1 Introduction

Shear-thickened fluid (STF), a colloidal suspension of nanoparticles in a carrier fluid, exhibits various rheological phenomena as a non-Newtonian fluid. It possesses good fluidity under low shear rates but undergoes an instantaneous transition to a solid state under high shear rates [1,2,3]. This transition is attributed to the shear-thickening effect of STF. Figure 1 visually represents the shear-thickening process of STF. At low shear rates, the viscosity of STF remains constant. As the shear rate gradually increases below the critical value, the viscosity of the STF initially decreases, indicating shear thinning. However, when the stress or rate of shear reaches a critical point, a further increase in stress or rate of shear leads to a sharp rise in the viscosity of STF, known as shear thickening. It is worth noting that this shear-thickening phenomenon is reversible, as the STF reverts to its initial liquid state once the external forces are removed [4]. Based on these properties of STF, it has been applied in vibration control [5,6], protective fabrics [7,8], safety batteries [9,10] and various other fields.

Figure 1
figure 1

STF exhibits a shear-thickening process with increasing shear rate (or shear stress) [11].

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In recent years, there have been several reviews on STF. Tan et al [8] highlighted the properties and future research issues of shear-thickening fabric composites. Liu et al [12] concentrated on the application of STF in impact protection. Serra et al [13] focused on the applications of STF in a number of fields and the potential of cork–STF composites. This work focuses on a more concise and accurate introduction of the basic mechanical properties of STF, provides a more complete summary of current research on the shear-thickening mechanism and the factors influencing the rheological performance, and presents more informative research recommendations.

2 Shear-thickening mechanism

The understanding of the shear-thickening mechanism of STF remains contentious. There is mainly the following: order-to-disorder transition (ODT), hydrodynamic clustering, dilation theory, jamming and the underlying to frictional theories.

2.1 Order-to-disorder transition theory

Hoffman’s research [14,15,16] focused on the rheological behaviour of dense suspensions (as seen in figure 2a), introducing the concept of the ODT theory. The theory posits that at low shear rates, STF does not exhibit shear thickening due to the particles’ ordered and layered structure. As shear rates rise, this structure is disrupted, giving rise to a disordered arrangement, which leads to a sharp increase in STF viscosity, reflecting shear thickening. This theory offers a comprehensive explanation for the existence of critical shear rates. The results of Küçüksönmez and Servantie [17] confirm the ODT theory, and as shown in figure 2b and c, shear thinning occurs as nanoparticles align in an ordered pattern with increasing shear rate at low volume fractions. Conversely, as volume fractions increase, shear thickening is attributed to the disordered arrangement of nanoparticles under stress, caused by spatial constraints. Consequently, the transition between ordered and disordered arrangements governs shear thinning and thickening in nanoparticle suspensions in simple fluids. However, It was found that shear thickening can occur independently of ODT [18,19], indicating that the ODT theory is not exhaustive in explaining all shear-thickening phenomena.

Figure 2
figure 2

(a) Microscopic view of a spherical nanoparticle suspension, (b) two-dimensional distribution of two nanoparticle suspensions with different volume fractions (φ) at different shear rates (\(\dot{\gamma }\)), (c) particle arrangement of suspensions with different particle volume fractions when subjected to shear [17].

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2.2 Hydrodynamic clustering theory

The hydrodynamic clustering theory, initially proposed by Brady and Bossis [20] based on the Stokesian dynamics model, posits that shear thickening occurs due to the formation of particle clusters within the suspension rather than as a consequence of an ODT. Figure 3 demonstrates this concept, illustrating that when a stable suspension undergoes shear, the stationary particles transition to a flowing state and come closer together, forming small clusters. At higher shear rates, this closer proximity reduces the distance between particles, resulting in substantial fluid lubrication forces that facilitate cluster formation. In theory, these particle clusters have minuscule gaps and exhibit incompressible characteristics. They arrange themselves into irregular chains consisting of rod-like particles reminiscent of a traffic jam, ultimately elevating the viscosity of the suspension [20,21]. However, when the shear rate fails to reach the critical shear rate, the system cannot generate sufficient lubrication forces, causing the particles to arrange themselves into bunches along the flow direction, consequently inducing shear thinning. This theory was supported by subsequent experimental observations [22,23]. Moreover, Maranzano and Wagner [24] demonstrated the crucial role of electrostatic repulsion in the onset of shear thickening, addressing the limitation of the initial model, which only considered fluid lubrication forces for particle cluster formation.

Figure 3
figure 3

Changes in particle structure inside the STF during shear thickening [25].

