Promising progress in the field of icephobicity has been made in the recent years. However, a majority of the reported icephobic surfaces rely on static mechanisms, and they maintain low ice adhesion on surfaces at extreme temperatures (as low as −60 °C), which is highly challenging. Dynamic anti-icing surfaces, which can melt ice or change the ice–substrate interfaces from the solid to liquid phase after the formation of ice, serve as a viable alternative. In this study, liquid layer generators (LLGs), which can release ethanol to the ice–solid interface and convert the ice–substrate contact from the solid–solid mode to the solid–liquid–solid mode, were introduced. Excellent icephobicity on surfaces with an ethanol lubricating layer was found to withstand extremely low temperatures (−60 °C), which was proven by both molecular dynamics simulations and experiments. Two prototypes of LLGs, one by packing ethanol inside and the other by storing replenishable ethanol below the substrate, were fabricated. These LLGs could constantly release ethanol for a maximum of 593 days without source replenishment. Both these prototypes exhibited super-low ice adhesion strengths of 1.0–4.6 kPa and 2.2–2.8 kPa at −18 °C. For select samples, by introducing an interfacial ethanol layer, the ice adhesion strength on the same surfaces decreased in an unprecedented manner from 709.2–760.9 kPa to 22.1–25.2 kPa at a low temperature of −60 °C.

Hydrogen-microvoid interactions were studied via unit cell analyses with different hydrogen concentrations. The absolute failure strain decreases with hydrogen concentration, but the failure loci were found to follow the same trend dependent only on stress triaxiality, in other words, the effects of geometric constraint and hydrogen on failure are decoupled. Guided by the decoupling principle, a hydrogen informed Gurson model is proposed. This model is the first practical hydrogen embrittlement simulation tool based on the hydrogen enhanced localized plasticity (HELP) mechanism. It introduces only one additional hydrogen related parameter into the Gurson model and is able to capture hydrogen enhanced internal necking failure of microvoids with accuracy; its parameter calibration procedure is straightforward and cost efficient for engineering purpose.

In a recent paper published at AIP Advances, for the first time we clearly demonstrated that the ice type itself has a strong influence on its adhesion to a solid surface. 

The brittle-to-ductile transition (BDT) is not an intrinsic phenomenon of material, and depends not only on the strain rate but also on the constraint at crack tip. By employing a dislocation mobility based continuum model, we found that the change of the stress distribution ahead of crack tip due to the T-stress dictates the fracture toughness in the transition region; lower constraint leads to a higher fracture toughness, a smoother transition curve and a lower critical transition temperature. A quantitative relation between fracture toughness and T-stress is established such that the transition curve with constraint can be estimated from a reference BDT curve.

Fluids flow in porous media is ubiquitous and has important application in numerous fields, such as groundwater remediation, oil recovery, water purification, CO₂ utilization, etc. In the paper, the oil field is taken as an example to study the fluid flow properties in porous media. Given that a reservoir with ultra-low permeability, abundances of nanopores interconnect large pores and control the permeability of reservoirs, and about 80% of them ranging from 0.5 to 100 nm in diameter (4500-5600 meter in depth). The fluids in confined nanopores will behave quite differently from the bulk ones, however, it is very challenging to control the thickness of oil film, the nanoscale diameter of porous media, pressure and temperature distribution by experiments, etc. Understanding dynamics of fluids flow in confinement, especially nanopores in reservoirs, is crucial for the design of flooding fluids. In this study, all-atom models of various oil components and modified silica NPs were put into MD simulations for investigating their influences on the displacement of fluid flow into silica nanopore. Via analyses of the relationship of displacement length (l) and time (t), dynamic water-oil displacement process, and pressure difference along capillary induced by silica NPs, a comprehensive mechanism of NP effects on displacement is proposed. Our findings shed light on resolving nanoscale water-oil displacement mechanism influenced by NPs in sandstone reservoirs, which is significant to design targeted NPs for applications.

We introduce an unprecedented route to fabricate nanoporous materials to circumvent the drawbacks in existing approaches (dealloying and template-mediated methods). We utilize focused ion beam milling of self-assembled magnetic superstructures as a novel approach to fabricate nanoporous materials with the possibility to tune the porosity. We find a fundamentally different behavior when superstructured materials consisting of ordered nanoparticles, as opposed to their bulk counterpart, are subjected to ion beam milling. Nanoparticles in the superstructures are not only subjected to milling, but also melting, causing particles to merge into each other thus forming a porous network. We find that the mean pore sizes increase linearly with increasing ion beam voltage, and also increase with decreasing packing factor within the superstructure which was in this study tuned by the shape of the nanoparticles in question (spherical versus cubic). Hence, our approach offers three-dimensional flexibility in terms of nanoparticle shape and material, in addition to the easily tunable ion beam voltage, enabling the fabrication of nanoporous materials with the possibility to both choose backbone material and control the porosity.

Understanding water wettability is essential for creating surfaces with varied hydrophobicity for practical applications. Two wetting states, namely the Wenzel and the Cassie-Baxter states, of surfaces have been classified for more than half a century. Their static force balances at the three phase contact line are analytically explained by the famous Young’s equation. In contrast, the water dewetting mechanics is rarely touched, especially on atomistic scale that applies to surfaces with nanoscale topography, for instance Lotus leaves. Our work focused on atomistic water dewetting at two wetting states, by employing atomistic modeling and molecular dynamics simulations.  We first realized the two wetting states, and applied force to detach water droplets from nanopillars and flat substrates, aiming for probing the nanoscale dewetting mechanics. Our results revealed the intermediate states in non-equilibrium dewetting, and quantified water adhesion stress in the dewetting process at two wetting states that was not covered in former studies. 

