New article on polycrystalline Molybdenum disulfide published in Nano Letters by Dr. Jianyang Wu
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.
New article in Acta Materialia by Dr. Kai Zhao
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.
New article in Physical Chemistry Chemical Physics by PhD student Xiao WANG
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.
New article in International Journal of Mechanical Sciences by PhD candidate Shengwen Tu
A method for determining material’s equivalent stress-strain curve with any axisymmetric notched tensile specimens without Bridgman correction.
Deformation and Stability of Core–Shell Microgels at Oil/Water Interface by Dr. Yi Gong
This paper investigates the behavior of P(NIPAM-co-AAc)@PTFMA core–shell microgels at the decane/water interface. The microgels were deposited at the interface to form a monolayer film, and the film’s compression behavior was measured using Langmuir trough. Typical compression isotherm embodies four regimes, weak interaction between microgels in regime I, viscoelastic deformation in regime II, elastic deformation of microgels with thin shell while still viscoelastic deformation with thick shell in regime III. Minor desorption of microgels takes place in regime III and massive in regime IV. The critical interfacial pressure for desorption of microgels is identified in the range of 43–45 mN/m, independent of the shell thickness. It shows that the deformability of the surrounding part rather than the mean deformation of the microgels dominates their stability at the interface. These results illustrate the behavior of microgels at the interface under loading, and deepen the understanding of the stability of microgel-stabilized emulsion.
New findings in organic semiconducting nanobelts by Dr. Mao Wang
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.
New article in Engineering Fracture Mechanics by PhD candidate Shengwen Tu
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
Super-Low Ice Adhesion Surfaces-Designed and Fabricated by NML
Research news Controlling problem ice — by cracking it
Research news Stopping problem ice -- by cracking it
Research news Får problematisk is til å sprekke opp
NTNU Nanomechanical Lab has recently reached a milestone in developing anti-icing technology in the FRINATEK project SLICE!
By introducing a novel concept, we reached for the first time for pure PDMS materials the super-low ice adhesion 5.7 kPa! The ice adhesion strength for common outdoor steel or aluminium surfaces is around 600-1000 kPa. A new paper published today at Soft Matter: Multiscale crack initiator promoted super-low ice adhesion surfaces.
New article in Journal of Physical Chemistry C by PhD candidate Mao Wang
Angle-Dependent Photoluminescence Spectroscopy of Solution-Processed Organic Semiconducting Nanobelts
Journal of Physical Chemistry C, DOI: 10.1021/acs.jpcc.7b02958
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 ‘magic’ notched tensile specimen that makes the Bridgman’s correction redundant - a new paper by PhD student Shengwen TU
Engineering Fracture Mechanics
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.
New article in Energies by PhD candidate Xiao Wang
Effect of Nanoparticles on Spontaneous Imbibition of Water into Ultraconfined Reservoir Capillary by Molecular Dynamics Simulation
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.
New article in Physical Chemistry Chemical Physics by PhD candidate Bjørn Strøm
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 article in Journal of Eletronic Materials by Dr. Sigurd Pettersen
Room-Temperature Curing and Grain Growth at High Humidity in Conductive Adhesives with Ultra-Low Silver Content
Can Hydrogen Embrittlement of High Strength Steels Even Occur at Low Temperature? New published paper gave an answer!
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 T28J, 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.
What are the Common Room Temperature Characteristics of Low Ice Adhesion Surfaces?
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.
New article in Journal of Applied Physics by Dr. Sigurd Pettersen
Investigation of thermal transport in polymer composites with percolating networks of silver thin films by the flash diffusivity method
by Sigurd R. Pettersen, Shijo Nagao, Helge Kristiansen, Susanne Helland, John Njagi, Katsuaki Suganuma, Zhiliang Zhang, and Jianying He*
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.
Fracture Mechanics and Nanomechanics
Tel: +47 73592530
Tel: +47 73594686
Adjunct Associate professor
Fracture Mechanics and Materials Technology
Postdoc research fellow
Postdoc research fellow
Fracture and Fatigue-Measurement Method