Taking a microfluidic approach to the production of self-healing construction materials

Abstract

Metal Powder ReportVol. 74, No. 3 Special FeatureFree AccessTaking a microfluidic approach to the production of self-healing construction materialsLívia Ribeiro de Souza, Abir Al-Tabbaa, Damiano RossiLívia Ribeiro de Souza⁎Corresponding author. email: E-mail Address: [email protected] aGeotechnical and Environmental Research Group, Department of Engineering, University of Cambridge, UKSearch for more papers by this author, Abir Al-Tabbaa aGeotechnical and Environmental Research Group, Department of Engineering, University of Cambridge, UKSearch for more papers by this author, Damiano Rossi bDolomite Microfluidics, Royston, UKSearch for more papers by this authorLívia Ribeiro de Souza; Abir Al-Tabbaa; Damiano RossiPublished Online:12 Nov 2021https://doi.org/10.1016/j.mprp.2019.01.001AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareShare onFacebookTwitterLinked InEmail Many composite building materials used in the construction industry – such as concrete – suffer fatigue over time, developing small cracks. Researchers at the University of Cambridge are aiming to overcome this problem by using a microfluidic device to produce microcapsules filled with ‘healing’ agents, which can be added to concrete prior to use. These microcapsules rupture as cracks begin to form, releasing their payload and stabilizing the material.Concrete is used in a wide range of structures, from roads, tunnels and car parks to bridges, buildings, dams and oil wells. However, this popular construction material is not entirely trouble free; over time, there is a tendency for microcracks to develop. When subjected to continuous environmental and mechanical stresses, these microcracks tend to coalesce and form larger cracks, allowing water and aggressive species – such as carbon dioxide, oxygen, chloride ions and sulphates – to enter the structure. This has the potential to cause degradation of the concrete itself, as well as corrosion of steel structural reinforcements.The scale of the problemIn general, there is a tendency for construction codes of practice to regard materials and structural degradation as unavoidable events, largely overlooking the long-term material behaviour and accepting the need for costly maintenance due to structural weathering. In the UK in 2017, £56,498 million was spent on repair and maintenance of existing structures [1]. Across the Atlantic, the United States has an estimated expenditure of between $18 and $21 billion/year for repairs, rehabilitation, strengthening and protection of concrete structures [2], with associated costs due to steel corrosion reaching $23 billion/year [3,4].Taking a self-healing approachRecently, researchers have begun to investigate self-healing concepts as a means of overcoming the issues associated with crack propagation and maintenance [5,6], as the formation of cracks is not problematic if is counteracted by a system that detects the damage and self-initiates a repair process [7]. One promising approach to self-healing is the addition of microcapsules containing a healing agent to the cement mix, which has been reported to successfully heal cracks up to 1 mm in size [8]. When a crack develops, the shell of the capsule ruptures, releasing the healing agent and effecting a repair (Fig. 1).Figure 1 Dr Lívia Ribeiro de Souza with a Dolomite Microfluidics system.To date, several microencapsulation techniques have been explored to produce aqueous or organic core materials – such as organic precursors for polymeric healing, bacterial spores suspended in an organic substrate, and mineral healing agents – including coacervation, in-situ polymerization and sol–gel reactions. However, it remains a concern that insufficient interfacial bonding between the microcapsule shell and the cement mix may lead to debonding, rather than rupture, of the capsules [9]. In general, the presence of hydrophilic groups – which are compatible with the water-based cementitious matrix – ensures good interfacial bonding between a polymeric material and cement. Conversely the formation of, for example, a poly(ureaformaldehyde) – PUF – shell relies on the deposition of water-insoluble prepolymer in the oil/water interface, which ultimately becomes highly cross-linked, forming a shell and encapsulating the core material. However, despite the inclusion of polar hydroxyl, amino and carboxyl groups in the PUF shell, the interfacial bonding is not always efficient, and debonding can occur. Similarly, the use of non-polar phenol–formaldehyde groups results in poor interfacial bonding, with the potential to de-bond on crack formation. Coacervation and sol–gel reactions offer alternative routes to the production of microcapsules, generating a shell that promotes interfacial bonding via chemical reactions [10]. However, conventional bulky emulsification methodologies tend to produce polydisperse microcapsules with a wide range of shell thicknesses. As the physical triggering of the self-healing process is dependent on the dimensions of the shell, this results in poor control of release of the healing agent.Exploring new techniquesAn alternative approach, that is increasingly attracting attention, is microfluidic encapsulation. This enables the production of monodisperse capsules, with precise control over the core to shell ratio and high encapsulation efficiency [11,12]. A wide variety of shell materials can be explored using the double emulsion template, experimenting to fine-tune the payload, permeability and shell properties of the microcapsules produced [13,14]. The technique can also be used to encapsulate potential self-healing agents, such as amines (for polymeric healing), biological