Self-healing Phenomenon Research Articles

Use of X-ray computed tomography for studying the desiccation cracking and self-healing of fine soil during drying–wetting paths

Managed aquifer recharge is an efficient approach using surface water for groundwater recharge. However, soil clogging in the infiltration systems represents a critical problem that reduces the efficiency of recharge systems. In recent years, much research has been conducted to investigate clogging in subsurface and surface soil (cake). Cake prevents infiltration into the water table and can drastically reduce the recharge rate. In arid and semi-arid areas, alternation of humid and dry seasons has an effect on cake cracking. In the present research, the propagation of desiccation cracking and self-healing of unconsolidated soil (cake), placed in Plexiglas columns, was investigated using X-ray computed tomography (CT) during drying–wetting (DW) paths. The results showed that during the drying path, cracks initiated at the base of the soil by friction with the bottom of the column and then propagated to the top surface of the cake. As the drying time increased, evaporation at the top surface led to more cracks growing from the surface than from the bottom of the cake. However, the cracks at the bottom tended to stabilize because the water content was greater than at the top surface. The initiation, propagation, and expansion of the cracks developed in the saturated condition of the cake. During the wetting path, some cracks were closed while others appeared. The cracks tended to close progressively with wetting time, highlighting the self-healing phenomenon, probably due to the high plasticity index of cake soil. The results showed that cake swelling is mainly related to an increase of the void ratio due to a decrease in suction between particles. The results demonstrate the capacity of the X-ray CT technique to investigate the evolution of cracks during DW paths.

Intrinsic Self-healing Characteristics of Electrical Tree in Silicone Rubber

The self-healing phenomenon of electrical tree in traditional silicone rubber (SiR) insulation material, which is called intrinsic self-healing, is demonstrated and investigated in this paper. For initial SiR samples with electrical trees, with the removal of applied voltage, it is found that some electrical tree branches naturally disappear in the visual field with time. A less branched morphology and decreasing fractal dimension of electrical tree are thus observed. The effects of temperature on the intrinsic self-healing characteristics are explored and the intrinsic self-healing process can be considered as taking place in three stages based on the temperature-dependent self-healing rate. It is proposed that the intrinsic self-healing behaviors are closely related to the gas motion in the tree channels and the retractive force of SiR, which is affected by the conformation of molecular chains. Re-growth performance of electrical tree is conducted on the self-healed samples and many tiny branches at the sidewall of main channels are found in the repeated electrical trees, while the tree length remains unchanged. Furthermore, some electrical trees vanish in self-healing regions after the re-growth process, which can be attributed to the shielding effect of dense branches and the reconstruction of some hydrogen bonds between nano-silica and molecular chains of SiR.

High-Strength, Strongly Bonded Nanocomposite Hydrogels for Cartilage Repair

Polyacrylamide-based hydrogels are widely used as potential candidates for cartilage replacement. However, their bioapplicability is sternly hampered due to their limited mechanical strength and puncture resistance. In the present work, the strength of polyacrylamide (PAM) hydrogels was increased using titanium oxide (TiO2) and carbon nanotubes (CNTs) separately and a combination of TiO2 with CNTs in a PAM matrix, which was interlinked by the bonding between nanoparticles and polymers with the deployment of density functional theory (DFT) approach. The synergistic effect and strong interfacial bonding of TiO2 and CNT nanoparticles with PAM are attributed to high compressive strength, elastic modulus (>0.43 and 2.340 MPa, respectively), and puncture resistance (estimated using the needle insertion test) for the PAM-TiO2-CNT hydrogel. The PAM-TiO2-CNT composite hydrogel revealed a significant self-healing phenomenon along with a sign toward the bioactivity and cytocompatibility by forming the apatite crystals in simulated body fluid as well as showing a cell viability of ∼99%, respectively. Furthermore, for new insights on interfacial bonding and structural and electronic features involved in the hydrogels, DFT was used. The PAM-TiO2-CNT composite model, constructed by two interfaces (PAM-TiO2 and PAM-CNT), was stabilized by H-bonding and van der Waals-type interactions. Employing the NCI plot, HOMO-LUMO gap, and natural population analysis tools, the PAM-TiO2-CNT composite has been found to be most stable. Therefore, the prepared polyacrylamide hydrogels in combination with the TiO2 and CNT can be a remarkable nanocomposite hydrogel for cartilage repair applications.