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2.3 Dilation theory

Non-Brownian particle suspensions (particle diameter >1000 nm), such as mixtures of cornstarch in water, may exhibit a sudden jump in stress with increasing shear rate, known as discontinuous shear thickening (DST), but the hydrodynamic clustering theory fails to fully elucidate this phenomenon [26,27]. Consequently, the dilation theory was introduced. This theory suggests that dilation of the particle packing causes the particles to penetrate the liquid–air interface of the sample and are pushed backwards by the air with a restoring force, which creates a confining stress on the suspension. The resulting normal stress is transferred in the packing via frictional interaction, and produces a proportional confining shear stress, causing DST [28]. The specific process is shown in figure 4a. Fall et al [29,30] confirmed the discontinuous shear thickening of a suspension of cornstarch and water as a result of dilatancy by an independent measurement of the dilation of the suspension as a function of the shear rate. Further, Brown and Jaeger [28] proposed that frictional stresses emerge when the dilation of the particle packing is constrained, causing increased dissipation in the shear-thickening regime, which can be described by a nonlocal constitutive relation. However, their research also found that dilation is necessary but insufficient for DST.

Figure 4
figure 4

(a) Time sequence of the behaviour of a suspension of cornstarch and water after a hit by a metal rod (top view) [31], and (b) STF forms local blockage during shearing [32].

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2.4 Jamming

Some research suggested that jamming is a significant mechanism for the shear thickening of non-Brown particles suspensions. The aggregation of nanoparticles under shear causes local blockages (figure 4b), which leads to an increase in viscosity and a transformation of the STF into a solid [33,34]. The shear stress jumps discontinuously by orders of magnitude beyond a certain shear rate, which suggests that DST is closely related to jamming. Fall et al [34] demonstrated that a necessary condition for the macroscopic DST is the flow separation into a low-density flowing and a high-density jammed region. Jiang et al [35] found that during the shearing of a dense suspension of cornstarch and water, the jammed state gradually deepens, stabilizes and eventually destroys with the increasing shear stress, and the DST also occurs according to the changing jammed state. Next, Cui et al [36] developed a computational fluid dynamics model that is able to simulate the formation and evolution of shear jamming in STF. Based on this theory, the transient DST can be analysed in multiple stages so that the relationship between shear stress, shear rate and apparent viscosity can be explained. Notably, jamming is insufficient to be considered as an independent theory to explain DST, but only a phenomenon that supports the research of DST mechanism [12].

2.5 Lubricated to frictional theory

For a more systematic explanation of CST (continuous shear thickening), DST and jamming, the lubricated-to-frictional theory was introduced [37]. This theory proposes that shear thickening is related to the transition from lubricated to frictional contact domination between particles. As seen in figure 5a and b, as shear stress or shear rate increases, there is a transition from the predominance of lubrication contact to the predominance of frictional contact between particles, which characterizes the entire shear-thickening process. Once the load is removed, the repulsive forces between particles regain dominance. Consequently, the particles separate and cease to make frictional contact, demonstrating the reversible nature of the shear-thickening phenomenon. The lubricated-to-frictional theory parallels the ODT theory in elucidating particle motion during shear thickening, distinguishing itself by emphasizing that the primary factor triggering shear thickening is the frictional force between particles rather than the disorder of the system structure. Frictional contact can provide force and torque without motion, so the lubricated-to-frictional theory provides a more comprehensive explanation of DST and jamming than the hydrodynamic clustering theory [38,39,40].

Figure 5
figure 5

Structure of the particle friction contact network formed during shear thickening for STF with different volume fractions (φ), φ = (a) 0.5 and (b) 0.56 [37].

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3 Rheological performance of STF

Accurate control of the rheological properties of STF is a fundamental requirement for its practical implementation. However, the performance of STF is affected by the dispersed phase particles, dispersion medium, additives, external environment and other factors.

3.1 Effect of dispersed phase particles

The dispersed phase particles of STF are micron-sized particles or nanoparticles, largely categorized into inorganic particles (such as SiO2 particles), natural polymer particles (such as cornstarch particles), and synthetic polymer particles (such as PMMA particles) by the material compositions. Various factors, including the concentration, particle size, shape and roughness of the dispersed phase particles, influence the rheological properties of STF. During a steady-state shear test of STF, increasing the volume fraction of dispersed phase particles leads to a decrease in the critical shear rate, an increase in both the initial and maximum viscosity, and no change in the critical stress at the onset of shear thickening [41,42]. These trends are illustrated in figure 6a. The decrease in the critical shear rate is due to the reduction in interparticle distance as the particle volume fraction increases, leading to a higher hydrodynamic force. Consequently, a lower shear rate is required for particles to overcome the mutual repulsive force, promoting shear thickening in the STF [43,44,45]. Moreover, the CST is likely to change into DST or even jamming when the concentration of particles is excessively high [46].

