Welcome to the NTNU Nanomechanical Lab


2018-10-11

T-stress effect on brittle-to-ductile transition by dislocation mobility

– Yang Li, Xiaobo Ren, Jianying He, Zhiliang Zhang

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.


2018-09-28

How can nanofluids enhance oil recovery

– Xiao Wang, Senbo Xiao, Zhiliang Zhang and Jianying He
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.

2018-09-19

New methodology for creating nanoporous materials

– Verner Håkonsen, Gurvinder Singh, Jianying He and Zhiliang Zhang
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.

2018-08-24

Atomistic Dewetting Mechanics revealed by molecular dynamics

– Senbo Xiao, Zhiliang Zhang, Jianying He
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. 

2018-08-16

Measuring electrical resistance of spherical thin films by van der Pauw methods

– Molly Bazilchuk, Otto Magnus Evenstad, Zhiliang Zhang, Helge Kristiansen and Jianying He
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.

2018-08-10

ISFAM2018 was successfully organized in Trondheim.


2018-08-08

Bioinspired Lubricant-Regenerable Icephobic Slippery Liquid-Infused Porous Surfaces

– Yizhi Zhuo, Feng Wang, Senbo Xiao, Jianying He and Zhiliang Zhang
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.

2018-06-15

Fabricated Anti-icing Coating Reached Ice Adhesion Lower than 1 kPa

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


2018-04-21

Joint article with Kyoto University by PhD student Molly Bazilchuk

– Molly S. Bazilchuk, Takashi Sumigawa, Takayuki Kitamura, Zhiliang Zhang, Helge Kristiansen, Jianying He
In situ imaging and analysis of the mechanical behavior of micron‐sized metal‐coated polymer particles under compression is reported. A nanoindentation set‐up mounted in a scanning electron microscope is used to observe the deformation and fracture of 10 μm polymer spheres with Ni, Ni/Au, Au, and Ag coatings. The spheres fracture in one of two metallization‐dependent modes, brittle, and ductile, depending only on the presence of a nickel layer. The metal coating always fractures parallel to the direction of compression. The mechanical properties up to the point of coating fracture are rate‐dependent due to the viscoelastic polymer core. Metal‐coated polymer spheres are an important composite material in electronics packaging, and this study demonstrates a novel method of evaluating the mechanical properties of particles to tailor them for electronic materials.

2018-04-12

PhD candidate Sigrid Rønneberg is elected new representative on the NTNU Board

Congratulations to Sigrid!

Hun blir nytt medlem i NTNU-styret


2018-03-29

Review article on Water Condensation and CO2 Condensation by PhD candidate Ingrid Snustad

– Ingrid Snustad, Ingeborg Treu Røe, Amy Brunsvold, Åsmund Ervik, Jianying He, Zhiliang Zhang

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.


2018-03-17

New article in ACS Applied Materials & Interfaces by PhD candidatet Yizhi Zhuo

– Zhuo, Yizhi; Håkonsen, Verner; He, Zhiwei; Xiao, Senbo; He, Jianying; Zhang, Zhiliang

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.


2018-03-07

New article in Fatigue & Fracture of Engineering Materials & Structures by PhD candidate Shengwen Tu

– Shengwen Tu, Xiaobo Ren, Tore Kristensen, Jianying He, Zhiliang Zhang

 

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.


2018-02-01

New article on polycrystalline Molybdenum disulfide published in Nano Letters by Dr. Jianyang Wu

– Jianyang Wu, Pinqiang Cao, Zhisen Zhang, Fulong Ning, songsheng zheng, Jianying He, and Zhiliang Zhang

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.


2018-01-27

New article in Acta Materialia by Dr. Kai Zhao

– Kai Zhao, Jianying He, A.E. Mayer, Zhiliang Zhang

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.


2018-01-12

New article in Physical Chemistry Chemical Physics by PhD student Xiao WANG

– Xiao Wang, Zhiliang Zhang, Ole Torsæter, Jianying He

 

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.


2018-01-02

New article in International Journal of Mechanical Sciences by PhD candidate Shengwen Tu

– Shengwen Tu, Xiaobo Ren, Jianying He, Zhiliang Zhang

 

Large deformation analyses of problems such as plastic forming, ductile fracture with finite element method need a full range of material's equivalent stress-strain curve or flow stress-strain curve. The equivalent stress-strain curve determined from the smooth round bar specimen should be corrected after diffuse necking, since tri-axial stress state occurs in the neck. The well-known Bridgman correction method is a candidate, however, it is not accurate as the strain increases. Furthermore, it is impossible to measure the equivalent stress-strain curve of each individual material zone in a weldment with cross weld tensile tests. To cope with these challenges, a correction function and an associated test procedure are proposed in this study. With the proposed procedure, the true stress-strain curve from any axisymmetric notched tensile specimen can be converted to the material's equivalent stress-strain curve accurately and no Bridgman correction is needed. The proposed procedure can be applied to both perfectly plastic and strain hardening materials. The equivalent stress-strain curve of each individual material zone in a weldment can also be measured with the proposed procedure.


Members

Zhiliang Zhang

Professor 

Fracture Mechanics and Nanomechanics

Zhiliang.zhang@ntnu.no

Tel: +47 73592530

 

Jianying He 

Professor 

Nanomechanics

jianying.he@ntnu.no

Tel: +47 73594686

Helge Kristiansen 

Adjunct Professor 

Nanomechanics

helge@conpart.no

 

Jim Stian Olsen

Adjunct Associate professor

Fracture Mechanics and Materials Technology

jim.stian.olsen@akersolutions.com

 

Postdoc research fellow
Atomistic modeling
 

Tong Li

Postdoc research fellow

Nanomechanics

tong.li@ntnu.no

 

Jianying Wu

Postdoc research fellow

Atomistic modeling

jianyang.wu@ntnu.no

 

Researcher
Fracture mechanics

 

Øyvind Othar Aunet Persvik

PhD student

Fracture and Fatigue-Measurement Method

oyvind.persvik@ntnu.no

 

PhD student
Nanomechanics for EOR
 
PhD student
Fracture mechanics
 
PhD student
Nanomechanics
 
PhD student
Nanomechanics
 
PhD student
Nanomechanics
 

PhD student
Nanomechanics
 
PhD student
Nanomechanics
 
PhD student
Nanomechanics
 
PhD student
Fracture mechanics
 
PhD student
nanomechanics
 
PhD student
nanomechanics
 
PhD student
nanomechanics
 
PhD student
Nanomechanics
 
Siqi Liu
PhD student
Nanomechanics