Calcium hydroxide influence in autogenous self-healing of cement-based materials in various environmental conditions

Abstract Several factors, such as chemical shrinkage or environmental attack can produce concrete cracks. Calcium hydroxide content from the environment or cement hydration can promote the self-healing phenomenon (SHP). This study evaluates the calcium hydroxide concentration influence on the self-healing phenomenon of cement-based materials. Cracked mortars with different types of cement, such as a filler, pozzolanic, and high initial strength, were exposed to four environmental conditions. Titration was the method proposed to determine the amount of calcium hydroxide content leached in the water, and the cracks closure were analyzed by optical microscopy. CPII F (Portland Cement, type II, compound with limestone) samples showed no trend to Ca(OH)2 leached, and to superficial cracking closure. Regarding CPV (Portland cement, type V, high initial strength cement) samples, their behavior showed the highest Ca(OH)2 content to all environmental exposure, and cracking closure to samples in submerged condition. Thus, a significant influence in the cracks´ closure regarding the presence of Ca2+ leached to the exposure environments, and a water source related to this behavior seems to be a primary factor to improve the SHP.

Coupled FEM-microstructural X-ray examination of a controlled internal damage approach for concrete samples

One of the critical factors that govern the technology of concrete self-healing evaluation at laboratory scale is the crack induction pattern within the concrete sample. Within the various techniques of inducing artificial cracks, such as the flexural testing or splitting testing methods, there are limitations of inducing the microcracks homogenously throughout the entire volume of the concrete sample. In this study, an innovative technique is utilised to induce the microcracks at a controlled damage level to further study the self-healing phenomena in concrete at the laboratory scale. By placing a concrete sample into an ad-hoc fabricated steel mould and applying fractional compressive strength, the axial-circumferential pressure induces microcracks in the concrete sample, homogenously. A Finite Element Model was also built to investigate the hypothesis on the cracking pattern at various damage levels; jointly, experimental work was conducted with X-ray µCT images to reconstruct the three-dimensional sections at the various damage levels. Qualitative analyses in relation to the two test methods were conducted. Furthermore, quantitative analyses on the individual—artificially generated—cracks were conducted in terms of the crack size crack geometry variation and the orientation of the newly formed cracks. Results revealed that the proposed crack-inducing methodology is highly efficient to induce uniform cracks in the sample, assisting for the evaluation of concrete self-healing process. The novel method can be adapted to identify the optimised strategies for enhancing the structural performance of concrete, thus facilitating the safe operation of concrete infrastructure.

A new approach for the evaluation of time–temperature superposition effects on the self-healing of bituminous binders

The experimental work described in this paper investigated the coupled effects of rest time and rest temperature on the self-healing potential of bituminous binders. A set of neat and polymer-modified materials were subjected to self-healing tests that included two oscillatory shear loading phases separated by a single rest period. Multiple testing conditions were analyzed by combining various rest times and rest temperatures. Self-healing was quantitatively assessed by introducing two healing indices, based on the magnitude of stiffness and fatigue endurance gain, in which time-dependent artefacts were properly quantified and removed. Experimental results were found to be consistent with the kinetics of self-healing phenomena and underscored the importance of considering rest time coupled with rest temperature for a reliable evaluation of true self-healing properties. A new model was proposed to describe the self-healing performance of bituminous binders and self-healing charts were constructed from generalized logistic functions incorporating the effects of both rest time and temperature.

