Effects of ionic structures on shear thickening fluids composed of ionic liquids and silica nanoparticles

Abstract

Shear thickening fluids (STFs) are energy dissipative materials and affected significantly by the properties of the dispersing media. Most of the dispersing media are polymeric or oligomeric fluids, and few studies have made ionic liquids (ILs) as the dispersing media. Since the properties of ILs are strongly dependent on its' ionic structures, the shear thickening behavior of STFs composed of ILs must be influenced by the ionic structures. The rheology and viscoelasticity of STFs of hydrophilic silica nanoparticles in ILs with different ionic structures were investigated systematically. The experimental results demonstrated that the dispersions in hydrophilic ILs displayed shear thickening behavior, while dispersions prepared by hydrophobic ones appeared as a colloidal gel and displayed shear thinning. With changing the length of the alkyl chain substituent on the organic cation, the shear thickening behavior was transformed. Shear thickening behavior was enhanced for the introduction of hydroxyl on the organic cation. Ionic structures influence the interactions between silica nanoparticles and ILs through the hydrogen bonds, and the shear thickening behavior may be induced by the formation of nanoparticle aggregation. The dynamic stabbing resistance, quasi-static tensile strength and conductivity of Kevlar fabric composites treated by STFs composed of ILs were enhanced observably, and the electromagnetic shielding property was improved to a certain degree.

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1 Introduction

Shear thickening fluid (STF) is a non-Newtonian concentrated dispersion that undergoes a sharp increase in viscosity by several orders of magnitude as soon as a critical shear rate is reached, and then recovers to a low viscosity with decreasing shear rate.^1–8 For the discontinuous shear thickening (DST) behavior, the STF even changes from a fluid-like into a solid-like state when the shear thickening is triggered.^9–13 In this case, the external energy is dissipated significantly for the shear thickening of STF. Food, paint and ceramic industries, and almost all processing industries, are among examples in which there is a concern with concentrated suspensions.¹⁴ Generally, the shear thickening phenomenon is not welcome in those industries. However, as an energy dissipative material, STFs have lots of gratifying achievements in the exploits of soft body armor, impacting resistance composites and damping devices in the past decade.^15–19 Young et al. researched the ballistic impact characteristics of Kevlar fabrics impregnated with STFs and proposed that the addition of STF to the fabric offered a significant enhancement in ballistic penetration resistance without any loss in material flexibility.¹⁵ Decker et al. investigated the stab resistance of STF-treated fabrics and found that the stab resistance exhibited significant improvements over neat fabric targets of equivalent areal density. Dramatic improvements in puncture resistance were observed under high and low speed loading conditions, while slight increases in cut protection were also observed.¹⁶ Fischer et al. studied the integration of STFs into composite structures and found that the vibration and damping properties were significantly modified under dynamic deformation.¹⁷ Kalman et al. researched the effect of particle hardness on the penetration of fabrics treated by STFs.¹⁸ Warren et al. reported the mitigation of STF composed of fumed silica and low molecular weight polymeric glycols on an aluminum facesheet sandwich composite under hypervelocity impact.¹⁹ These novel applications of STFs in different fields drive us to exploit novel STFs.

STF is composed of a particulate dispersing phase and a polymeric or oligomeric dispersing media. The properties of STF can be adjusted by changing the dispersing phase, such as volume fraction, particle size and shape, hardness and surface chemistries, as well as the dispersing media. Some investigations have been made and demonstrated that the shear thickening behavior of STF depended on the property of the dispersing media.^20–22 Shenoy and Wagner studied the influence of dispersed media viscosity on the shear thickening transition and found that the critical stress was independent of media viscosity.²⁰ Xu et al. found that the shear thickening effect was significantly enhanced with the increase of the concentration and the molecular chain length of additives.²¹ Jiang et al. investigated the influence of interactions between glycerin–water mixtures and PMMA particles on shear thickening and investigated by varying the ratio of glycerin to water. They proposed that shear thickening will be more drastic with an increase in the proportion of glycerin in glycerin–water mixtures.²² Justin Warren et al. investigated STFs composed of hydroxyl-terminated polyethylene glycol with a molecular weight of 200 g mol⁻¹ (PEG-200), a polyethylene glycol with a molecular weight of 400 g mol⁻¹ (PEG-400) and a three-armed glycerol polypropylene oxide having a molecular weight of 700 g mol⁻¹ (PPG-700) over a temperature range spanning approximately 100 °C.¹⁹