The classic van der Pauw method is extended to measure the electrical resistance and determine the resistivity of spherical thin metal films coated on the polymer sphere. The resistivity is used to interpret resistance contributions in single particle electromechanical nanoindentation measurements, which simulate the compression particles undergo in application. The resistivity was found to be coating thickness dependent for thin films in the range 60-270 nm. Estimation of the resistance of the metal shell using the measured resistivity did not account for the total resistance measured in electromechanical nanoindentation. We therefore deduce a significant contribution of contact resistance at the interfaces of the particle. The contact resistance is both coating thickness and particle deformation dependent.

The conventional slippery liquid-infused porous surfaces (SLIPS) possess low ice adhesion strength thanks to the existence of lubricant on the interface. However, the SLIPS are still far from being applicable in real environments owing to their low durability. Herein, inspired by the functionality of amphibians’ skin, we used one-step method to fabricate a series of skin-like SLIPS, which can regenerate lubricant on the surfaces after 15 wiping/regeneration tests. We studied the behaviour of regenerable lubricant, and proposed mechanism for that. Thanks to the regenerability of the surface lubricant, the skin-like SLIPS presented durable icephobicity, showing a long-term low ice adhesion strength below 70 kPa, which is only 43% of 160 kPa, that for the pristine polydimethylsiloxane (PDMS, Sylgard 184), after 15 icing/deicing cycles.

PDMS sponge structures-based coatings fabricated by NTNU Nanomechanical Lab reached a record super-low ice adhesion strength below 1 kPa! Details can be found in a new paper published in the June issue of  Soft Matter.

Design and preparation of sandwich-like polydimethylsiloxane (PDMS) sponges with super-low ice adhesion

Liquefaction of vapor is a necessary, but energy intensive step in several important process industries. This review identifies possible materials and surface structures for promoting dropwise condensation, known to increase efficiency of condensation heat transfer. Research on superhydrophobic and superomniphobic surfaces promoting dropwise condensation constitutes the basis of the review. In extension of this, knowledge is extrapolated to condensation of CO₂. Global emissions of CO₂ need to be minimized in order to reduce global warming, and liquefaction of CO₂ is a necessary step in some carbon capture, transport and storage (CCS) technologies. The review is divided into three main parts: 1) An overview of recent research on superhydrophobicity and promotion of dropwise condensation of water, 2) An overview of recent research on superomniphobicity and dropwise condensation of low surface tension substances, and 3) Suggested materials and surface structures for dropwise CO₂ condensation based on the two first parts.

Icephobic surfaces are crucial to all cold-condition applications, ranging from nano to macro scales. The study presented in this manuscript focuses on enabling new functionality, namely self-healing, in passive icephobic surfaces. The aim of the work is to improve the common durability issue of all the state-of-the-art icephobic surfaces. Here, we designed and fabricated a novel icephobic material by integrating interpenetrating polymer network (IPN) into autonomous self-healing elastomer. The material showed great potentials in anti-icing applications with an ultralow ice adhesion and long-term durability. Most importantly, the material was able to demonstrate self-healing from mechanical damages in a sufficiently short time, which shed light on the longevity of icephobic surface in practical applications. Moreover, we studied the creep behaviours of the elastomer that were absent in most relevant studies on self-healing materials. We also provided molecular mechanisms of the self-healing and creep resistance of the IPN in the manuscript.

Quasi-static tensile tests with smooth round bar and axisymmetric notched tensile specimens have been performed to study the low-temperature effect on the fracture locus of a 420-MPa structural steel. Combined with a digital high-speed camera and a 2-plane mirror system, specimen deformation was recorded in 2 orthogonal planes. Pictures taken were then analysed with the edge tracing method to calculate the minimum cross-section diameter reduction of the necked/notched specimen. Obvious temperature effect was observed on the load-strain curves for smooth and notched specimens. Both the strength and strain hardening characterized by the strain at maximum load increase with temperature decrease down to −60°C. Somewhat unexpected, the fracture strains (ductility) of both smooth and notched specimens at temperatures down to −60°C do not deteriorate, compared with those at room temperature. Combined with numerical analyses, it shows that the effect of low temperatures (down to −60°C) on fracture locus is insignificant. These findings shed new light on material selection for Arctic operation.

Large-area chemical-vapor-deposited monolayer MoS2 tends to be polycrystalline with intrinsic grain boundaries (GBs). Topological defects and grain size skillfully alter its physical properties in a variety of materials; however, the polycrystallinity and its role played in the mechanical performance of the emerging single-layer MoS2 remain largely unknown. Using large-scale atomistic simulations, GB structures and mechanical characteristics of realistic single-layered polycrystalline MoS2 of varying grain size prepared by confinement-quenched method are investigated. Depending on misorientation angle, structural energetics of polar-GBs in polycrystals favor diverse dislocation cores, consistent with experimental observations. Polycrystals exhibit grain size dependent thermally-induced global out-of-plane deformation, although defective GBs in MoS2 show planar structures that are in contrast to the graphene. Tensile tests show that presence of cohesive GBs pronouncedly deteriorates the in-plane mechanical properties of MoS2. Both stiffness and strength follow an inverse pseudo Hall-Petch relation to grain size, which is shown to be governed by the weakest link mechanism. Under uniaxial tension, transgranular crack propagates with small deflection, whereas upon biaxial stretching the crack kinkily grows with large deflection. These findings shed new light in GB-based engineering and control of mechanical properties of MoS2 crystals towards real-world applications in flexible electronics and nanoelectromechanical systems.