Figure 6
figure 6

(a) Shear-thickening characteristic curves of STF with different dispersed phase particle concentrations [45], (b) shapes [47], (c) sizes [48], (d) and roughness [49].

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The impact of particle shape on the performance of STF primarily manifests through particle size ratio and roughness. Spherical particles typically have a length-to-diameter ratio of about 1. Nonetheless, research indicates that STF composed of particles with higher length-to-diameter ratios, such as ellipsoidal and rod-shaped particles, exhibit lower shear-thickening rates and superior shear-thickening properties to spherical particles [47,50]. This is illustrated in figure 6b. Mari [51] conducted tests and comparisons using dimeric particles formed by joining two spherical particles and indicated that an increased aspect ratio markedly enhanced the shear-thickening performance. Furthermore, experimental studies have shown that at the same particle concentration and temperature conditions, STFs exhibit larger initial and peak viscosity when the particle size of the dispersed phase particles is smaller [24,48,52] (see figure 6c). Talreja et al [53] also observed that as the particle size decreases and the particle size distribution becomes uniform, the peak viscosity of STF increases, leading to improved impact resistance in fabrics impregnated with the prepared STF. Figure 6d [49] shows that increasing the roughness of the dispersed phase particles in a suspension has been found to reduce the minimum particle volume fraction necessary for shear thickening to occur. Notably, this phenomenon provides a robust foundation for the lubricated-to-frictional theory, elucidating the shear-thickening mechanism.

3.2 Effect of dispersion medium

The rheological performance of STF is also influenced by the dispersion medium, which is an essential factor to consider. Studies have demonstrated that higher molecular weight of the dispersion medium results in a lower critical shear rate, higher initial and peak viscosities, and enhanced shear-thickening performance of the STF when all other conditions are equal [45,54,55]. The dispersion medium is generally a polar solvent, in STF containing SiO2 particles or some other nanoparticles (for example, polystyrene microspheres) as the dispersed phase, polyethylene glycol (PEG) is commonly used as the dispersion medium. However, the rheological properties of STF can vary depending on the dispersion medium used. Shan et al [56] compared the rheological performance of STFs using four different dispersion mediums: ethylene glycol (EG), PEG400, propylene glycol (PG) and polypropylene glycol 400 (PPG400), in combination with SiO2 particles. Their findings revealed that the four STFs exhibited varying degrees of continuous or discontinuous shear-thickening behaviour, as depicted in figure 7a–d. One possible explanation for this disparity is the varying thickness of the solvent layer formed around the particles by different dispersion mediums, resulting in variations in interparticle distances and repulsion forces. Consequently, the critical shear rate required for shear thickening also differs accordingly, consistent with the shear-thickening mechanism mentioned earlier. Another possible reason is the difference in viscosity of dispersion mediums, which affects the rheological performance of the suspension. The desired shear-thickening effect can be achieved by controlling the composition and parameters of the dispersion medium in the STF [57]. Moreover, besides the STF of the SiO2 system, some STFs made with other dispersed phase particles and dispersion medium may provide better performance, e.g., the styrene/acrylate particle-based STF tested by Fu et al [58,59], and a new STF with PMMA as dispersed phase and [BMIM]PF(6) as dispersing medium produced by Cao et al [60].

Figure 7
figure 7

Shear-thickening characteristic curves of STFs made with different dispersion mediums: (a) SiO2/EG suspension, (b) SiO2/PEG400 suspension, (c) SiO2/PG suspension and (d) SiO2/PPG400 suspension [56].

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3.3 Effect of additives

Adding additives to STF is crucial for effectively enhancing their rheological properties. These additives can be categorized into two main types based on their mode of action. The first type includes additives that modify the rheological properties of STF by altering the properties of dispersed phase particles or carrier fluid, such as polyurethane, silicone oil and polystyrene–polyacrylamide particle [61,62,63]. The second type consists of additives that do not alter the particles or the carrier fluid themselves but only affect the motion of the dispersed phase, such as carbon nanotube, halloysite nanotube and carbide particle [48,64,65]. Gürgen [66] unveiled some significant changes of Si3N4 in the critical shear rate and initial and peak viscosity of the Si3N4/SiO2-STF, as depicted in figure 8a–c. Sharma et al [67] fine-tuned the rheological properties of a SiO2-based STF by introducing boric acid as an additive. Their steady-state shear tests revealed that the STF with boric acid demonstrated a fourfold increase in peak viscosity compared to the pure STF and exhibited a more pronounced shear-thickening behaviour. Islam et al [68] conducted experiments to examine the shear-thickening behaviour of SiO2-based STF incorporated with porous cellulose particles. They discovered that the rheological properties of the modified STF were over three times higher than those of pure STFs at the same concentration. This significant enhancement can be attributed to the interaction between the excess PEG molecules in the STF and the porous cellulose through hydrogen bonding, as depicted in figure 8d. The addition of additives to STFs has proven to be a rapid and visually evident method to modify the rheological properties, making it a common approach in STF modification.