However, the dispersing media of existing STFs commonly consist of polymeric or oligomeric fluids, such as low molecular weight polyethylene glycol, ethylene glycol and propylene glycol.^15–22 Compared to the dispersing media of traditional STFs, ionic liquids (ILs) are comprised entirely of ions and are fluids under ambient conditions. They have attracted considerable interest in many fields of chemistry, and in the chemical industry, for their unique liquid and physicochemical properties, which include low vapor pressure, low flammability, high ionic conductivity and wide electrochemical window, high chemical stability and good thermal stability.^23–26 For colloidal nanomaterials, ILs have been used in preparing functional nanomaterials,²⁷ enhancement of colloidal stability^28,29 and magnetorheological fluids based on dispersions of magnetic particles.³⁰ Especially in recent years, a few studies have reported that some dispersions composed of ionic liquids (ILs) displayed shear thickening behavior.^31–37 STFs based on hydrophilic silica nanoparticles and 1-butyl-3-methylimidizolium tetrafluoroborate have been investigated systemically in recent times and it has been demonstrated that the shear thickening of STFs composed of ILs was enhanced over that of traditional STFs comprised of low molecular weight polyethylene glycol (PEG-400).³⁷ These stimulate us to investigate the application of more different ILs in STFs.

As reported by Eric Brown, the discontinuous shear thickening transition of STF-based on cornstarch and water even can produce a fluid so resistant to flow that a person could run on it or bounce a bowling ball off its surface.³⁸ For the particular interesting phenomenon of STFs, lots of researchers have made investigations to study it in recent decades, and three mechanisms have been proposed. The first mechanism about STFs, the order-to-disorder transition, was proposed by Hoffman.^39,40 Hoffman suggested that particles would be orderly packed at the initial low shear rates and as the viscosity decreases slightly, shear thinning is displayed. As the critical shear rate is reached, the ordered packing particles would be broken up into less ordered arrays, and the viscosity increases quickly, displaying shear thickening. Brady et al. proposed that the reversible continue shear thickening (CST) in concentrated colloidal suspensions was due to the formation of a non-equilibrium and self-organized microstructure that is bound together by hydrodynamic forces, denoting by the term of clusters.^1–3,5–8 They proposed that the long-range hydrodynamic force overcomes the short-range repulsive force between nanoparticles at the critical shear rate, and nanoparticles aggregate into clusters that lead to the increase of viscosity. The hydroclusters have been visualized in the shear thickening region in concentrated colloidal suspensions with fast confocal microscopy, as reported by Xiang Cheng et al.⁷ The jamming mechanism of the discontinuous shear thickening behavior in concentrated suspensions has been discussed.^9–13,38 Particles jam the confined space under shearing and lead to the dilation of suspensions, which makes the viscosity increase discontinuously. The three mechanisms are independent to one another. For the order-to-disorder mechanism, the order state is not necessary before shear thickening The hydroclustering mechanism has been demonstrated for Brownian dispersions of nanoparticles with low concentration, and the jamming mechanism has been conformed for concentrated non-Brownian suspensions. Particles' movements under the external shear intrinsically lead to shear thickening in the concentrated suspensions systems. The movements of particles are governed by the interactions between the dispersing particles and the dispersing media as well as between themselves. Several interactions are important such as van der Waals, electrostatic, steric, frictional, hydrodynamic, and Brownian. The hydrocluster formation in STFs has been proposed and believed to be governed by a complex balance of inter-particle forces.^1–3,5–8 Forces that enforce particles to aggregate mainly include hydrodynamic force, van der Waals force and electrostatic force, while the repulsive forces between particles mainly include the steric hindrance force (corresponding to solvation force), Brownian force and electrostatic force.^5,14,19 When it comes to dispersions of ILs, different solvation layers on the surface of particles will form different ionic structures, and the interactions between particles and ILs as well as between particles would change. Such that the shear thickening behavior of dispersions composed of ILs may be transformed when ionic structures are changed. The effective rule of ionic structure on shear thickening fluids needs study.

Since ionic liquids possess a lot of advantages, and are suitable for exploiting novel STFs, and the shear thickening behavior of STFs composed of ILs must be influenced by the ionic structures, the effects of ionic structures on STFs composed of ILs is investigated systematically in this paper for the first time. A theoretical understanding about shear thickening in dispersions of hydrophilic silica nanoparticles in ILs with different ionic structures has been proposed here to explore the complex underlying contributions of the various factors leading to shear thickening. The new Kevlar fabric composites treated by STFs composed of different ILs were prepared, and the dynamic stabbing resistance, quasi-static tensile strength, conductivity and electromagnetic interference shielding effectiveness (EMI SE) of these composites were measured for the first time. The experimental results were compared with neat Kevlar fabrics and those treated with traditional STFs composed of low molecular weight polyethylene glycol.

