Shear thickening fluid (STF) in engineering applications and the potential of cork in STF-based composites

Shear thickening fluids (STFs) are a unique type of fluids that can quickly transform into a solid-like state when subjected to forces (rate dependent). These fluids are created by dispersing micro and nanoparticles within a medium. When the force is removed, they return to their original liquid state. Shear thickening fluids can absorb a significant amount of impact energy, making them useful for reducing vibrations and serving as a damper. This study provides a comprehensive and brief overview of existing literature on shear thickening fluids, including their properties, classification, and the rheological mechanisms behind the shear thickening behaviour. It also examines the use of these fluids in various applications, such as improving resistance to stabs and spikes, protecting against low- and high-velocity impacts, and as a new medium for energy dissipation in industries such as battery safety, vibration control and adaptive structures. Lastly, this work reviews the promising combination of STFs with cork. Given the sustainability of cork and its energy absorption capacity, cork-STF composites are a promising solution for various impact-absorbing applications. Overall, the paper underscores the versatility and potential of STFs, and advocates for further research and exploration.

Shear Thickening Fluid and Its Application in Impact Protection: A Review

Shear thickening fluid (STF) is a dense colloidal suspension of nanoparticles in a carrier fluid in which the viscosity increases dramatically with a rise in shear rate. Due to the excellent energy absorption and energy dissipation of STF, there is a desire to employ STFs in a variety of impact applications. In this study, a comprehensive review on STFs' applications is presented. First, several common shear thickening mechanisms are discussed in this paper. The applications of different STF impregnated fabric composites and the STF's contributions on improving the impact, ballistic and stab resistance performance have also been presented. Moreover, recent developments of STF's applications, including dampers and shock absorbers, are included in this review. In addition, some novel applications (acoustic structure, STF-TENG and electrospun nonwoven mats) based on STF are summarized, to suggest the challenges of future research and propose some more deterministic research directions, e.g., potential trends for applications of STF.

Shear-induced phase transition in the aqueous solution of an imidazolium-based ionic liquid

An ionic liquid (IL) is a salt in the liquid state that consists of a cation and an anion, one of which possesses an organic component. Because of their non-volatile property, these solvents have a high recovery rate, and, hence, they are considered as environment-friendly green solvents. It is necessary to study the detailed physicochemical properties of these liquids for designing and processing techniques and find suitable operating conditions for IL-based systems. In the present work, the flow behavior of aqueous solutions of an imidazolium-based IL, 1-methyl-3-octylimidazolium chloride, is investigated, where the dynamic viscosity measurements indicate non-Newtonian shear thickening behavior in the solutions. Polarizing optical microscopy shows that the pristine samples are isotropic and transform into anisotropic after shear. These shear thickened liquid crystalline samples change into an isotropic phase upon heating, which is quantified by the differential scanning calorimetry. The small angle x-ray scattering study revealed that the pristine isotropic cubic phase of spherical micelles distort into non-spherical micelles. This has provided the detailed structural evolution of mesoscopic aggregates of the IL in an aqueous solution and the corresponding viscoelastic property of the solution.

Insight into the inhibitory mechanism of soluble ionic liquids on the transport of TiO2 nanoparticles in saturated porous media: Roles of alkyl chain lengths and counteranion types

Column experiments were carried out to investigate the transport of TiO₂ nanoparticles (nTiO₂) in water-saturated porous media in the presence of various imidazolium-based ionic liquids (ILs) with different alkyl chain lengths and counteranions. The results indicated that the effects of ILs on nTiO₂ transport were considerably dependent upon IL species. In general, the transport-inhibition effects increased with the increasing length of branched alkyl chain on the ILs (i.e., [C₆mim]Cl > [C₄mim]Cl > [C₂mim]Cl). The trend was dominated by the hydrophobicity effects of ILs. Meanwhile, the inhibitory effects of ILs were strongly related to the counteranions and followed the order of [C₄mim]Cl > [C₄mim][TOS] > [C₄mim][PF₆], mainly due to different electrostatic repulsion force between nanoparticles and porous media in the presence of various ILs. Furthermore, the inhibitory role of [C₄mim][TOS] in nTiO₂ transport under acidic conditions (i.e., pH 6.5) was greater than that under alkaline conditions (i.e., pH 8.0). The dominant mechanism was that the differences in the extent of electrostatic repulsion between sand grains and nTiO₂ with or without ILs at pH 6.5 were larger than that at pH 8.0. Moreover, two-site kinetic retention model and DLVO theory provided good descriptions for the transport behaviors of nTiO₂ with different ILs.