Figure 8
figure 8

Variation of the indices of STF with the addition of different mass fractions of silicon nitride particles: (a) critical shear rate, (b) peak viscosity, (c) thickening rate [66], and (d) enhancement of the shear-thickening effect of STF by the addition of porous cellulose [68].

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3.4 Effect of external environment

External factors such as temperature, pH value, magnetic fields and electric fields can impact the rheological properties of STF. As depicted in figure 9a, as the temperature rises, the critical shear rate of the STF increases, while the initial and peak viscosities experience significant decreases, resulting in an overall reduction in the shear-thickening effect [52,69,70]. Shan et al [71] experimented to investigate the impact of pH on the shear-thickening behaviour of STF by using fumed silica suspensions with the addition of HCl and NaOH. They observed changes in the critical shear rate of STF corresponding to variations in pH, as illustrated in figure 9b. The reason may be that pH alteration affected nanoparticles’ surface properties, resulting in an isoelectric point in both acidic and alkaline environments. Deviating the pH from the isoelectric point, either by decreasing or increasing it, led to a reduction in the critical shear rate of the STF, thereby increasing the likelihood of shear thickening. Researchers have successfully developed magnetorheological shear-thickening fluid and effectively controlled their rheological properties by applying a magnetic field [72,73,74]. The impact of this method is depicted in figure 9c. Furthermore, Brown et al [75] observed that an increase hindered the shear thickening ability of STFs with dielectric effects in electric field strength. They accomplished this by incorporating spherical particles with dielectric effects into STFs and subjecting them to an electric field, as illustrated in figure 9d. Therefore, applying an electric field is also an available method to control the rheological performance of STF [76].

Figure 9
figure 9

(a) Rheological performance of STFs at different temperatures [52], (b) pH values [71], (c) magnetic fields [72] and (d) electric fields [75].

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4 Conclusions and perspectives

STF has exhibited considerable potential as an energy-absorbing material across numerous applications, including composite fabrics, body armour, sandwich plates and other potential usage fields. Nevertheless, as research advances and demand for STFs increases, it is anticipated that various technical challenges will arise.

4.1 Stability of STF at different temperatures

In the natural temperature range, STF typically decreases initial viscosity, peak viscosity and shear-thickening effect as the temperature increases. This decrease is quite significant and can have a detrimental impact on the practical performance of STF. Although this issue is negligible when STF is used in protective gear like body armour due to the stable human surface temperature, it becomes a significant problem when STF is applied in sandwich plates and other fields with a prominent difference in natural outdoor temperatures. This poor thermal stability of STF can lead to numerous application difficulties. Therefore, the primary focus of future research is to develop STFs with better thermal stability by adjusting the composition of STFs, such as changing the dispersed phase particles and media or adding additives.

4.2 Performance control of STF

The shear-thickening process of STF exhibits nonlinear behaviour. This presents challenges in controlling the nonlinear rheological properties of STF, particularly in situations involving unstable shear rates. Moreover, the rheological performance of STF is also influenced by dispersed phase particles, dispersion medium and other variables, which makes it difficult to achieve the same performance in different working scenarios. Therefore, it is necessary to further explore the interaction and control mechanisms of factors that affect the rheological properties of STF. It is important to establish a constitutive model for predicting the mechanical properties of STF when applied in a certain field.

4.3 Long-term stability of STF

Long-term stability poses a critical challenge for STF. The interaction between the dispersed phase particles and the dispersion medium can lead to undesired particle settling or coalescence, resulting in unstable rheological properties. Moreover, the shear-thickening reversibility constitutes a vital property for practical applications, but the factors above can also compromise it. To overcome this challenge, the main goal of future research is to improve the formulation of STF, specifically, we suggest to develop a synthetic polymer particle that is not easy to settle or coalesce as a dispersed phase particle or a dispersion medium that can mitigate the particle settlement or coalescence.