2 Experimental section

2.1 Materials

Fumed silica nanoparticles with surface silanol groups were purchased from Aladdin Industrial Corporation. Fig. 1 shows the field emission scanning electronic microscopy (SEM, Quant FEG 450) images of them. The fumed silica nanoparticles had an average diameter of about 50 nm and aggregated because of the hydrogen bonds between the surface silanol groups. The shear thickening behavior of STFs containing different particle sizes was different.^2,3 Five different ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([C₂mim][BF₄]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C₆mim][BF₄]) and 1-ethoxyl-3-methylimidazolium tetrafluoroborate ([C₂OHmim][BF₄]) were purchased from Sinopharm Chemical Reagent Co., Ltd. Fig. 2 shows the chemical structures of these ILs. The steady shear rheology of pure ILs was measured and shown in Fig. 3. The conductivity was measured with a digital conductivity meter with Pt/Pt black electrodes (DDSJ-318, INESA instrument Co., Ltd). All of the ILs had good conductivity (see the Table S1 in the (ESI†)). It is clear that all ILs in this study were Newtonian fluids. The viscosities of the different ILs were different from each other because of the ionic structures. [C₄mim][PF₆] is a hydrophobic IL due to its hydrophobic cation and anion.⁴¹ The viscosity of [C₄mim][PF₆] is about 0.1 Pa s and bigger than other ILs for the existing PF₆ anion.⁴² The viscosity of pure ILs containing the [BF₄] anion increases with increasing the length of alkyl chain substituent on the organic cation (as shown in Fig. 2b, d and e). This corresponds with previous reports.⁴³ Compared with [C₂mim][BF₄], the polarity of [C₂OHmim][BF₄] is enhanced significantly because of the introduction of hydroxyl on the alkyl chain of the organic cation.³² PEG-400 was purchased from Sinopharm Chemical Reagent Co., Ltd and used as received. Plain woven Kevlar fabrics with K49 were supplied by the Dupont Company, and the area density was 240 g cm⁻².

Fig. 1 SEM of silica nanoparticles presenting the aggregations are composed of primary spherical silica nanoparticles with about 50 nm diameter.

Fig. 2 Chemical structures of (a) [C₄mim][PF₆], (b) [C₂mim][BF₄], (c) [C₂OHmim][BF₄], (d) [C₄mim][BF₄] and (e) [C₆mim][BF₄].

Fig. 3 Shear rate dependencies of viscosity for the five pure ILs at 30 °C under steady shear.

2.2 Preparation of dispersions

Fumed silica nanoparticles were treated at 200 °C for 12 h in a muffle furnace to get rid of the surface adsorbed water. ILs were dried under vacuum at 80 °C over 24 h before use. Then, STFs were prepared by dispersing silica nanoparticles in ILs through ultrasonication for no less than 24 h until they were homogeneous. The concentrations of all STFs were calculated by volume percentage (vol%). All samples were dried under vacuum at 80 °C for 24 h and sealed. Because of the hydrophobic property of [C₄mim][PF₆], it was difficult to get high concentration dispersions, and the dispersions with 5 vol% silica nanoparticles was prepared for steady shear and oscillatory shear, respectively. The dispersions with 8 vol% and 12 vol% silica nanoparticles in [C₄mim][BF₄], and other dispersions with 12 vol% silica nanoparticles in [C₂mim][BF₄] and [C₂OHmim][BF₄] were prepared, respectively.

2.3 Measurements

Both steady shear rheological experiments and oscillatory shear viscoelastic experiments were performed with a stress controlled rheometer (Anton Paar Rheometer Physica MCR 301) at 30 °C, which controlled by the environmental chamber of the rheometer. The cone and plate system with a diameter of 50 mm, a cone angle of 2° and a gap size of 0.99 mm was used. To allow the samples to establish their equilibrium structures and eliminate any previous shear history, a steady shear was performed with a shear rate of 10 s⁻¹ sustained for 30 s before each measurement. The values are void because some samples are thrown out from the measuring system. The ranges of shear rate and strain are confined by either the maximum torque limit of the rheometer or whether the samples were thrown out from the gap between the cone and the plate.