Hydrogen embrittlement of metallic materials is far from being understood. In this work, we develop a hydrogen-informed expanding cavity model for the first time to describe the dynamic evolution of load-displacement curve obtained from nanoindentation tests. What we want to specially mention is that the proposed model takes into account the kinetic diffusion of H atoms towards the plastic region and the H-induced decrease of the formation energy of dislocations. This new model allows us to make comparison with atomistic simulations and nanoindentation experiments, and bridge the gap between the knowledge obtained from nano and micro-scales.

The nanofluids or nanoparticles (NPs) transport in confined channel is of great importance for many biological and industrial processes. In this study, molecular dynamics simulation has been employed to investigate spontaneous two-phase displacement process in ultra-confined capillary controlled by surface wettability of NPs. The results clearly show that the presence of NPs modulates the fluid-fluid meniscus and hinders displacement process compared with NP-free case. From the perspective of motion behavior, hydrophilic NPs disperse in water phase or adsorb on the capillary, while hydrophobic and mixed-wet NPs are mainly distributed in the fluid phase. The NPs dispersed into fluids tend to increase the viscosity of fluids, while the adsorbed NPs contribute to wettability alteration of solid capillary. Via capillary number calculation, it is uncovered that the viscosity increase of fluids is responsible for hindered spontaneous displacement process by hydrophobic and mixed NPs. Wettability alteration of capillary induced by adsorbed NPs is dominating the enhanced displacement in the case of hydrophilic NPs. Our findings provide the guidance to modify the rate of capillary filling and reveal microscopic mechanism of transporting NPs into porous media, which is significant to the design of NPs for target applications.

In this manuscript, we present the Raman antenna effect from organic semiconducting nanobelts of 6,13-dichloropentacene (DCP). Under resonant excitation, DCP nanobelts act like a nearly perfect dipole antenna, and all Raman signals from the intramolecular phonons of DCP exhibit the same angle-dependent behaviour. It is the first time that Raman antenna effect in organic semiconducting materials is reported. The underlying mechanism (exciton-phonon coupling) is intrinsically different from that in inorganic semiconducting materials and carbon nanotube (geometry effect). The Raman antenna phenomenon is attributed to the intramolecular exciton‒phonon coupling, which dominates over the intrinsic Raman selection rule of DCP molecules and leads to all the Raman modes possessing the same angular dependence behaviour. The formation of intermolecular exciton in DCP nanobelts further results in maximum Raman emission perpendicular to the nanobelt’s long-axis. Besides, the Raman antenna effect also amplifies the anisotropy of Raman scattering of the one-dimensional DCP nanobelts. These findings give new physical insights into the light interaction with organic semiconductors and enrich our knowledge on the exciton‒phonon coupling and its effects on the optical properties of organic semiconductors.

Running ductile fracture is one of the most catastrophic accidents of pipelines for natural gas transportation. Crack arrest toughness is important for preventing crack extension to a long distance along pipeline. Critical crack tip opening angle (C

We present the anisotropic optical properties of 1D nanobelts of 6,13-dichloropentacene (DCP). High-quality large-area well-aligned DCP nanobelt arrays were readily obtained through self-assembly utilizing the strong π-π interaction between the molecular cores by simple solution processing method. The comparison of absorption and emission spectra of DCP in solution and DCP nanobelt indicated the co-existence of intramolecular and intermolecular excitons in the aggregation state of DCP. The photoluminescence (PL) from individual DCP nanobelt exhibited strong anisotropic property and the measured polarization ratio is on average 0.92±0.05, superior to that of the prior-art organic semiconductors. Beyond that, the angle-dependent photoluminescence clearly verified that the emission arose only from the relaxation of intramolecular exciton in spite of the strong electronic coupling along the π-π stacking direction. We believe these findings will enrich our knowledge of the exciton behaviour in 1D π-π stacking organic semiconductors and demonstrate DCP’s great potential for low-cost large-scale organic optoelectronic.

A novel tensile testing method is proposed, and a  ‘magic’ specimen with a special notch geometry has been identified. By using this special notched tensile specimen, material’s flow stress-strain curve can be DIRECTLY obtained from the recorded load versus diameter reduction curve and no Bridgman correction is needed.

Imbibition in porous media is ubiquitous and has important application in oil fields. Understanding the fundamental imbibition mechanism for nanofluids is very crucial to enhanced oil recovery (EOR) by nanoparticles. As it is difficult to disentangle the specific role of different interfaces in imbibition process by experimental trials, atomistic and molecular simulations hold the key to explore the migration mechanism of nanofluids into porous media and identify the dominating driving force for nanoparticles application in EOR.

In this study, we employ molecular dynamics simulations to study the spontaneous water imbibition into ultraconfined reservoir channels influenced by nanoparticles. By combining the dynamic process of imbibition, the water contact angle in capillary and the relationship of displacement (l) and time (t), a competitive mechanism of nanoparticle effects and fluid properties on spontaneous imbibition is proposed. Our findings provide new physical insights into the roles of nanoparticles in fluid imbibition, which is the core process in a number of technologies, including enhanced oil recovery.