Rheological and Ionic Transport Properties of Nanocomposite Electrolytes Based on Protic Ionic Liquids and Silica Nanoparticles

In this study, the effect of hydrophilic silica nanoparticle (AEROSIL 200) addition on the rheological and transport properties of several protic ionic liquids (PILs) consisting of protonated 1,8-diazabicyclo[5.4.0]undec-7-ene cation (DBU) was studied. Interactions between the surface silanol groups of the silica nanoparticles and the ions of these PILs affected the nature of particle aggregation and the hydrogen bonding environment, which was reflected in the nonlinear rheological behaviors and transport properties of their colloidal suspensions. In contrast to shear-thinning gels formed by colloidal suspensions of the silica nanoparticles in [DBU][TFSA] ([TFSA] = [N(SO₂CF₃)₂]), [DBU][TfO] ([TfO] = [CF₃SO₃]), and [DBU][TFA] ([TFA] = [CF₃CO₂]), a shear-thickening stable suspension was formed in the [DBU][MSA] ([MSA] = [CH₃SO₃]) system. A relatively strong interaction between the silanol groups and the ions of [DBU][MSA] and the ability of this PIL to form a thicker solvation layer through hydrogen bonding were assumed to be responsible for this unique behavior. Moreover, the [DBU][MSA]-silica system showed a large enhancement in the conductivity at a certain silica concentration. This enhancement was not observed in the other PIL-silica composites that exhibited shear-thinning behavior. Even though diffusion of ions was found to be restricted in the presence of silica, a preferentially stronger interaction between [MSA] anions and the silica surface resulted in an increase in the number of charge carriers.

Interparticle hydrogen bonding can elicit shear jamming in dense suspensions

Dense suspensions of hard particles in a liquid can exhibit strikingly counter-intuitive behaviour, such as discontinuous shear thickening (DST)^1-7 and reversible shear jamming (SJ) into a state where flow is arrested and the suspension is solid-like^8-12. A stress-activated crossover from hydrodynamic interactions to frictional particle contacts is key for these behaviours^2-4,6,7,9,13. However, in experiments, many suspensions show only DST, not SJ. Here we show that particle surface chemistry plays a central role in creating conditions that make SJ readily observable. We find the system's ability to form interparticle hydrogen bonds when sheared into contact elicits SJ. We demonstrate this with charge-stabilized polymer microspheres and non-spherical cornstarch particles, controlling hydrogen bond formation with solvents. The propensity for SJ is quantified by tensile tests¹² and linked to an enhanced friction by atomic force microscopy. Our results extend the fundamental understanding of the SJ mechanism and open avenues for designing strongly non-Newtonian fluids.

Anisotropic Nanoparticles Contributing to Shear-Thickening Behavior of Fumed Silica Suspensions

Rheological characteristics of a concentrated suspension can be tuned using anisotropic particles having various shapes and sizes. Here, the role of anisotropic nanoparticles, such as surface-functionalized multiwall carbon nanotubes (MWNTs) and graphene oxide nanoplatelets (GONPs), on the rheological behavior of fumed silica suspensions in poly(ethylene glycol) (PEG) is investigated. In these mixed-particle suspensions, the concentrations of MWNTs and GONPs are much lower than the fumed silica concentration. The suspensions are stable, and hydrogen-bonded PEG solvation layers around the particles inhibit their flocculation. Fumed silica suspensions over the concentration range considered here display shear-thickening behavior. However, for a larger concentration of MWNTs and with increasing aspect ratios, the shear-thickening behavior diminishes. In contrast, a distinct shear-thickening response has been observed for the GONP-containing suspensions for similar mass fractions (MFs) of MWNTs. For these suspensions, shear thickening is achieved at a lower solid MFs compared to the suspensions consisting of only fumed silica. A significant weight reduction of shear-thickening fluids that can be achieved by this approach is beneficial for many applications. Our results provide guiding principles for controlling the rheological behavior of mixed-particle systems relevant in many fields.

Functionalization of silica particles to tune the impact resistance of shear thickening fluid treated aramid fabrics

Kevlar fabrics treated with MTMS modified silica based STF showed better impact energy absorption as compared to APTES modified and control silica based STF treated fabrics, attributed to changes in interactions between fabrics and silica particles.

Dynamic/quasi-static stab-resistance and mechanical properties of soft body armour composites constructed from Kevlar fabrics and shear thickening fluids

Soft body armour composites were constructed by combining Kevlar fabrics with different quantities of shear thickening fluid (STF).

Body armour materials: from steel to contemporary biomimetic systems

The history of armour is as old as evolution of mankind; indeed it is an intrinsic instinct of humanity to protect themselves from critical environment as well as other human in the battlefield setting.