No ice, ice, baby!

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

Congratulations to Sigrid!

Hun blir nytt medlem i NTNU-styret

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.