cargo and mineral agents, making it an effective platform to evaluate the importance of core retention and interfacial bonding for physical triggering.Microfluidic production of polymeric shell microcapsulesResearchers at the University of Cambridge have investigated a microfluidic approach to encapsulating self-healing agents in microcapsules with polymeric shells, using a flow-focusing microfluidic device (Dolomite Microfluidics, Fig. 2) to produce a double emulsion template [15,16]. The inner phase is formed by injecting the encapsulant through a fluorophilic capillary tube while pumping the dispersed phase – a photocurable oil – through the central channel and flowing in the continuous phase from two side channels. Jets of dispersed phase containing the inner phase form at the cross-junction where the fluids stream into the main outlet channel (Fig. 3) and are UV-polymerized. The resulting monodisperse microcapsules encapsulate aqueous or non-aqueous compounds within an acrylate shell, which is functionalized with carboxylic groups to increase its hydrophilic nature and enhance interfacial bonding with the cement.Figure 2 Schematic of mechanically triggered capsule-based self-healing in cementitious matrix.Figure 3 The microfluidics platform used to produce the w/o/w double emulsion template (a) set-up used to produce microcapsules; (b) microfluidic device.Aqueous core microcapsulesWater-in-oil-in-water droplets (w/o/w) were generated using a fluorophilic-coated capillary placed before a hydrophilic flow-focusing droplet junction. The monodisperse double emulsion template was comprised of a 50 wt% 1,6-hexanediol diacrylate and 50 wt% bisphenol A glycerolate dimethacrylate solution, with an aqueous core containing colloidal silica (CS) – which enhances the mechanical properties and durability of the cementitious matrix and can promote self-healing in cement-based mortars – and polyvinyl alcohol (PVA). Water plays a key role in the biological and mineral self-healing of cement-based materials, promoting the chemical reactions between the healing agent and the host cementitious material. In encapsulated systems, it acts as the dispersal medium of the healing compound; a healing agent dispersed in a liquid will have greater mobility than encapsulated solid particles, covering a larger surface area of the damage location. However, the acrylate shell is permeable to water and impermeable to ions and high molecular weight molecules. Consequently, water encapsulated as core material evaporates, leaving just CS and PVA within the microcapsule (Fig. 4). As water permeability increases with decreasing shell thickness, a better approach may be to use non-aqueous liquid healing agents, as these are more likely to be retained.Figure 4 Schematic illustration of the formation of the double emulsion template.Non-aqueous core microcapsulesMineral oil was investigated as a model organic core material using an oil-in-oil-in-water (o/o/w) double emulsion droplet template for the formation of the microcapsules. The versatility of the microfluidic device to produce microcapsules with different core and shell materials was demonstrated by using a mixture of 50 wt% isobornyl acrylate and 50 wt% bisphenol A glycerolate dimethacrylate solution. Microcapsules were obtained by cross-linking the photocurable oil phase in the double emulsion droplets at the exit of the collection tube. The decreased interfacial tension between the inner and middle phase in an o/o/w double emulsion means that the template is more stable compared to a w/o/w model, allowing the production of microcapsules with decreased shell thicknesses. This increases the payload of the microcapsules, maximizing the amount of core material and healing agent (Fig. 4).Mechanical triggering and interfacial bondingFine-tuning of the key factors governing mechanical triggering – good interfacial bonding, thin shelled microcapsules, retention of the core material and low fracture toughness and strength – enhances rupturing of the capsule and promotes self-healing (see Fig. 5). Compared to encapsulation of non-aqueous core materials, the thicker shell of the aqueous core microcapsules reduces rupturing during cracking, while the loss of water during hydration of the cement paste causes the capsules to buckle and collapse. In contrast, non-aqueous core microcapsules have a thinner shell and retain the core material, maintaining their spherical shape. However, the interfacial bonding between the hydrophobic shell and the cement matrix is poor, and the microcapsules are prone to debonding without rupture. This is overcome by functionalizing the shell with hydrophilic groups, enhancing the bonding between the microcapsule and the cement paste, and enabling physical triggering on crack formation.Figure 5 The differences between microcapsules with aqueous and organic cores.SummaryLooking forward, there is scope to further extend this approach to evaluate the encapsulation of other agents typically used for self-healing of cementitious materials. Combined with functionalization of the microcapsule surface, which increases the interfacial bonding and broadens the types of shell that can be used for mechanical triggering, this microfluidic approach is set to open up new opportunities to promote self-healing of construction materials, extending the service life of concrete infrastructure.AcknowledgementsFinancial support from CAPES Foundation Ministry of Education of Brazil (BEX 9185/13-5) is gratefully acknowledged. The authors would like to acknowledge the support from the UK Engineering and Physical Sciences Research Council (EPSRC) for the Materials for Life (M4L) grant (EP/K026631/1) and the programme grant Resilient Materials for Life (RM4L, EP/02081X/1).