Scientific publications

Transition from an amorphous to a crystalline phase and stabilization of amorphous phases is a common strategy in biomineralization. Although no such phenomenon has yet been reported for biogenic calcium oxalate systems, it was recently demonstrated for synthetic calcium oxalate monohydrate (COM). Here we focused on COM raphides—needle shaped biominerals—synthesized by Duckweed. Although these raphides show some birefringence in polarized light, implying their crystallinity, they diffracted poorly when examined by x-ray diffraction in our experiments. By means of transmission electron microscopy coupled with electron diffraction experiments we demonstrated that raphides from Duckweed are completely amorphous in their tip region and transform into a crystalline phase under the electron beam after a few seconds of exposure. To the best of our knowledge, this is the first report on biogenic amorphous calcium oxalate produced by a living organism.

Hexadecane exhibits pronounced molecular layering upon confinement to gaps of a few nanometer width which is discussed for its role in boundary lubrication. We have probed the mechanical properties of the confined layers with the help of an atomic force microscope, by quasi-static normal force measurements and by analyzing the lateral tip motion of a magnetically actuated torsional cantilever oscillation. The molecular layering is modeled by a oscillatory force curve and the tip approach is simulated assuming thermal equilibrium correlations in the liquid. The shear response of the confined layers reveals gradually increasing stiffness and viscous dissipation for a decreasing number of confined layers.

In order to reveal fundamental tribological mechanisms in polymer/steel sliding pairs, the pin-on-flat configuration of classical macroscopic tribotests was transferred into a high-resolution tribometer designed for scratch tests. Experiments were performed with a polyetheretherketone (PEEK) pin sliding on a steel disk in straight unidirectional movement mode. The surface morphology was monitored by interrupting the tests every 10,000 sliding strokes. The evolving surface morphology of PEEK was correlated with the transfer layer formed on steel counter surface. Scratching grooves in the PEEK surface were induced by asperities at the counter steel surface covered with transfer layers. Transfer layers were composed of lumpy polymer material accompanied by fine wear debris in areas of lower roughness. These smooth areas limit the penetration of large asperities and distinguish the scratching mechanism in macroscopic sliding from typical single-asperity scratching tests. The results reveal the mechanisms leading to inhomogeneity in the transfer layers as consequence of the asperity distribution.

The human skin comprises a complex multi-scale layered structure with hierarchical organization of different cells within the extracellular matrix (ECM). This supportive fiber-reinforced structure provides a dynamically changing microenvironment with specific topographical, mechanical and biochemical cell recognition sites to facilitate cell attachment and proliferation. Current advances in developing artificial matrices for cultivation of human cells concentrate on surface functionalizing of biocompatible materials with different biomolecules like growth factors to enhance cell attachment. However, an often neglected aspect for efficient modulation of cell-matrix interactions is posed by the mechanical characteristics of such artificial matrices. To address this issue, we fabricated biocompatible hybrid fibers simulating the complex biomechanical characteristics of native ECM in human skin. Subsequently, we analyzed interactions of such fibers with human skin cells focusing on the identification of key fiber characteristics for optimized cell-matrix interactions. We successfully identified the mediating effect of bio-adaptive elasto-plastic stiffness paired with hydrophilic surface properties as key factors for cell attachment and proliferation, thus elucidating the synergistic role of these parameters to induce cellular responses. Co-cultivation of fibroblasts and keratinocytes on such fiber mats representing the specific cells in dermis and epidermis resulted in a hierarchical organization of dermal and epidermal tissue layers. In addition, terminal differentiation of keratinocytes at the air interface was observed. These findings provide valuable new insights into cell behaviour in three-dimensional structures and cell-material interactions which can be used for rational development of bio-inspired functional materials for advanced biomedical applications.

