Transport of Janus nanoparticles in confined channels
New article in Environmental Science Nano
Janus nanoparticles (JNPs) have drawn significant attention due to their unique surface with dual character. The transportation of two-phase fluids with JNPs in an ultra-confined channel was studied by molecular dynamics (MD) simulations. The results indicated that the fluid displacement was hindered by JNPs, which was significantly dependent on the concentration of the NPs self-assembled at the fluid interface; compared to the NPs with uniform surface properties, the determining migration states for JNPs that influenced the displacement process were self-assembled at the fluid interface and aggregated in the three-phase contact region; this modified the interfacial tension and the three-phase contact angle. These key migration states were validated by the potential of the mean force of JNPs transporting from water to the oil phase. The capillary pressure calculated by the local pressure distribution was found to be the key factor driving the displacement process of the nanofluids with JNPs.
Liquid Layer Generators for Excellent Icephobicity at Extremely Low Temperatures
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-Informed Gurson Model Proposed
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.
Ice Type Strongly Affects its Adhesion to a Solid
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.
International Symposium on Materials for Anti-Icing - very successful
The NML successfully organized the 1st International Symposium on Materials for Anti-icing! International experts from USA, Canada and Europe, researchers and students from NTNU and NML, together with industrial partners from more than 8 Norwegian and Scandinavia anti-icing relevant companies engaged in very lively discussions and debates about the state-of-the-art as well as future perspectives of anti-icing materials during the 2 days symposium.
Understanding the role of hollow sub-surface structures in reducing ice adhesion strength
In our previous studies, incorporation of hollow sub-surface structures which act as macro-scale crack initiators has been shown to drastically lower the ice adhesion on PDMS surfaces. In this study, the effects of hollow sub-surface structure geometry, such as the heights, shapes, and distributions, as well as the directions of the applied shear force, are experimentally investigated. Our results show that the number of potential macro-scale crack initiation sites dictates ice adhesion strength. The directions of the applied shear force also influence the ice adhesion strength when the potential crack length is dependent on the applied shear force direction. The inter-locking effect between ice and the coating, caused by the pre-deformation, needs to be considered if one of the dimensions of the hollow sub-surface structures approaches the millimeter scale. These results improve the understanding of the role of hollow sub-surface structures in reducing ice adhesion, providing new insights into the design principles for multi-scale crack initiator-promoted icephobic surfaces.
Seasonal greetings from NML
Phase transition enabled durable anti-icing surfaces and its DIY design
Anti-icing surfaces are crucial to all cold-condition applications, ranging from nano- to macro scales. The pitcher-plants-inspired slippery liquid-infused porous surfaces (SLIPS) show positive effects in lowering ice adhesion strength. However, the longevity and durability of SLIPS applied for anti-icing purpose are of great challenge. Hereby we propose to use phase transformable oil lubricant in the design of SLIPS to overcome this tough barrier. The underlying mechanism relies on the physical property of lubricants that enables the transformation to solid state before water freezing. Peanut oil infused porous PDMS substrates show low ice adhesion strength (4.45 ⁓ 22.43kPa) as well as excellent durability. For selected samples, low ice adhesion strength around ⁓16kPa maintains after 30 icing/de-icing. Phase transformable slippery liquid infused porous surfaces (PTSLIPS) also suit to various substrates regardless of hydrophobic or hydrophilic materials, wide pore size distributions and diverse pore morphologies. We show the possibility of creating anti-icing surfaces by Do-It-Yourself (DIY) with porous materials (wipers, foams and paper) that can be found easily from household garbage and lab supplies. The results of this work motivates designing numerous anti-icing surfaces from various substrates. The idea of phase transition oil is also promising in other fields of SLIPS, like heat transmission, water collection, cell growth, and so on.
T-stress effect on brittle-to-ductile transition by dislocation mobility
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.
How can nanofluids enhance oil recovery
New methodology for creating nanoporous materials
Focused ion beam milling of self-assembled magnetic superstructures: an approach to fabricate nanoporous materials with tunable porosity
Atomistic Dewetting Mechanics revealed by molecular dynamics
Physical Chemistry Chemical Physics -- PCCP, 2018, 20: 24759-24767
Measuring electrical resistance of spherical thin films by van der Pauw methods
Bioinspired Lubricant-Regenerable Icephobic Slippery Liquid-Infused Porous Surfaces
One-Step Fabrication of Bioinspired Lubricant-Regenerable Icephobic Slippery Liquid-Infused Porous Surfaces (SLIPS)
NTNU Fabricated Anti-icing Coating Materials 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.
Joint article with Kyoto University by PhD student Molly Bazilchuk
Review article on Water Condensation and CO2 Condensation by PhD candidate Ingrid Snustad
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.
New article in ACS Applied Materials & Interfaces by PhD candidatet Yizhi Zhuo
Enhancing mechanical durability of icephobic surfaces by introducing autonomous self-healing function
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.
New article in Fatigue & Fracture of Engineering Materials & Structures by PhD candidate Shengwen Tu
Study of low-temperature effect on the fracture locus of a 420-MPa structural steel with the edge tracing method
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.
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.
Fracture Mechanics and Nanomechanics
Tel: +47 73592530
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Adjunct Associate professor
Fracture Mechanics and Materials Technology
Postdoc research fellow
Postdoc research fellow
Fracture and Fatigue-Measurement Method