Small systems are known to deviate from the classical thermodynamic description, among other things due to their large surface area to volume ratio compared to corresponding big systems. As a consequence, extensive thermodynamic properties are no longer proportional to the volume, but are instead higher order functions of size and shape. We investigate such functions for second moments of probability distributions of fluctuating properties in the grand-canonical ensemble, focusing specifically on the volume and surface terms as proposed by Hadwiger [Hadwiger, Springer, 1957]. We resolve the shape dependence of the surface term and show, using Hill’s nanothermodynamics [Hill, J. Chem. Phys., 1962, 36, 3182], that the surface satisfies the thermodynamics of a flat surface as described by Gibbs [Gibbs, Ox Bow Press, 1993, Vol. 1]. The Small System Method (SSM), first derived by Schnell et al. [Schnell et al., J. Phys. Chem. B, 2011, 115, 10911], is extended and used to analyze simulation data on small systems of water. We simulate water as an example to illustrate the method, using the TIP4P/2005 and other models, and compute the isothermal compressibility and thermodynamic factor. We are able to retrieve the experimental value of the bulk phase compressibility within 2 %, and show that the compressibility of nanosized volumes increases by up to a factor of two as the number of molecules in the volume decreases. The value for a tetrahedron, cube, sphere, polygon, etc. can be predicted from the same scaling law, as long as second order effects (nook and corner effects) are negligible. Lastly, we propose a general formula for finite reservoir correction to fluctuations in subvolumes.

New paper published at Materials Science & Engineering A.

Our recent results show that hydrogen embrittlement is present at sub-zero temperatures, causing an increase in fracture toughness reference temperature T₀ and a small decrease in deformation capability. The relationship between the T₀ and the impact toughness transition temperature T_28J, which, in the case of ultra-high-strength steel, deviates from that observed for lower strength steels, is proposed to be affected by the hydrogen content.

New paper published in Scientific Reports.

Our results show that low ice adhesion strength does not correlate well with water contact angle and its variants, surface roughness and hardness. Low elastic modulus does not guarantee low ice adhesion, however, surfaces with low ice adhesion always show low elastic modulus. Low ice adhesion (below 60 kPa) of commercial surfaces uniquely associates with small water adhesion force.

The flash diffusivity method/laser flash analysis (LFA) is one of the most popular methods for finding the thermal conductivity of a large range of materials, including polymer composites for thermal and electronic interconnects. With standardized, commercial instruments available, it has become common practice even in peer-reviewed journal publications to only state the instrument model and manufacturer, and then give the estimated thermal conductivity as an absolute value without discussing the intermediate factors. In this paper, we show that both the absolute values and temperature-dependent behavior of the specific heat capacity of polymer composite materials varies significantly with the three most common methods used to estimate this input factor for the LFA method, and that this further has a significant impact on the estimated thermal conductivity. We also give a systematic theoretical overview of the methods used in the manuscript, as this to our best knowledge has not before been published in one single paper. We expect that this paper can be of large value for researchers interested in investigating thermal properties of polymer composites, and as a general starting point for researchers interested in using the LFA method.

The reverse micelles (RMs) in supercritical CO₂ (scCO₂) are promising alternatives for organic solvents, especially for  both polar and non-polar components are involved. Fluorinated surfactants, particularly the double-chain fluorocarbon surfactants, are appropriate to form well-structured RMs in scCO₂. The mechanisms inherent to the self-assembly of the surfactants in scCO₂ are still subject to discussion. In this study, molecular dynamics simulations were performed to investigate the self-aggregation behavior of di-CF4 based RM in scCO₂ and a stable and spherical RM is formed. The dynamics process and the self-assembly structure in the RM reveal a three-step mechanism to  form the RM, that is, small RMs, rod-like RMs and the fusion of rod-like RMs. The Hydrogen-bonds between headgroups and water molecules, and the salt-bridges linking Na⁺, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The result shows the di-CF4 molecule has the high surfactant coverage at the RM interface, implying the high CO2-philicity. Ths mainly results from the bend of the short chain (C-COO-CH2-(CF2)3-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful to understand the surfactant self-assembly phenomena and design new CO₂-philic surfactants.

Colloidosomes have attracted great attention due to its special structure and broad applications, and the permeability is one of the key parameter of colloidosomes. In the manuscript, an effective and straightforward approach for fabricating novel colloidosomes from pH-responsive core-shell microgels is presented. One-pot surfactant-free synthesis of the microgels with hydrophobic core and hydrophilic shell is developed. The Model drug release results show that the permeability of colloidosomes can be coarsely controlled by pH and fine-tuned by the ratio of shell to core in microgels.

Compare to the works reported previously, the synthesis of microgels in one-pot process could avoid the preparation, isolation and purification of raw products, while microgels with well-defined core-shell structure are still obtained. Moreover, the core and shell of microgels can be tailor-designed by choosing different monomers; thus the method can be commonly used to integrate various functional materials into colloidosomes. The methodology revealed in the study not only provides a unique technique to control the permeability of colloidosomes but also opens a platform pathway to integrate multi-functionalities to the colloidosomes.