2.4 Preparation and measurements of STF-treated fabrics

Highly concentrated STFs (the rheology of them are shown in Fig. S1 in the ESI†) with 15 vol% silica nanoparticle in PEG-400, [C₂mim][BF₄], [C₄mim][BF₄] and [C₂OHmim][BF₄] were prepared and combined with Kevlar fabrics. STFs were coated on Kevlar fabrics and then pressed repeatedly with moderate pressure to guarantee thorough infiltration. All STF-treated Kevlar fabrics were sealed by films. The sample parameters (see Table S2 in the ESI†) of the dynamic stabbing resistance and testing installation were performed according to the rule of NIJ Standard-0115.00 (see Fig. S2 in the ESI†). The sample parameters (see Table S3 in the ESI†) and procedures for the quasi-static tensile strength testing were performed with an electronic universal testing machine (CMT 7204) according to the rule of ASTM D5035-2006. The conductivity and EMI SE of neat Kevlar fabrics and STF-treated Kevlar fabric composites were measured by a Tera-Ohmmeter (TO3, Germany) and an Agilent vector network analyzer (N5232A, USA), respectively.

3 Results and discussion

3.1 Effects of the hydrophilic/hydrophobic property of ILs

The effect of the hydrophilic/hydrophobic property of ILs on the STFs was investigated at first. As shown in Fig. 4a, the rheology of dispersions prepared by [C₄mim][PF₆] was distinctive from that of dispersions composed of [C₄mim][BF₄] completely. The dispersions of [C₄mim][PF₆] with 5 vol% silica nanoparticles displayed shear thinning for the whole range of shear rates at 30 °C, which was the same with other dispersions containing hydrophobic ILs.^31,32 The dispersions in [C₄mim][BF₄] with 8 vol% silica nanoparticles present a representative shear thickening behavior at 30 °C, as shown in the insert plot of Fig. 4a. A first shear thinning was observed for shear rates in the range of 10⁻² to 10⁰ s⁻¹. The viscosity will increase as soon as a critical shear rate is reached. A second shear thinning was observed beyond the shear rate of 10³ s⁻¹. Although the concentration of silica nanoparticles in [C₄mim][BF₄] was higher compared to that of silica in [C₄mim][PF₆], the viscosity of dispersions in [C₄mim][BF₄] was much lower than the dispersions of [C₄mim][PF₆]. Especially for low shear rates, the viscosity of dispersions in [C₄mim][PF₆] even reached 5000 Pa s, and it was higher than that of dispersions in [C₄mim][BF₄] by almost four orders of magnitude. With increasing shear rate, the viscosity of dispersions in [C₄mim][PF₆] decreased sharply and continuously. Fig. 4b shows the strain dependencies of the dynamic viscoelastic properties for the dispersions in [C₄mim][PF₆] and [C₄mim][BF₄], respectively. The dispersions consisting of 8 vol% silica nanoparticles in [C₄mim][BF₄] exhibited a strain-independent loss modulus (G′′) that was higher than its storage modulus (G′) by one order of magnitude before shear thickening, indicating that the dispersions were non-flocculent³² and behaves as a fluid, as shown in the inset of Fig. 4b. Both the G′ and viscosity declined seriously in the dispersions of [C₄mim][PF₆] with 5 vol% silica nanoparticles, implying that silica nanoparticles are flocculent just as shown in the inset of Fig. 4b.^31,34,35

Fig. 4 (a) Shear rate dependencies of viscosity for the dispersions with 5 vol% silica nanoparticles in [C₄mim][PF₆] and with 8 vol% silica nanoparticles in [C₄mim][BF₄] at 30 °C under a steady shear, respectively. (b) Strain dependencies of storage modulus (G′) and loss modulus (G′′) for the dispersions at 30 °C under an oscillatory shear. Insets: photographs of the dispersions.

The interaction between fumed silica nanoparticles and the hydrophobic [C₄mim][PF₆] was weak. The hydrogen bonds between silanol groups on the surface of silica nanoparticles may result in the formation of a three-dimensional particulate network of silica nanoparticles in the [C₄mim][PF₆], as shown in Fig. 5a. The dispersions of silica nanoparticles in [C₄mim][PF₆] appear as a colloidal gel, and both the viscosity and the G′ display very high values at the initial application of shear. The hydrogen bonds between silanol groups are weak. With increasing shear rate or strain, corresponding to the increase of shear stress, the particulate network is disrupted, resulting in the significant decline of viscosity, G′ and G′′. When it comes to the dispersions of silica nanoparticles in [C₄mim][BF₄], [BF₄] anions interact with hydrophilic silica particles through hydrogen bonding between the F atoms of [BF₄] anions and the surface silanol groups.⁴⁴ Ionic solvation layers around the silica nanoparticles are generated,^35,45–47 and steric hindrance forces arising from solvation layers make silica nanoparticles non-flocculent in [C₄mim][BF₄]. However, the [BF₄] anions cannot be closely arranged on the particle surface because of the repulsive forces between them, and some silanol groups may not form hydrogen bonds as shown in Fig. 5b. So that some bare silanol groups may form hydrogen bonds with adjacent silica nanoparticles, and some very small aggregations (Fig. 5b) may exist in the dispersions of [C₄mim][BF₄]. The increase of G′ and G′′ in the shear thickening region demonstrates that silica nanoparticles aggregate.^14,32 At the same time, their increase was accompanied by the increase of viscosity. We can make an ansatz that silica nanoparticles are forced to aggregate and form clusters at high shear rates, which makes the dispersions display a shear thickening behavior. Silica nanoparticle aggregations will be destroyed when the shear stress is strong enough at very high shear rates. So that, the viscosity decreases, displaying a second shear thinning, and the G′ declines.