Bio-inspired fibrillar adhesives rely on the utilization of short-range intermolecular forces harnessed by intimate contact at fibril tips. The combined adhesive strength of multiple fibrils can only be utilized if equal load sharing (ELS) is obtained at detachment. Previous investigations have highlighted that mechanical coupling of fibrils through a compliant backing layer gives rise to load concentration and the nucleation and propagation of interfacial flaws. However, misalignment of the adhesive and contacting surface has not been considered in theoretical treatments of load sharing with backing layer interactions. Alignment imperfections are difficult to avoid for a flat-on-flat interfacial configuration. In this work we demonstrate that interfacial misalignment can significantly alter load sharing and the kinematics of detachment in a model adhesive system. Load sharing regimes dominated by backing layer interactions and misalignment are revealed, the transition between which is controlled by the misalignment angle, fibril separation, and fibril compliance. In the regime dominated by misalignment, backing layer deformation can counteract misalignment giving rise to improved load sharing when compared to an identical fibrillar array with a rigid backing layer. This result challenges the conventional belief that stiffer (and thinner) backing layers consistently reduce load concentration among fibrils. Finally, we obtain analytically the fibril compliance distribution required to harness backing layer interactions to obtain ELS. Through fibril compliance optimization, ELS can be obtained even with misalignment. However, since misalignment is typically not deterministic, it is of greater practical significance that the array optimized for perfect alignment exhibits load sharing superior to that of a homogeneous array subject to misalignment. These results inform the design of fibrillar arrays with graded compliance capable of exhibiting improved load sharing over large areas.

During plastic deformation, most of the dissipated work is released as heat, but a fraction of it, usually small, is stored in the microstructure, is called latent heat and is associated with the network of dislocations that develops. The rate of energy storage in the microstructure divided by the rate of plastic dissipation is defined as the latent heat capacity. Latent heat remains stored in the microstructure of cold worked specimens after quenching. This energy is associated with modified mechanical properties, e.g. hardness, and is released upon annealing. Saturation of this stored energy has been observed in experiments after a specific amount of plastic deformation is reached. A thermodynamically consistent model for continuous dynamic recrystallization is proposed in this paper with the aim of explaining the phenomenon of latent heat saturation and relating it to grain refinement. The proposed model has three essential features: (i) the latent heat increases in the specimen during plastic deformation as plastic work is continuously dissipated; (ii) the rate of latent heat storage per unit work, i.e. the latent heat capacity, is related to the internal architecture of the microstructure and decreases to zero as a consequence of microstructural evolution; (iii) the relationship between the latent heat and the microstructure is described through the use of two parameters: (a) the dislocation density and (b) the average grain diameter. A comparison of the proposed model with experiments is reported and a validation for the prediction of microstructural evolution, as well as the evolution of the latent heat and latent heat capacity, is provided.

We construct a constitutive law for the response of dielectric elastomers subject to high levels of stretch during combined electrostatic and mechanical loading. The constitutive law is based on a statistical mechanics analysis of a freely jointed chain, due to Kuhn and Grün [1–3], that relates the force of extension and polarizability anisotropy of a polymer chain to its fractional extension, r / n l , through the inverse Langevin function. We utilize a Padé [4] approximant that accurately represents the inverse Langevin function through the entire range of fractional extensions. Thereafter, we cast this machinery into the 8-chain lattice [5,6] and model an elastomer as a heavily interpenetrated network of 8-chain lattices. We assume that the motions of each lattice are affine with the overall deformation of the elastomer. In this fashion, the fractional extension of each chain, r / n l , is linked to the stretch ratios. With such an approach, we obtain a materially objective free energy density and an expression for the dielectric permittivity of the elastomer that depends on the current state of deformation and the overall stretch level. The elastic free energy density depends on two parameters, the small deformation shear modulus and the chain extensibility limit. We observe that the present model and the well established Arruda and Boyce [5], Gent [7], and neoHookean models are all special cases of the eight chain model of the elastic free energy density presented in this work. The isotropic part of the dielectric permittivity and the electrostrictive coefficient depend on the dilatation. The dielectric permittivity remains isotropic under a pure dilatation, but otherwise becomes anisotropic during deformation. The form of the permittivity resembles that of the deformation dependent permittivity presented by Jiménez and McMeeking [8]. However, in the model presented in this work, the electrostrictive coefficient is not only affected by dilatation but also becomes a function of the current level of deformation through the first invariant of the left Green-Cauchy tensor. We utilize the free energy density of the dielectric elastomer to compute the response of a thin film actuator subject to electrostatic and mechanical loading. In this model, the actuator is allowed to have different levels of in-plane limit stretch, and the through thickness permittivity is allowed to increase or decrease with in-plane extension of the actuator. We establish a parameter space map, extensibility limit versus electrostrictive coefficient of the elastomer, for which our constitutive law is relevant to the behavior of dielectric elastomers. With this approach, we study the actuation, electric charge storage, and stability characteristics of the actuator. From the results of our calculations we clearly identify two types of actuator behavior: actuators that exhibit electromechanical instability (type A), and actuators that do not exhibit this instability (type B). We establish that type A actuators develop hysteresis loops in a similar manner to those identified by Zhao, Hong and Suo [9] and Jiménez and McMeeking [8], for dielectric elastomers with constant isotropic permittivity that stiffen during straining, in the case of the former, and for dielectric elastomers that do not stiffen but exhibit a through thickness permittivity that increases/decreases with straining, in the case of the latter. Finally, we show that, while pre-stretch reduces the electric potential and electric charge levels required to operate the actuator, and simultaneously enhances the sensitivity of the actuator to electric potential, it has a detrimental effect on the sensitivity to electric charge.