The void growth in monocrystalline Cu and Fe are investigated by molecular dynamics simulations to reveal the ductile mechanisms based on dislocation emission and propagation. The results show that the void growth in Cu is governed by the collective interaction of stacking faults along four (111) planes. Three dominant mechanisms of void growth in Fe are identified: (i) for small voids, nucleation of twinning boundaries; (ii) for intermediate voids, emission of shear loops; (iii) for large voids, stacking faults nucleate at the void surface and then degenerate into shear loops. The slip-twinning transition rate of Fe at room temperature calculated according to Zerrili-Armstrong model is in the range measured by our atomistic simulations. Vacancy generation which promotes void growth results from the intersection of more than two stacking faults in Cu, while in Fe it is attributed to the jog dragging of screw dislocations. An analytical model based on nudged elastic band calculation is developed to include the strain rate dependence of the nanovoid-incorporated incipient yielding. This new model demonstrates that the critical radius of shear loop in Cu under a strain rate of 10⁸ s^-1 is on the order of Burgers vector. For both metals, the dislocation density has been calculated to elucidate the plastic hardening coupled with void growth. This work sheds new lights in exploring the atomistic origins of the void size and strain rate dependent mechanisms associated with dislocation activities close to void surface

The convergence problem during the cohesive zone modelling of hydrogen embrittlement in constant displacement scenario is attributed to the numerical instability which is studied analytically in the present work. The property of numerical stability is directly associated with the number of solutions for the controlling equations from the failure initiation point. It is shown that all the cases with a non-unique solution are numerically unstable thereby having convergence problem. Linear elastic and elasto-plastic material models are considered in the derivation, and the convergence properties for both models are proved essentially the same. The viscous regularization proposed by Gao and Bower proves effective in solving the convergence problem with good accuracy under constant displacement, provided that the viscosity is small enough. This is further supported by a pipeline engineering case study where the viscosity regularized cohesive zone approach is applied to the hydrogen embrittlement simulation. The stabilizing mechanism of the viscous regularization is attributed to its capacity to enforce a single solution by modifying the controlling equations. The influence of viscous regularization on symmetry modelling is also discussed.

New article published in Applied Physics Letters. In this manuscript, we are presenting a novel method for performing four-point electrical measurements on spherical thin films with micron-scale diameters. Such measurements have not been reported previously, as four-point measurements are normally performed on flat surfaces and with complete symmetry in probe positions. The method takes advantage of recent advances in commercially available micro-robots, which allows the positioning of four separately controlled electrical probes on very fine structures. Using finite element models to obtain geometric corrections factors yields the opportunity to estimate the resistivity of materials and structures where symmetric probe positioning is difficult to achieve. By this method, we show that the intrinsic resistivities of spherical thin films are higher than that of bulk metal. The findings are of large significance to the electronic packaging industry, where cost-efficiency and getting large gain from the consumed amount of precious metals are of large importance.

Excessive icing is a general problem to human activities in low temperature environment. The aim of creating anti-icing materials, surfaces and applications rely on understanding the fundamental nanoscale ice adhesion mechanics. As increasing experimental trials on manufacturing anti-icing coatings have been carried out, theoretical knowledge on the atomistic determinants of ice adhesion is in urgent need. In this study, we employ all-atom modeling and molecular dynamics simulations to study ice adhesion, detaching and shearing on smooth silicon and graphene surfaces, aiming to decipher the basis of ice adhesion strength. We also study the mechanical effects of an aqueous water layer that sandwiched between ice and substrate, giving results to support previous experiments. Our results for the first time provide atomistic view on the key events of nanoscale deicing processes, and supply strong theoretical references for further anti-icing studies.

PhD candidate Verner Håkonsen has won the NTNU NanoLab Image contest 2016. We congratulate Verner!

Image description: Self-assembled magnetic nanocubes into superstructured tubes, which again have self-assembled into "leaf-like" micropatterns. Instrument used: Hitachi S5500 S(T)EM.

PhD candidate Molly Bazilchuk received the Best Student Talk Award from the 7th annual workshop of The Norwegian PhD Network on Nanotechnology for Microsystems. We congratulate Molly!

New paper published in Journal of Applied Physics. In the paper we present a method of simultaneous electrical resistance and compression measurements of single micron-size metal coated polymer particles. The method allows fundamental physical insight into the mechanisms of electrical resistance in the interconnect where such particles are applied.

Recently, there has been an increasing interest in silver thin film coated polymer spheres as conductive fillers in isotropic conductive adhesives (ICAs). Such ICAs yield resistivities similar to conventional silver flake based ICAs while requiring only a fraction of the silver content. In this work, effects of the nanostructure of silver thin films on inter-particle contact resistance were investigated. The electrical resistivity of ICAs with similar particle content was shown to decrease with increasing coating thickness. Scanning electron micrographs of ion milled cross-sections revealed that the silver coatings formed continuous metallurgical connections at the contacts between the filler particles after adhesive curing at 150 °C. The electrical resistivity decreased for all samples after environmental treatment for three weeks at 85 °C /85 % relative humidity. It was concluded that after the metallurgical connections formed, the bulk resistance of these ICAs were no longer dominated by the contact resistance, but by the geometry and nanostructure of the silver coatings. A figure of merit (FoM) was defined based on the ratio between bulk silver resistivity and the ICA resistivity, and this showed that although the resistivity was lowest in the ICAs containing most silver, the volume of silver was more effectively utilized in the ICAs with intermediate silver contents. This was attributed to a size effect due to smaller grains in the thickest coating.

CuO/Cu based superhydrophobic surfaces with ordered micro/nanostructures have been prepared via a solution-immersion process combining with photolithography and argon ion beam etching. CuO nanoneedles grow only inside microholes on copper substrate due to delaying effect of both residual photoresist and carbon layer produced during Ar etching. The hierarchical structures and surfaces show a water contact angle of 152⁰, a contact angle hysteresis of 3⁰, and a low water adhesion force of 15 μN, indicating a good superhydrophobicity. The obtained surfaces also keep itself clean from carbon black, chalk dust and water, enabling a great potential in self-cleaning and anti-fouling applications.