Fig. 5 Schematic illustrations of silica nanoparticles dispersed in (a) hydrophobic ILs, such as [C₄mim][PF₆]; in (b) hydrophilic ILs consisting of BF₄⁻ and different length alkyl chain substituents on the cation, such as [C₂mim][BF₄], [C₄mim][BF₄] and [C₆mim][BF₄]; and in (c) hydrophilic ILs containing [BF₄]⁻ and the organic cation branched with hydroxyl, such as [C₂OHmim][BF₄].

3.2 Effects of the length of alkyl chain substituents

To understand the effect of the ILs' organic cationic structure on the STFs, the rheology and viscoelasticity of dispersions based on three different ILs, consisting of different length alkyl chain substituents on the methylimidazolium and [BF₄] anion, were investigated. As shown in Fig. 6a, the dispersions containing 12 vol% silica nanoparticles in [C₂mim][BF₄], [C₄mim][BF₄] and [C₆mim][BF₄] all display shear thickening behavior. The shear thickening behavior in [C₆mim][BF₄] was different from that in [C₂mim][BF₄] and [C₄mim][BF₄]. The initial viscosity of STFs increases observably with increasing the length of branched alkyl chain on the methylimidazolium at low shear rates. The first shear thinning behavior becomes much more remarkable with increasing the length of the alkyl chain. The increase of viscosity in the shear thickening region for the STF in [C₄mim][BF₄] was much more prominent. Fig. 6b show the viscoelasticity of them. For the STFs in [C₂mim][BF₄] and [C₄mim][BF₄], both G′ and G′′ were independent with the increase of strain, and the G′′ was higher than G′ by nearly one order of magnitude before shear thickening. This indicates that silica nanoparticles in both ILs were non-flocculent (as shown in the insets of (A) and (B) in Fig. 6b).³² For the STF in [C₆mim][BF₄], both G′ and G′′ were much higher than those in [C₂mim][BF₄] and [C₄mim][BF₄]. The G′ was even higher than G′′ at low strain values, and both of them declined quickly with increasing strain before shear thickening, implying that the STF of silica nanoparticles in [C₆mim][BF₄] may form a weak gel (the mobility was poor, as shown in the inset of (C) in Fig. 6b) for the too-long branched alkyl chain. Both G′ and G′′ began to increase as soon as a critical strain was reached, and the critical strain increased with the increased of the branched alkyl chain length. The shear thickening behavior of STFs were strongly related to the length of alkyl chain substituent on the organic cation. Compared to the suspension in [C₄mim][BF₄], the critical shear rate was higher with short branched alkyl chain ([C₂mim][BF₄]), while the shear thickening behavior was weaker with the longer branched alkyl chain ([C₆mim][BF₄]). That is to say, many more silica nanoparticles aggregated in the shear thickening region in [C₄mim][BF₄], compared with [C₂mim][BF₄] and [C₆mim][BF₄].^14,32 The possible explanation for those differences are discussed based on the following equations and theories.

Fig. 6 (a) Shear rate dependencies of viscosity for the dispersions with 12 vol% silica nanoparticles in [C₂mim][BF₄], [C₄mim][BF₄] and [C₆mim][BF₄] at 30 °C under the steady shear. (b) Strain dependencies of storage modulus (G′) and loss modulus (G′′) at 30 °C under oscillatory shear. Insets: the photographs of (A), (B) and (C) are dispersions with 12 vol% silica nanoparticles in [C₂mim][BF₄], [C₄mim][BF₄] and [C₆mim][BF₄], respectively.

