate-dependent mechanical response of 3D aggregations, confined in rheometers. We here investigate the mechanical response of 2D, planar ant rafts roughly as they form in nature. Specifically, we load rafts under uniaxial tension to failure, as well as to 50% strain for two cycles with various recovery times between. We do so while measuring raft reaction force (to estimate network-scale stress), as well as the networks' instantaneous velocity fields and topological damage responses to elucidate the ant-scale origins of global mechanics. The rafts display brittle-like behavior even at slow strain rates (relative to the unloaded bond detachment rate) for which Transient Network Theory predicts steady-state creep. This provides evidence that loaded ant-to-ant bonds undergo mechanosensitive bond stabilization or act as \say{catch bonds}. This is further supported by the coalescence of voids that nucleate due to biaxial stress conditions and merge due to bond dissociation. The characteristic timescales of void coalescence due to chain dissociation provide evidence that the local detachment of stretched bonds is predominantly strain- (as opposed to bond lifetime-) dependent, even at slow strain rates, implying that bond detachment rates diminish significantly under stretch. Significantly, when the voids are closed by restoring the rafts to unstressed conditions, mechanical recovery occurs, confirming the presence of concentration-dependent bond association that - combined with force-diminished dissociation - could further bolster network cohesion under certain stress states

Helical growth during the phototropic response, avoidance response, and in stiff mutants of Phycomyces blakesleeanus

The sporangiophores of Phycomyces blakesleeanus have been used as a model system to study sensory transduction, helical growth, and to establish global biophysical equations for expansive growth of walled cells. More recently, local statistical biophysical models of the cell wall are being constructed to better understand the molecular underpinnings of helical growth and its behavior during the many growth responses of the sporangiophores to sensory stimuli. Previous experimental and theoretical findings guide the development of these local models. Future development requires an investigation of explicit and implicit assumptions made in the prior research. Here, experiments are conducted to test three assumptions made in prior research, that (a) elongation rate, (b) rotation rate, and (c) helical growth steepness, R, of the sporangiophore remain constant during the phototropic response (bending toward unilateral light) and the avoidance response (bending away from solid barriers). The experimental results reveal that all three assumptions are incorrect for the phototropic response and probably incorrect for the avoidance response but the results are less conclusive. Generally, the experimental results indicate that the elongation and rotation rates increase during these responses, as does R, indicating that the helical growth steepness become flatter. The implications of these findings on prior research, the &ldquo;fibril reorientation and slippage&rdquo; hypothesis, global biophysical equations, and local statistical biophysical models are discussed. </div

A theoretical treatment on the mechanics of interfaces in deformable porous media

AbstractThe presence of interfaces (such as cracks, membranes and bi-material boundaries) in hydrated porous media may have a significant effect on the nature of their deformation and interstitial fluid flow. In this context, the present paper introduces a mathematical framework to describe the mechanical behavior of interfaces in an elastic porous media filled with an inviscid fluid. While bulk deformation and flow are characterized by displacement gradient and variations in the fluid chemical potential, their counterpart in the interface are derived by defining adequate projections of strains and flow onto the plane of the interface. This operation results in the definition of three interface deformation and stress measures describing decohesion, mean tangential strain and relative tangential strain, as well as three interface fluid driving forces and flux representing normal flux, mean tangential flux and relative tangential flux. Consistent with this macroscopic description of interface behavior, a set of governing equations are then introduced by considering the conservation of mass and the balance of momentum in the mixture. In particular, we show that the coupled mechanisms of interface deformation and fluid flow are described by six differential equations for fluid flow and three equations for solid deformation. It is also shown that a simpler set of governing equation can be derived when incompressible constituents are considered. The behavior of the mixture is finally specified through a general linear constitutive relation that relies on the definition of quadratic strain energy and dissipation functions. While an large number of material constants are needed in the general case, we show that under simplifying assumptions, the behavior of the interface can be written in terms of only eight material constants. A summary and discussion is then provided on the proposed formulation and potential applications are suggested