The glass beads utilised in this study were made of soda glass, a product commercially known as SiLibeads^®, supplied by Sigmund Lindner, Warmensteinach, Germany. The beads have a size range of 90–150 μm, making them suitable for the intended application. Nevertheless, they were sieved again to ensure that they were in the designated sieve size range.

The aluminium mono-stearate was obtained from Merck Life Science (based in Dorset, United Kingdom), while the aluminium di-stearate and aluminium tri-stearate were obtained from Peter Greven (based in Bad Münstereifel, Germany). These differences in the number of alkyl chains could influence the way the different types interact with the treated surfaces, and thus their influence in charge mitigation.

Both the acid-washed and silanised glass beads were subdivided into smaller batches of 40 g to be blended with the three types of aluminium stearate powder. Different amounts of aluminium stearate were weighted and added to the glass beads, producing samples with 0.5 and 3 wt% of aluminium stearate, respectively. The coating process was performed using the Formatrix 4M8-Trix ProCepT high shear mixer, which has a three-bladed impeller that induces mechanical agitation to the glass beads and aluminium stearate mixture, promoting a homogeneous mixing. Following the procedure described by Goh et al. (2021), the glass beads-AlSt mixtures were agitated under an impeller rotational speed of 400 RPM for 20 and 40 min, respectively.

The tribocharging rig, based on the work of Ali and Ghadiri (2017) and Zafar et al. (2018), was used in this work to assess the performance of aluminium mono-, di- and tri-stearates in mitigating the charge built-up on functionalised glass beads. A schematic of the setup is shown in Figure 1. The tribocharging rig makes use of the aerodynamic dispersion unit of Malvern Panalytical Morphologi G3 to induce charge to the particles fed into system. A short pulse of pressurised dry air of 20 ms is released into the dispersion capsule, upon which particles fully are dispersed and experience a large number of collisions against the inner confining walls. This causes charge transfer between the particles and the dispersion capsule, following which the particles exit the dispersion spool and fall into a Faraday cup, where their net charge is measured by a Keithley 6514 electrometer. The dispersion capsule and post-dispersion duct are interchangeable to provide different contact surfaces. In this work, we evaluated the charge mitigating effect of AlSt-coated glass beads against stainless steel, copper, aluminium and polytetrafluoroethylene (PTFE). The sample volume was kept constant at 5 mm³ for each test using a volumetric sampling spoon, and a dispersion pressure of 4 barg was used to disperse them. At least three measurements were made for each sample and an average charge level was calculated.

Surface modification via acid washing and silanisation changes the surface functional group of the glass beads. It is well known that acid washing hydroxylates the glass surfaces and exposes the -OH groups, while subsequent silanisation process introduces methyl groups (-CH₃) which replace the hydroxyl group on the glass surfaces. Having these vastly different surface functional groups, one can therefore expect these treated glass beads to differ in their charging behaviour. The specific charge per unit mass acquired by the surface-treated glass beads when they collide against different materials is shown in Figure 2. Untreated glass beads charge positively against aluminium, copper and PTFE. According to the classical triboelectric series of materials, glass is the most electropositive, followed by aluminium, steel, copper and PTFE (Gooding and Kaufman, 2019; Zou et al., 2020) and this is in agreement with the results obtained here. Interestingly, when charging against stainless steel, untreated glass beads acquire a negative charge. It is worth taking note that carbon steel is more electronegative compared to glass according to the triboelectric series; it was therefore intuitively assumed that stainless steel, while having other components in the alloy mix, should somehow exhibit similar triboelectric charging properties. The result obtained here, albeit slightly unexpected, is not unjustifiable. Unlike steel, stainless steel oxidises and a protective layer of chromium oxide (Cr₂O₃) is formed on the surface. This metal oxide surface has not been extensively studied in the literature and given a position in the triboelectric series. However, our finding of glass beads charging negatively against stainless steel is not unique. Choi et al. (2020) reported the same observation, where they also found glass beads charged negatively against stainless steel. They also measured the work function of both glass beads and stainless steel and found that the former has a higher work function. When two materials are interacting, the one with lower work function tends to donate electrons to the one with higher work function.

Acid-washed glass beads charge negatively against aluminium, copper and stainless steel, but positively against PTFE. Based on these results, the exposed hydroxyl groups on the glass surfaces reduce their electropositivity, as indicated by the shift in their relative position (from being the most electropositive to the third in the ranking) in the triboelectric series. The silanised glass beads have very similar triboelectric charging characteristics as compared to acid-washed glass beads, charging negatively against aluminium, copper and stainless steel, but positively against PTFE. The methyl groups appear to have a retarding effect on the chargeability. The only difference is that silanised glass beads charge slightly less compared to acid-washed glass beads against the four materials tested.

In terms of the charge level, whilst both acid-washed and silanised glass beads charge to the same polarity, their charge levels differ. Acid-washed glass beads appear to charge more compared to silanised glass beads regardless of the charge polarity. Against aluminium, acid washing and silanisation both change the polarity of the glass beads and bring their charge level to an almost similar value around −40 nC/g. Against copper, similar behaviour is seen, but the charge level of silanised glass beads is lower than that of acid-washed (−22 vs. −40 nC/g, respectively). Against PTFE, whilst keeping the same polarity, acid washing reduces the charge of glass beads by ∼10% from the untreated case, but for the silanised glass beads the charge level is reduced by ∼80%, down to 19 nC/g. Against stainless steel, both surface treatments actually enhance the charge level of the glass beads from 12 nC/g (untreated case) to around 30 nC/g for silanised and 48 nC/g for acid-washed cases.