Abstract: Present ab initio study was focussed on a response of NiTi martensite to a superposition of shear and tensile or compressive stresses acting normally to the shear planes. The theoretically predicted base-centered orthorhombic (BCO) ground-state structure was found unstable under uniaxial compression and two transformations, one from orthorhombic to a monoclinic symmetry and the other back from monoclinic to orthorhombic symmetry, were observed in the computational model. The former transformation shows that the uniaxial compressive stress of about 4GPa destabilizes the BCO structure by reducing its symmetry to the experimentally observed monoclinic one. However, superposition of small shear stresses remarkably lowers the compressive stress necessary for this destabilization. The latter transformation then draws the crystal lattice to the B19 structure. The theoretical shear strength of NiTi martensite was subsequently computed as a function of the normal stress. The results obtained show that the e_ect of the normal stress is surprisingly opposite to that calculated for NiTi austenite and other cubic metals, i.e., that the shear strength is lowered by the compressive normal stress and vice versa.

Abstract: The degrading effect of hydrogen on high strength steels is well recognized. The hydrogen degradation is dependent not only on hydrogen content, but also on geometric constraints or equivalently, level of stress triaxiality, which means the hydrogen degradation locus is not likely to be a unique material property. Experimental data on notched tensile tests reported by Wang et al. are analyzed via cohesive zone modelling, and a cohesive strength based uniform hydrogen degradation law is proposed upon normalization of hydrogen degradation loci with different specimen geometries. Since the e ects of hydrogen content and geometric constraints are decoupled during normalization, the proposed law is applicable to all the specimen geometries as a material property. This law is subsequently applied to simulate the constant loading tests performed on the same material. Excellent agreement is observed between the simulation and test results in terms of incubation time for fracture initiation and highest permissible initial hydrogen content. The inconsistency observed in one of the cases is discussed, suggesting that the effects of strain rate and stress relaxation need to be taken into account in order to improve the transferability of the degradation law calibrated from tensile tests to constant loading situations. 

NTNU Nanomechanical Lab received two projects from Research Council of Norway through the highly competitive FRIPRO program. One project titled "Engineering Metal-Polymer Interface for Enhanced Heat Transfer (HEFACE)” is led by Jianying He and funded by the FRINATEK Young Research Talents program. Another FRINATEK project “Towards Design of Super-Low Ice Adhesion Surfaces (SLICE)” is led Zhiliang Zhang. Contracts for both projects have been signed.

Engineering Fracture Mechanics, 2015, v150, p209-221

The mechanical behavior of Ni-coated polyethylene (PE) nanoparticles subjected to compression loading is systemically investigated by classical molecular dynamics simulation. Results show that the Ni coatings on PE nanoparticles lead to a densification of particle surface and remarkably enhance the compression strength. The particle size-effect and the coating thickness effect on the compression responses in terms of compressive strength and particle burst are observed. Burst of Ni-coated PE nanoparticles initiates at the high stress-concentrated grain-boundaries zone of polycrystalline Ni-shell, and propagates along the compression direction to the flattened contact surface, resulting in release of core PE molecules.

Sediment-hosted gas hydrates have profound impacts on global energy sources and climate change. Their mechanical properties play a crucial role in gas recovery and understanding their evolution in nature, however, the deformation mechanisms of gas hydrates have not yet been elucidated owing to the difficulties in experimental measurements. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected ductile ultimate strength as a result of crossover from grain-size strengthening to weakening in polycrystals. Upon uniaxial depressurization, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilizes the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insights into the destabilization mechanisms of gas hydrates caused by deformation beyond the conventionally thermodynamic instability.

Axi-symmetric and 3D unit cell analyses with continuous non-proportional loading paths are performed to investigate the path dependence of the fracture loci in a 3D space. The loading pattern utilized is the generalization of a number of non-proportional paths recorded in real tests. Failure of the unit cell is predicted when localization of plastic flow occurs, and the failure strains are plotted against the strain history averaged stress triaxiality and Lode parameter to construct fracture loci in a 3D space. The fracture locus with a non-proportional loading path deviates from that with a proportional loading path along the axis of stress triaxiality and becomes non-monotonic in high triaxiality regime. Meanwhile, such deviation occurs only when a certain level of triaxiality is reached. Agreement with the proportional locus as well as monotonicity maintains over a large range of stress triaxiality that covers most cases in reality, as long as the non –proportionality of the loading path is su_ciently low. This provides the rationale for utilizing the average triaxiality based fracture locus as an acceptable approximation in practice. Deviations of the non-proportional loci along the axis of Lode parameter are also observed. Further study on the Lode history dependence suggests using the final value of Lode parameter instead of the averaged one as the Lode axis in the fracture loci, which can alleviate the severity of path dependence for the loading patterns concerned. Based on these results, the e_ectiveness of the average stress state based fracture loci reported in the literature is discussed.