During operation of a lithium ion cell electrode storage particles experience an inhomogeneous volume change due to local differences in the internal lithium concentration. The resulting mechanical stress can become large enough to provoke particle fracture, an aging mechanism considered to have a severe detrimental impact on the life time of lithium ion cells. In this work, we use a coupled model of mechanical stress, lithium diffusion and crack growth to study the problem of fracture in storage particles. The model was successfully applied to study crack growth during a single half cycle of lithium insertion or extraction in earlier investigations. It was demonstrated that, under specific circumstances, particle breakage may occur in a single half cycle. Here, we consider the second half cycle and examine under which conditions cracks that either remained stable or underwent growth in the first half cycle, can lead to particle fragmentation during the subsequent half cycle. From both numerical results and supportive analytic solutions, we find that growth of a through crack during Li insertion is a strong indicator for particle breakage, either in one or two half cycles. Such a relationship is not found for growth of a surface crack during Li extraction.

Fracture of storage particles is considered to be one of the major reasons for capacity fade and increasing power loss in many commercial lithium ion batteries. The appearance of fracture and cracks in the particles is commonly ascribed to mechanical stress, which evolves from inhomogeneous swelling and shrinkage of the material when lithium is inserted or extracted. Here, a coupled model of lithium diffusion, mechanical stress and crack growth using a phase field method is applied to investigate how the formation of cracks depends on the size of the particle and the presence or absence of an initial crack, as well as the applied flux at the boundary. The model shows great versatility in that it is free of constraints with respect to particle geometry, dimension or crack path and allows simultaneous observation of the evolution of lithium diffusion and crack growth. In this work, we focus on the insertion process. In particular, we demonstrate the presence of intricate fracture phenomena, such as, crack branching or complete breakage of storage particles within just a single half cycle of lithium insertion, a phenomenon that was only speculated about before.

Storage particles of lithium ion batteries undergo significant mechanical stress during charging and discharging due to the inhomogeneous volume change within the particles when lithium is inserted and extracted. This stress potentially leads to fracture of the particles resulting in detrimental effects for the capacity and internal resistance of a lithium ion battery, such as the growth of additional solid electrolyte interface, loss of contact in conductive pathways or complete disintegration of the electrode. Here, we tackle the problem of fracture in storage particles by merging a coupled model of mechanical stress and diffusion of lithium ions with a phase field description of an evolving crack. This approach allows the simultaneous study of the evolution of the lithium concentration together with the growth of a crack without restrictions to specific geometries, simulation dimensions and presuppositions regarding the crack path. The model was successfully applied to study crack growth during lithium insertion in an earlier work. It was shown that inertia effects play a crucial role with respect to a possible fragmentation of the storage particles. Here, we focus on the opposite charging condition and examine the circumstances under which unstable crack growth and particle breakage can occur during lithium extraction.