Self-healing characterization of UHPFRCC with crystalline admixture: Experimental assessment via multi-test/multi-parameter approach

Within the framework of the Research Project ReSHEALience, new advanced Ultra High-Performance Fibre-Reinforced Cement Composites with enhanced durability, hereafter denoted as Ultra High Durability Concretes, are under investigation to characterize their tensile behaviour and durability performance in aggressive conditions, devoting particular attention to the phenomenon of self-healing. Three different mixes are under scrutiny, based on the combination of cement (CEM I or CEM III), slag, small aggregates (sand with a maximum size of 2 mm), and steel or metallic-alloy amorphous fibre. Self-healing capability has been investigated in aggressive environment (namely, under immersion in geothermal water) via 3 different test setups: (1) water permeability test on pre-cracked concrete disks, (2) 4-Point Bending Test – 4PBT on 100 × 100 × 500mm3 prismatic beam specimens and (3) on 25 × 100 × 500mm3 thin beams. In the case of beams, self-healing has been assessed via visual inspection of cracks trough digital microscope and via mechanical re-loading, so to investigate both crack-sealing capability and mechanical recovery. The results of this assessment aim at providing the starting point for a data base finalized at defining a design approach explicitly taking into account self-healing in the evaluation of structural durability. In particular, it has been observed as the adoption of strain-hardening cement composites significantly promotes self-healing phenomenon, thanks to smeared cracking in the tensile region and to consequent low values of crack opening. Self-healing proved to be very effective already after 1 month of curing.

Experimental investigation of bentonite impact on self-healing of clay soils

Compacted clay layers are known as earth dams’ most common impermeable layer. However, owing to the specific nature of clay and Atterberg limits, an increase in age damage, these layers by cracking them. Despite this, the crack-healing property of clay is the main factor in closing external cracks. In this regard, in this study, a new method is used to evaluate clays’ self-healing characteristic by pinhole tests with the variation of the Plasticity Index (PI). For this purpose, three soil samples from the Vanyar Dam located in Iran were treated to obtain the PI between 7 and 26 by adding different percentages of bentonite (i.e., 5%, 10%, 15%, and 20%) and the impact of bentonite percentage increase was investigated on self-healing property and outflow rate of clay soil. Subsequent to geotechnical tests, the obtained results showed that adding bentonite to the soil samples with optimum water content and 2% below it, decreases soil dispersion and makes it non-dispersive. In addition, the self-healing phenomenon was visible and predictable with the increase of bentonite in natural soil; as for the sample with 20% bentonite, this phenomenon was observed from an early age due to high PI and the potential for high inflation. Also, the sample with 20% bentonite and a moisture content of 2% less than the optimum showed the most reduction in the outflow (38%) when compared with the natural soil sample. Therefore, it can be concluded that the PI variation (between 7 and 26) can cause non-dispersivity of clay and increase the self-healing ability.

Healable, highly sensitive LPG sensor based on Ni0.4Zn0.6Fe2O4 nanohybrid grown by autocombustion process

In this study, dual-layered self-healable LPG sensor which can successfully re-establish both of its structural and sensing properties after an external deformation occurred, has been reported. Here Ni0.4Zn0.6Fe2O4 (NZF) works as a highly sensitive LPG sensor which can detect LPG well below the lowest explosion limit (LEL) efficiently. The self-healing phenomenon has been achieved by urea-formaldehyde (U-F) microcapsules encapsulating NZF and linseed oil. The dual-layered self-healable sensing film is prepared by forming a lower layer using urea-formaldehyde (U-F) microcapsules capturing linseed oil and NZF, serving as healing layer, and an upper layer using NZF aiding sensing phenomenon. LPG sensing parameters like sensitivity, sensor response, recovery time and response time were investigated in case of both uncracked sample and healed sample to get a clear idea of how much sensing properties were recovered after healing took place. The minimum response and recovery times of the uncracked film corresponding to 0.5 vol.% LPG concentrations were found to be 28.08 s and 52.46 s respectively, whereas, the same after healing were as 34.39 s and 45.17 s respectively. After healing, ∼ 86.4 % sensitivity was restored. This is the first dual-layered microcapsule-based LPG sensor, which has healing property.