For traditional STF, the formation of a cluster is governed by the hydrodynamic, van der Waals, electrostatic, steric hindrance and Brownian forces.^5,14,19 However, the inter-particle forces in ILs under steady shear are different from those of traditional STFs. The van der Waals forces between particles will not be significant and can be neglected in non-aqueous highly particle-concentrated dispersions.⁴⁸ Meanwhile, the inter-particle electrostatic force is depressed and neglected in the dispersions of ILs.¹⁴ Brownian force scales with k_BT/r, where k_B is the Boltzmann constant, T is the temperature of the dispersions, and r is the hydrodynamic radius of particle. In our study, the temperature is constant and, therefore, the effect of Brownian force is ignored. It is clear that the cluster formation in the dispersions of ILs containing silica nanoparticles is really governed by the balance of hydrodynamic forces and the steric hindrance forces for a constant temperature system. The dispersing media is deformed under shear, which induces hydrodynamic forces on the particles. The hydrodynamic force has been estimated for two particles by determining the pressure field upon solving the Stokes equation,

where, η₀ is the viscosity of the dispersed media, a is the particle radius, γ is the shear rate and h is the distance between two particles.^19,49 The solvation layers of ILs on the surface of hydrophilic silica nanoparticles has been confirmed in the Brownian dispersions.^47,50–52 The solvation layers induce a steric hindrance for approaching particles. The steric hindrance force can be estimated by eqn (2) through differentiating steric hindrance potential functions for approaching particles with polymer solvation layers (see eqn (S1)–(S3) in the ESI†).⁵³where, δ is the thickness of solvation layer. As the distance, h, decreased continuously, and when h < 2δ, a steric hindrance force arises for the compression of the approaching solvation layers. The steric hindrance force, F_steric, will become infinite when h is much less than the thickness of solvation layer, as expressed in eqn (2). The F_steric will increase when the thickness of solvation layer increases for either 0 < h < δ or δ < h < 2δ. It is clear that the hydrodynamic force, F_hydrodynamic, will increase as the viscosity of ILs increases, and the steric hindrance force, F_steric will increase as the length of alkyl chain substituent increases. For STFs in [C₂mim][BF₄], the hydrodynamic force, F_hydrodynamic, may be very weak due to the much lower viscosity of [C₂mim][BF₄] liquid. Therefore, a few silica nanoparticles aggregate in [C₂mim][BF₄] in the shear thickening region, leading to the increase of the critical shear rate and the decrease of the critical viscosity. For STFs in [C₆mim][BF₄], the steric hindrance force, F_steric, may be very strong due to the increasing alkyl length of the ionic liquid and then a few silica nanoparticles aggregate in the shear thickening region, also leading to the increase of the critical shear rate.

3.3 Effects of the introduction of hydroxyl

The rheological and viscoelastic properties of STFs with 12 vol% silica nanoparticles in [C₂mim][BF₄] and [C₂OHmim][BF₄] are shown in Fig. 7a and b, respectively. The initial viscosity of STF in [C₂OHmim][BF₄] was much higher than that of STF in [C₂mim][BF₄]. The critical shear rate of STF in [C₂OHmim][BF₄] was less than that in [C₂mim][BF₄]. The maximum viscosity in the shear thickening region of STF in [C₂OHmim][BF₄] was much higher than that of STF in [C₂mim][BF₄]. The first shear thinning was weak for both of them. Both G′ and G′′ were independent with strain and the G′′ was higher than G′ by almost one order of magnitude before shear thickening for STF in [C₂OHmim][BF₄], also implying that silica nanoparticles were non-flocculent in [C₂OHmim][BF₄] even at high concentration. Both G′ and G′′ will increase as soon as a critical strain is reached, indicating that silica nanoparticles aggregate in the shear thickening region. As previously described, the density of [BF₄] anions on the surface of silica nanoparticles may be limited to the electrostatic repulsion for STFs in [C₂mim][BF₄], [C₄mim][BF₄] and [C₆mim][BF₄]. On the one hand, the [C₂OHmim] cations containing the OH end-groups have a strong interaction with the surface silanol groups through hydrogen bonding for higher polarity parameters.³⁵ On the other hand, an electrostatic attraction exists between the [BF₄] anions and the [C₂OHmim] cations. These are beneficial to the formation of the hydrogen bonding between the [C₂OHmim] cations and the bare silanol groups, as shown in Fig. 5c. In this case, the interactions between the dispersing media and silica nanoparticles is enhanced in [C₂OHmim][BF₄] for the introduction of OH on the cation, which leads to both G′ and G′′ of STF in [C₂OHmim][BF₄] being higher than in [C₂mim][BF₄] by one order of magnitude before shear thickening. Meanwhile, more hydrogen bonds formed on the surface of silica nanoparticles inhibit the aggregation of silica nanoparticles in the dispersing media, as shown in Fig. 5c. The average inter-particle distance, h, is shorter in [C₂OHmim][BF₄] than in [C₂mim][BF₄] for STFs with the same concentration, and the formation of nanoparticle aggregation needs less time for STF in [C₂OHmim][BF₄] than in [C₂mim][BF₄], such that the critical shear rate for STF in [C₂OHmim][BF₄] decreases. At the same time, the viscosity of [C₂OHmim][BF₄] is higher than that of [C₂mim][BF₄], which makes the hydrodynamic force of STF in [C₂OHmim][BF₄] higher than in [C₂mim][BF₄] under steady shear, as eqn (1) expressed, so that more nanoparticles will be aggregated in the dispersions of [C₂OHmim][BF₄] in the shear thickening region, and the shear thickening behavior of STF in [C₂OHmim][BF₄] is more significant than in [C₂mim][BF₄]. As shown in Fig. 6 and 7, the increase of viscosity in the shear thickening region of STF in [C₂OHmim][BF₄] is the highest compared with STFs in [C₂mim][BF₄], [C₄mim][BF₄] and [C₆mim][BF₄]. Both G′ and G′′ of STF in [C₂OHmim][BF₄] in the shear thickening region are much higher than in other ILs, indicating more nanoparticles aggregate for STF in [C₂OHmim][BF₄]. This demonstrates that the introduction of hydroxyl on the organic cation of hydrophilic ILs has a very positive effect on the improvement of the shear thickening property of STFs.