Herein, the assembly behavior of gold nanoparticles (AuNPs) at oil/water interface is studied by surface-enhanced Raman scattering (SERS) spectroscopy. Two selected chemicals (1-dodecanethiol (DDT) and tetramethylammonium ion (TMA⁺)) are applied to tune the surface properties of AuNPs and the corresponding assembly behaviors at oil/water interface are thoroughly investigated. Various AuNPs films, namely sparse 2D film, perfect monolayer, and multilayers are obtained. The SERS spectra analyses show that the surface composition of AuNPs is strongly dependent on the chemical environment around AuNPs and results in different morphologies of AuNPs film at oil/water interface. Accordingly, we propose a rational relationship between AuNPs assembly behavior at oil/water interface and their surrounding chemical environment, and thus reveal the physical mechanism underlying the nanoparticle assembly.

Molybdenum disulfide (MoS2) nanostructures have received considerable research attentions due to their outstanding physical and chemical properties. Recently, a form of MoS2 ring structure exhibiting unique transport properties has been experimentally identified. Herein, we present the first report describing direct molecular dynamics (MD) simulations of structural instability and mechanical properties of hypothetical MoS2 nanotube (NT) toroidal nanostructures. Nanorings with small MoS2 NTs' diameter retain their circular shape because of higher bending stability of NTs, while for those with large diameter of MoS2 NTs buckling/kinking and displacive phase transformation appear to effectively reduce bending stress as a mechanism for stabilizing the nanorings. However, the nanorings which have to polygonize maintain a circular shape as thick multi-walled inner nanorings are presented. Furthermore, mechanical responses of various nanoweaves (nanochains, nanomailles, and nanochainmailles) by linking nanorings together are also studied. Results show that Young’s modulus, stretchability and tensile strength of such nanoweaves depend not only on the helicity of MoS2 NTs but also the woven pattern. For example, nanostructures with 4-in-1 weave of nanorings exhibit much higher tensile strength and stiffness but lower extensibility than those with 2-in-1 weave. The finding suggests that MoS2 NT nanorings and their woven hierarchical structures may be used in the development of new flexible, light-weight electromechanical and optoelectronic nanodevices.

During October, 2015 we have started an international cooperation with CEITEC - Brno Univ. of Technology from the Czech Republic within the frame of the Norway Grants (proj. num. NF-CZ07-ICP-3-199-2015). The aim of this project is an international collaborative work between the Nanomechanical lab and the group of Advanced Metallic Materials and Metal Based Composites at the CEITEC in Brno. The main project objectives are focused to a collective work on ongoing research projects at the Nanomechanical lab, high quality scientific publications and knowledge exchange in atomistic modelling of materials properties using Ab initio and MD calculations. The project period is planned for twelve months and, at its beginning, Dr. Petr Šesták from CEITEC (the fifth person from the left site) has visited the Nanomechanical lab.

Selective growth of metallic micro/nanostructures on desired micropatterns was achieved via a simple solution-immersion process. Interestingly, metallic micro/nanostructures directly grow only inside the hollow copper micropatterns due to the different surface properties. Furthremore, these hierarchical micro/nanostructured Cu/CuO surfaces possess superhydrophobicity, low water adhesion forces and self-cleaning properties. 

Abstract: The extraordinary deformation and loading capacity of nine different [∞]carbohelicene springs under uniaxial tension up to their fracture were computed using the density functional theory. The simulations comprised either the experimentally synthetized springs of hexagonal rings or the hypothetical ones that contained irregularities (defects) as, for example, pentagons replacing the hexagons. The results revealed that the presence of such defects can significantly improve mechanical properties. The maximum reversible strain varied from 78% to 222%, the maximum tensile force varied in the range of 5 nN to 7 nN and, moreover, the replacement of hexagonal rings by pentagons or heptagons significantly changed the location of double bonds in the helicenes. The fracture analysis revealed two different fracture mechanisms that could be related to the configurations of double and single bonds located at the internal atomic chain. Simulations performed with and without van der Waals interactions between intramolecular atoms showed that these interactions played an important role only in the first deformation stage.

A delagation from Singapore Polytechnic visited NTNU Nanomechanical Lab to discuss the cooperation in Arctic Icephobic Coatings.

MSc student Emil Stokkeland developed a four wire measurement setup to investigate the electric properties of a silver coated polymer sphere, Ø 30 µm, with a 150 nm silver coating. Copyringht©Emil Stokkeland.

MSc student Jon Oddvar Kolnes developed  Carbon nanotube arrays for anti-icing applications. Cross-sectional edge image of a desired structure: Carbon nanotubes, grown with an iron catalyst on a silicon substrate, with an aluminium oxide barrier layer. Copyringht©JOn Oddvar Kolnes.

Contact resistance measurement of nano-coated polymer particles. Experiment conducted by  master student August Emil Stokkeland and PhD student Sigurd Pettersen.

Associate Professor Jianying He became one of members of NTNU's pilot Star Program (2014-2018) which consists young and promising research talents from whole NTNU. Jianying He represents the IVT faculty! We congratulate!!!!

The NTNU Nanomechanical Lab logo made on a polymer parictcle by using the Focused Ion Beam. Image created by PhD student Sigurd Pettersen.

A NTNU-SINTEF-University of Oslo cooperation project titled "Hydrogen-induced degradation of offshore steels in ageing infrastructure - models for prevention and prediction(HIPP)" received 17 mkr from the Norwegian Research Council of Norway. The project is led by Zhiliang Zhang and will start from Spring 2014 and end Des 2017.

The primary objective of the HIPP project is to develop a model framework which describes and couples environment-assisted hydrogen degradation mechanisms at different length and time scales towards a predictive mechanism-based integrity assessment approach for oil and gas steel infrastructure. 