Application of Self-Healing Intelligent Polymer Materials in Anti-aging Research

The self-healing materials come from the bionic research of the self-healing phenomenon of the organism. Self-healing materials can self-repair and restore some mechanical properties when damaged. With the development of material science in recent years, more and more attention has been paid to intellectual materials. Self-healing polymer materials play an important role in the field of intellectual materials because of their unique properties. It is significant for polymer materials in the engineering field to repair micro-cracks in time. This article summarizes the related characteristics of self-healing polymer intellectual materials and the causes and coping modes leading to the aging of polymer materials, as well as the application of self-repairing polymer intelligent materials at home and abroad in anti-aging research, and expounds its broad prospects for development.

Evaluation of Self-Healing in Concrete by Means of Analytical Techniques

In the present work, the self-healing process in concrete was evaluated using analytical techniques. For this purpose, two concrete mixes of different composition (one used as control) were prepared with a water-to-binder ratio of 0.45. The self-healing process was triggered by the introduction in the concrete mix of a commercial expansive admixture (calcium sulfo-aluminate), two dicarboxylic acids, and sodium carbonate salt. After 28 days curing in water, the specimens were artificially cracked (crack width ≤ 900 μm) and then again water-cured for further 60 days until self-healing occurred. The progress of self-healing was investigated with a stereomicroscope at 40, 50, and 60 days, after cracking, to identify the quality and the degree of the healing. The efficiency of the self-healing process was also evaluated using micro-Raman spectroscopy and X-ray micro-computed tomography. Significant reduction in the crack width was observed as a result of a calcite filling, generated during the self-healing process. In some cases (crack width < 400 μm), the crack was completely healed. The experimental methodology used provided new insights into the evolution of the self-healing phenomenon in concrete.

Bioinspired Hydrogels with Muscle-Like Structure for AIEgen-Guided Selective Self-Healing

Living systems, including human beings, animals, and plants, display the power to self-heal spontaneously after being damaged. The self-healing is usually selective, which means that the healing ef...

The leaching mechanism of hydraulic mortars as part of autogenic self-healing process

Historic mortars have exhibited long term durability which has been largely attributed to their self-healing properties. The aim of this work is the study of the autogenic self-healing mechanism as observed in historic mortars as well as the simulation of the phenomenon in laboratory prepared lime-pozzolan and natural hydraulic lime mortars. One of the most significant parameters in autogenous self-healing phenomenon is the source of the calcium ions, which controls the formation of new healing phases inside cracks. In this study, an electrochemically accelerated leaching procedure was designed to examine the influence of mineralogy and curing time on the dissolution of Ca-bearing phases of lime-pozzolan and NHL mortars, which are characterized by high portlandite content. It was found that, apart from portlandite, Ca+2 are also leached from C2S and C–S–H phases and it was shown that this phenomenon takes place in two separate time-frames: an early period (t < 28 d), which is controlled by the portlandite content of the binder and, a later period (t > 28 d) where decalcification of C–S–H and dissolution of secondary portlandite are dominant. The phenomenon was further studied on mortar specimens where healing of micro cracks was observed by precipitation of secondary calcite inside cracks and voids, highlighting the role of leached Ca+2 on the autogenic self-healing mechanism. It is proved that, contrary to what is believed about the autogenic healing, besides portlandite, the contribution of hydraulic phases is also significant. Overall, this study confirms the significant role of the Ca-leaching process in the autogenic self-healing mechanism and highlights the different sources of calcium ions, as well as the different time frames of the events. Finally, the findings elucidate the high self-healing potential of traditional mortars which contribute to their prolonged service life.

Self-Healing Mechanisms for 3D-Printed Polymeric Structures: From Lab to Reality.

Existing self-healing mechanisms are still very far from full-scale implementation, and most published work has only demonstrated damage cure at the laboratory level. Their rheological nature makes the mechanisms for damage cure difficult to implement, as the component or structure is expected to continue performing its function. In most cases, a molecular bond level chemical reaction is required for complete healing with external stimulations such as heating, light and temperature change. Such requirements of external stimulations and reactions make the existing self-healing mechanism almost impossible to implement in 3D printed products, particularly in critical applications. In this paper, a conceptual description of the self-healing phenomenon in polymeric structures is provided. This is followed by how the concept of self-healing is motivated by the observation of nature. Next, the requirements of self-healing in modern polymeric structures and components are described. The existing self-healing mechanisms for 3D printed polymeric structures are also detailed, with a special emphasis on their working principles and advantages of the self-healing mechanism. A critical discussion on the challenges and limitations in the existing working principles is provided at the end. A novel self-healing idea is also proposed. Its ability to address current challenges is assessed in the conclusions.