Fig. 7 (a) Shear rate dependencies of viscosity for dispersions with 12 vol% silica nanoparticles in [C₂mim][BF₄] and [C₂OHmim][BF₄] at 30 °C under steady shear. (b) Strain dependencies of G′ and G′′ for dispersions at 30 °C under oscillatory shear.

3.4 Dilatancy of STFs

Based on the rheological experiments, the upper cone of the rheometer will thus be attracted toward the bottom plate for negative values of the normal force and pushed away for the positive values (schematic illustration shown in the inset in Fig. 8). The normal forces of the cone present negative values before shear thickening for all STFs, and the absolute value of them increases with increasing the viscosity of the dispersed media. The negative normal forces are attributed to the strong hydrodynamic forces at low shear rates,⁹ demonstrating that the hydrodynamic force in STFs increases with increasing the length of alky chain substituent as well as with introducing the hydroxyl on the cation for hydrophilic ILs. The normal force is constant and then declines sharply with increasing the shear rate for STF in [C₂mim][BF₄]. The sharp decline results from the strong centrifugal force. As shown in Fig. 8, the normal forces increase jumpily with increasing shear rate in the shear thickening region for STFs in [C₄mim][BF₄], [C₆mim][BF₄] and [C₂OHmim][BF₄]. The increase of normal forces on the cone is accompanied by the increase of viscosity, G′ and G′′ in STFs. It is possible that nanoparticles aggregate in the shear thickening region, and they are large and concentrated enough to jam the measuring system, but do not organize in such a way that the STF becomes dilatant, displaying dilatancy. Both the amount and the volume of nanoparticle aggregation increases with increasing shear rate in the shear thickening region, which makes the dilatancy intensify and the normal force increase. More nanoparticles aggregate for the high hydrodynamic forces for STFs in [C₄mim][BF₄] and [C₂OHmim][BF₄], leading to the prominent dilatancy. For the STF in [C₂mim][BF₄], both the amount and the volume of nanoparticle aggregations are so small for the low hydrodynamic forces that dilatancy does not take place. The nanoparticle aggregation is a metastable conformation,⁵⁴ and it will be destroyed and rebuilt continuously under tangential shear stress, so that the normal force is fluctuant.

Fig. 8 Shear rate dependencies of normal force for the dispersions with 12 vol% silica nanoparticles in [C₂mim][BF₄], [C₄mim][BF₄], [C₆mim][BF₄] and [C₂OHmim][BF₄] at 30 ^oC under a steady shear application. Inset: schematic illustration for the normal force of the cone.