A new KPN research project "Wettability alteration and improved flow transport by engineered nanoparticles for petroleum application" led by Professor Jianying He received 7.2 mkr funding from the Research Council of Norway's NANO2021 and PETROMAKS II programmes, the industrial partners Det norske oljeselskap ASA and Wintershall Holding GmbH. The project involves two PhD positions and will start spring 2014.

Nanoteknologi får ut gjenstridig olje

In our previous research published at SMALL and JACS, it was demonstrated that defects incorporated in a proper manner can enhance the mechanical properties of helical CNTs. However, in our paper recently published at Applied Physics Letters  we found that the thermal conductivity of the same type HCNTs will be dramatically reduced by the presence of defects. Details see the paper.

In a recent paper published at Journal of Mechanics and Physics of Solids (JMPS) a  novel constitutive model was presented.

The traditional model of Alexander and Haasen poses several limitations. We introduce in this work a novel constitutive model for covalent single crystals and its implementation into a rate-dependent crystal plasticity framework. It is entirely physically based on the dislocation generation, storage and annihilation processes taking place during plastic flow.

We show in a recent paper published at the high impact jounral Small that helical nanotubes could be tougher than the toughest materials.

Helical carbon nanotubes with intentionally incorporated non-hexagonal defects have unexpectedly high toughness and plasticity, in addition to the well-recognized extreme elasticity. The obtained toughness approaches 5000 J g^-1 with decreasing spring radius. The high toughness originates from the plastic nanohinge formation as a result of distributed partial fractures. A strong spring size effect, contradictory to the continuum solution, is precisely described by an atomistic bond-breaking model.

Carbon-based nanomaterials have attracted significant attention due to their unique optical, electrical, thermal and mechanical properties. In this study, various multi-dimensional graphitic architectures are constructed by spanning fullerenes with carbon nanotube (CNT) super-bonds. The mechanical properties of these novel architectures are systematically investigated by full atomistic simulations. Surprising negative Poisson's ratio observed in 2D and 3D networks is revealed to originate as a result of curvature-flattening or rigid mechanical model. The magnitude of Poisson's ratio is strongly dependent on the level of strain, CNT length as well as temperature. The insight on the deformation mechanism of these periodic graphitic nanostructures will facilitate the integration of low-dimensional materials towards high-dimensional organized structures to realize targeted multi-functional properties.

Aker Solutions is co-funding a research programme together with the Norwegian University of Science and Technology on the subjects of materials technology and mechanical engineering. The aim of the programme is to understand and assess high-performance materials for use in high pressure, high temperature and corrosive environments. A project agreement between Aker Solutions and NTNU was signed on the 12th of Oct 2013. The project involves two PhD positions.

Photo: professor Zhiliang Zhang (NTNU), Jim Stian Olsen (Aker Solutions), Kjartan Pedersen (Aker Solutions), Krista Amato (Aker Solutions), professor Roy Johnsen (NTNU).

It has been found that the crosslinking density significantly influences the fracture property as well as the failure morphology. Slightly crosslinked particles become permanently deformed after compression, while highly crosslinked ones are entirely fragmented once a critical strain is reached.

Our recent work has been published at the premier Journal - the Journal of  the American Chemical Society (JACS). 

 There is a surging interest in 3D graphitic nanostructures which possess outstanding properties enabling them to be prime candidates for a new generation of nanodevices and energy-absorbing materials. Here we study the stretching instability and reversibility of tightly-wound helical carbon nanotubes (HCNTs) by atomistic simulations. The inter-coil vdW interaction-induced flattening of HCNT walls prior to loading is constrained by the defects coordinated for the curvature formation of helices. The HCNTs exhibit extensive stretchability in the range from 400% to 1000% as a result of two distinct deformation mechanisms depending on HCNT size. For small HCNTs tremendous deformation is achieved by domino-type partial fracture events, whereas for large HCNTs this is accomplished by stepwise buckling of coils. The formation and fracture of edge-closed graphene ribbons occur at lower temperatures while at elevated temperatures the highly distributed fracture realizes a phenomenal stretchability. Results of cyclic stretching-reversing simulations of large HCNTs display pronounced hysteresis loops, which produce large energy dissipation via full recovery of buckling and vdW bondings. This study provides physical insights into the origins of high ductility and superior reversibility of hybrid CNT structures

In this work, the role of five-fold twin boundary on the structural and mechanical properties of fcc Fe nanowires is explored by classical molecular dynamics simulation with well-established embedded atom method (EAM) potential. Size-dependent mechanical properties of nanowires attributed to the surface tension in the literature. We identified, however, a new origin to the remarkable size effect on Young's modulus and tensile strength upon loss of elasticity of the novel five-fold twinned Fe nanowires. The inhomogeneous intrinsic stress due to the built-in five-fold twin structure contribute to the monatomic Young's modulus – elastic response per atom – distributed over the cross section, and results in size dependent Young's modulus. The five-fold twin boundaries strengthen the smaller nanowires by prohibiting dislocation nucleations in the fcc structure. Instead, phase transformation of fcc → bcc occurs at the elastic limit, and creates multi-grain structure that exhibits ductility. Yielding by dislocation nucleation is exclusively observed for larger nanowires at elevated temperature, followed simultaneously by the fcc → bcc phase transformation. Because of the five-fold twin structure, the central region of the nanowires experiences local expansion under elastic tension (negative Poisson's ratio). The critical stress/strain upon los of elasticity displays U-type temperature dependence.