Influence of thermal oxidative aging on temperature induced self-healing transition of polymer modified bitumens

In this study, the fatigue-healing-fatigue (FHF) test is conducted to investigate influence pattern of temperature and thermal oxidative aging on the self-healing ability of styrene-butadiene-styrene (SBS) modified bitumen (SBS-MB) and polyethylene (PE) modified bitumen (PE-MB). Results display that the optimum healing temperature (OHT) of each bitumen (wherever unaged or aged) is in the phase transition (PT) zone, and is lower than its softening point, followed by its Newtonian flow temperature. Two types of healing crossover temperature points are found in the healing-temperature curves of polymer modified bitumen (PMB) samples with various aging degrees, indicating the complexity of self-healing transition phenomenon in the aged PMB samples. Hence, the evaluation of aging effects on the self-healing ability of PMBs should be performed in a wide temperature range. Furthermore, the self-healing ability of PMBs is gradually weakened with the deepening of aging extents. Notably, the thermal oxidative aging imparts totally different effects to the self-healing ability of SBS-MB and PE-MB at lower temperatures. Moreover, the results of softening point test, fluorescence microscopy test, frequency sweep test and differential scanning calorimeter test convey that the altered phase structure and polymer distribution of PMBs during aging elicit the healing-temperature curves shift and self-healing transition.

Multi-gradient analysis of temperature self-healing of asphalt nano-cracks based on molecular simulation

In order to accurately explain the temperature self-healing mechanism of asphalt nano-cracks, the correlation of temperature self-healing of asphalt nano-cracks with different gradients was studied. Molecular dynamics (MD) on nano-gradient was used to simulate the temperature self-healing process of asphalt nano-cracks, and it was proposed that activation energy plays a key role in self-healing. The experimental results of the atomic force microscope (AFM) and scanning electron microscope (SEM) were interpreted by the asphalt molecular model. Simulation and experiment can be correlated and verified. The temperature self-healing phenomenon of asphalt nano-cracks was studied by three different resolution gradient methods, and the uniformity of different gradients was formed. The results show that the width of nano-cracks has a greater effect on self-healing than the temperature. Aging has a negative effect on the self-healing, and the self-healing of aging asphalt requires longer time and more activation energy. In addition, the shape of the wave crest in AFM is influenced by the number and shape of the voids on the asphalt model surface, and the folding in SEM is also closely related to the concave and convex changes on the asphalt model surface. Therefore, the multi-gradient explanation and unified demonstration of the self-healing mechanism of asphalt nano-cracks widths at different temperatures are carried out in this paper. The research results have certain theoretical and application value for promoting the further development of asphalt materials.

Laboratory Investigation on Physical, Rheological Thermal and Microscopic Characteristics of Water-Foamed Asphalt under Three Environmental Conditions

Applications of water-foamed binders have received widespread attention due to its environmental and economic benefits. This study aims to evaluate the properties of water-foamed asphalt under three environmental conditions (high-temperature evaporation, low-temperature frozen and the freeze–thaw cycle). Conventional physical properties tests, dynamic shear rheometer test (DSR), differential scanning calorimetry test (DSC) and scanning electron microscope test (SEM) are employed to assess the physical, rheological thermal and microscopic characteristics of samples. Conventional physical properties test results showed that the performance of a foamed binder had declined under three environmental processes and the foamed asphalt gradually returned to the characteristics before being foamed, with the increase of process time. A comprehensive evaluation index, deterioration degree was proposed based on the test results and entropy theory, and the deterioration process of the foamed binder under three environmental conditions was quantified. Moreover, freeze–thaw (F–T) cycles had been proved to have the most significant influence on the performance of a foamed binder among three environmental factors, which was a key issue that limits the application and promotion of foamed asphalt in seasonal frozen regions. The DSR test showed that the resistance to high-temperature permanent deformation of the foamed binder was improved after F–T cycles, and the fatigue resistance became worse. The sensitivity analysis of complex modulus and frequency illustrated that foamed asphalt after F–T cycles were more sensitive to the loading frequency and less sensitive to the temperature. The DSC test indicated that the thermal stability of foamed asphalt was improved after F–T cycles. The disappearance of circular “cavitation” observed through SEM test revealed that moisture of foamed asphalt was gradually precipitated and self-healing phenomenon occurred during F–T cycles.