3.5 Properties of fabric composites treated by STFs containing ILs

The dynamic stabbing resistance and quasi-static tensile strength of neat Kevlar fabrics and Kevlar fabric composites treated by STFs are shown in Fig. 9 and 10, respectively. It is clear that both the dynamic stab resistance and the quasi-static tensile strength of Kevlar fabrics are enhanced after treatment with STFs. The tensile strength of Kevlar fabrics treated by STFs containing [C₄mim][BF₄] and [C₂OHmim][BF₄] were even one time higher than that of neat ones. Comparing with Kevlar fabric composites treated with traditional STFs, the penetration depth of a knife decreased on the whole for those treated with STFs composed of ILs; the quasi-static tensile strength of composites treated by STFs composed of [C₄mim][BF₄] and [C₂OHmim][BF₄] were enhanced by 1000–2000 N. On the one hand, STFs dissipate energy under the application of impact or shear. On the other hand, the mobility of fibers is restricted because of the shear thickening of STFs between fibers, which destroys more fibers at the same time, as shown in Fig. S2b–e.†^15,16,18 The conductivity values of neat Kevlar fabrics and those treated by STFs composed of PEG-400 were about 0.568 × 10⁻¹⁰ S m⁻¹ and 0.312 × 10⁻¹⁰ S m⁻¹, respectively. They increased to 0.178–0.327 × 10⁻⁴ S m⁻¹ (see Table S4 in the ESI†) after treating with STFs composed of [C₂mim][BF₄], [C₄mim][BF₄] and [C₂OHmim][BF₄]. It is beneficial to eliminate the adverse effect of static electricity on practical applications. The EMI SE of neat Kevlar fabrics and STF-treated Kevlar fabric composites is shown in Fig. 11. For neat Kevlar fabrics and those treated by STF composed of PEG-400, the EMI SE was about −0.5 dB for the whole range of frequencies. For Kevlar fabric composites treated by STFs composed of [C₂mim][BF₄], [C₄mim][BF₄] and [C₂OHmim][BF₄], the EMI SE was about −3.0 dB and was enhanced so that they had a certain degree of EMI shielding ability.

Fig. 9 Knife stabbing test results for neat Kevlar fabrics and Kevlar fabric composites treated with STFs composed of 15 vol% silica nanoparticles in PEG-400, [C₂mim][BF₄], [C₄mim][BF₄] and [C₂OHmim][BF₄].

Fig. 10 Tensile strength testing results for neat Kevlar fabrics and Kevlar fabric composites treated with STFs containing 15 vol% silica nanoparticles in PEG-400, [C₂mim][BF₄], [C₄mim][BF₄] and [C₂OHmim][BF₄].

Fig. 11 EMI SE of neat Kevlar fabrics and those treated by STFs containing 15 vol% silica nanoparticles in PEG-400, [C₂mim][BF₄], [C₄mim][BF₄] and [C₂OHmim][BF₄].

Conclusions

Effects of ionic structures on STFs composed of five different ILs and hydrophilic silica nanoparticles were investigated systemically for the first time. Firstly, STFs were affected significantly by the hydrophilic property of ILs. The dispersions of hydrophilic ILs display shear thickening behavior, while those of hydrophobic ones appear as a colloidal gels and display shear thinning. Secondly, the shear thickening behavior of STFs were strongly related to the length of alkyl chain substituent on the organic cation. Thirdly, the shear thickening behaviors of STFs were enhanced with the introduction of hydroxyl on the organic cation. Ionic structures influence the interactions between silica nanoparticles and ILs through the hydrogen bonds between silanol groups and ions, which influence the hydrodynamic forces and steric hindrance forces of silica nanoparticles in ILs. The formation of nanoparticle aggregations leads to shear thickening in STFs consisting of hydrophilic silica nanoparticles and ILs, and is intrinsically controlled by the hydrodynamic forces and steric hindrance forces for a constant temperature. For the customizability of ILs, by introducing more polar groups on both anions and cations, excellent STFs composed of ILs can be exploited by referencing the results of this paper.

Compared with traditional STFs prepared by low molecular weight PEG and hydrophilic silica nanoparticles,⁴⁰ the shear thickening behavior was enhanced observably for STFs prepared with ILs consisting of hydrophilic anions and hydrophilic cations. Compared with neat Kevlar fabrics and those treated with traditional STFs, the dynamic stabbing resistance, quasi-static tensile strength and conductivity of Kevlar fabric composites treated by STFs composed with ILs were enhanced observably; the EMI SE was improved to a certain degree. For the above demonstrated improvements of STFs composed of ILs in composites and other particular properties of ILs, STFs consisting of ILs may provide more chances than traditional ones to exploit functional impact-resistance composites, such as soft body armor and spacecraft shielding, to mitigate the highly energetic orbital debris impacts in high vacuum environments. Moreover, functional damping materials and devices based on STFs composed of ILs can be exploited. However some experiments, such as small-angle neutron scattering and confocal microscopy, should to be done to discover more objective evidences of nanoparticle aggregation in the shear thickening region. Effects of more ILs containing different ionic structures on STFs composed of silica nanoparticles or other nanoparticles should be considered.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors are grateful for the financial support provided by the National Science Foundation of China (No. 51303149), foundation for the Fundamental Research Funds for Central Universities (3102014JC01095).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12460g

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