Reliability modeling and optimal random preventive maintenance policy for parallel systems with damage self-healing

Materials with intrinsic self-healing phenomenon possess the ability to heal in response to external random shocks. Introducing a recovery factor to quantitatively measure the damage self-recovery efficiency, this paper designs a self-healing mechanism corresponding to both damage load and shock arrival numbers for a parallel redundant system consisting of multiple non-identical components. From the actual engineering perspective, each shock arriving on the system selectively affects one component or more but not necessarily all units in parallel, and consequently, random shocks are categorized according to their sizes, attributes and affected components. This study investigates novel reliability models and schedules optimal preventive maintenance policies, in which the closed-form reliability quantities are derived analytically and the optimum preventive replacement interval is demonstrated theoretically. In addition, a Nelder-Mead downhill simplex method is introduced to seek the optimal replacement age in minimizing the long-run average maintenance cost rate for the condition system failure distribution is rather complex. A micro-electro-mechanical system (MEMS) whose constitutional materials are integrated by microcrystalline silicon, where polymer binders with self-healing capability are always synthesized, is designed to verify the results we obtained numerically, illustrating the significance of considering damage self-healing phenomena.

Self-healing polymers with nanomaterials and nanostructures

Abstract Self-healing polymers have attracted a lot of attentions in the past two decades, driven by their intriguing applications, new synthetic approaches and understanding of nanoscale mechanism and discovery of nanomaterials. Nanomaterials and nanostructures in polymers provide large surface area, rich functional groups and unique properties that facilitate the healing process. This review provides an introduction of the key studies from a historical standpoint and the chronological advancement of the design philosophy behind self-healing phenomena of polymers. Recent advance in utilizing nanomaterials and nanostructures to facilitate the healing and introduce novel functionalities in self-healing polymers is extensively reviewed. In addition, innovative characterization methods are employed to analyze and understand the underlying polymer chain interactions occurring at the interface at micro- and nanoscale. The knowledge of the healing process at the nanoscopic level and the contribution of nanomaterials and nanostructures in self-healing has greatly advanced the design, fabrication and application of self-healing polymers.

Temperature induced self-healing capability transition phenomenon of bitumens

The temperature may induce the inconsistent order of self-healing capability between two bituminous materials, which could be called as the temperature induced self-healing capability transition phenomenon. To confirm this phenomenon, two base bitumens and four different styrene–butadiene-styrene polymer modified bitumen (SPMB) binders are selected to systematically investigate their self-healing capability through establishing the healing rate temperature (HR-T) curves and healing index temperature (HI-T) curves. Fluorescence microscopy (FM) is applied to study the internal morphology of different SPMB binders. The phase transition (PT) temperature range is determined by the differential scanning calorimeter (DSC) test. Within the PT temperature range, three kinds of special temperatures are discovered, i.e. healing peak temperature (HPT), healing critical temperature range (HCTR) and healing cross temperature (HCT). The HPT and HCTR are both lower than the high temperature PT range, followed by the softening point, and can be applied as the optimum healing temperature to guide the formulation of energy efficient pavement induction healing strategies. The HCT clearly divides the HR-T curves or HI-T curves of bitumens into different parts, thus leading to an opposite order of self-healing capability between two bitumens, which confirms the self-healing capability transition phenomenon. Therefore, the self-healing capability can not be evaluated comprehensively and accurately at only one given temperature. Most notably, it can be inferred that the temperature induced phase structure and property transition of bitumen is the essential reason for the self-healing capability transition phenomenon, which is significantly affected by the bitumen type, polymer type and content.