For reference, the three forms of AlSt were tested on their own. The specific charge acquired per unit mass of the aluminium stearate powders against different materials is shown in Figure 3. Aluminium mono- and di-stearate charge positively against aluminium, copper and PTFE, but negatively against stainless steel. In contrast, aluminium tri-stearate powder acquires negative charge from aluminium, copper and stainless steel, but a positive charge from PTFE. Looking at the chemical structure of aluminium mono- and di-stearate (Figure 4), aluminium tri-stearate is far more ionic than the other forms of aluminium stearate. Hydroxyl groups absent in aluminium tri-stearate but that is not the case for aluminium mono- and di-stearates. The metal atom and the stearic acid chains of aluminium tri-stearate are held together by ionic bonding. It is therefore speculated that this could have led to the disparity seen in the charge polarity between aluminium tri-stearate and the rest, though further investigation is needed to support this.

The effect of coating acid-washed glass beads with aluminium stearate powders on their chargeability is shown in Figure 5. In general, at 0.5 wt%, all the three aluminium stearate powders show a good level of mitigating effect in the charge acquired by the acid-washed glass beads. Their effect is most prominent when the coated acid-washed glass beads are charged against stainless steel, showing a reduction in specific charge of ∼80%. On PTFE, the presence of aluminium stearate reduces the specific charge of acid-washed glass beads by ∼44%. A change in charge polarity is observed when the coated acid-washed glass beads are charged against aluminium and copper. In the case of aluminium, the level of charge acquired by the coated acid-washed glass beads, albeit of different polarity, is very small. This shows that the addition of aluminium stearate is very effective in retarding the charge transfer between the acid-washed glass beads and aluminium. The same cannot be extended to the case of copper, where the acid-washed glass beads acquire a considerable amount of opposite charge in the presence of aluminium stearate powder. While the difference is not very discernible amongst the three types of aluminium stearate, in most cases the tri-stearate form appears to show the most effective charge mitigating effect in acid-washed glass beads.

Increasing the concentration of aluminium stearate in acid-washed glass beads does not enhance their charge mitigating properties. In fact, this has an opposite effect where the acid-washed glass beads get even more charged as shown in Figure 5B. At 3 wt%, aluminium mono- and di-stearate exacerbate the specific charge acquired per unit mass of the acid-washed glass beads against aluminium, copper and PTFE. At its extreme, the charge level almost trebles when aluminium mono-stearate is added to the acid-washed glass beads and charged against aluminium. On the other hand, aluminium tri-stearate produces a good degree of charge reduction in the acid-washed glass beads when they are charged against aluminium and stainless steel. Against PTFE, the trend reverses, and the results show that aluminium tri-stearate actually aggravates the charge level of the acid-washed glass beads. In contrast, Aluminium mono-stearate, when added to the acid-washed glass beads at 3 wt%, produces a much better charge reduction effect compared to 0.5 wt% for the PTE case.

The effect of adding aluminium stearate to silanised glass beads on their charging behaviour is shown in Figure 6. While both the acid-washed and silanised glass beads exhibit similar charging tendency as previously discussed, they do not behave the same way when aluminium stearate is added to them. At 0.5 wt%, the silanised glass beads actually acquire higher charge levels in the presence of aluminium stearate for most of the cases. Against aluminium, all three forms of aluminium stearate show varying degrees of charge enhancement effect, with the mono-stearate form at the top of the list, followed by the di-stearate and tri-stearate forms. No change in the charge polarity of the silanised glass beads is observed when aluminium stearate is added to them. The only exception is when the silanised glass beads are coated with aluminium tri-stearate and charged against PTFE, as they charge negatively.

A notable increase in the charge acquired by the silanised glass beads is noted for aluminium mono- and distearates when their concentration is increased to 3 wt%. An interesting observation here is that the charge level does not increase much for the case of aluminium tri-stearate. Even though the amount of aluminium-tri-stearate added to the silanised glass beads is almost sixfold of the lower concentration, the charge acquired remain at almost the same level when they are charged against aluminium, copper and stainless steel. While the exact reason is not known, this highlights the special effect of aluminium tri-stearate. However, aluminium tri-stearate performs rather poorly when it is added to the silanised glass beads and charged against PTFE. It actually increases the charge level of the silanised glass beads the most.

To provide a more direct comparison of the charge mitigating effect of aluminium stearate in acid-washed and silanised glass beads, the test results are grouped together as illustrated in Figure 7. At low concentration (0.5 wt%), all the three forms of aluminium stearate reverse the charge polarity of the acid-washed glass beads from negative to positive when they are charged against aluminium. However, for silanised glass beads, while maintaining the same charge polarity, the charge level actually increases when aluminium stearate is added to them. Note that both acid-washed and silanised glass beads charge negatively against aluminium. The fact that aluminium stearate coated acid-washed and silanised glass beads have opposite charge polarity is an indication that the charge transfer mechanism entailed here is a complicated one. The same behaviour is also seen when they are charged against copper. One possible explanation to this behaviour is the restructuring of aluminium stearate molecules on the surface of the acid-washed and silanised glass beads. Aluminium stearate molecules are amphipathic, having a hydrophilic head and hydrophobic tails. The hydrophilic surface of acid-washed glass beads could attract the hydrophilic stearic acid chains of the aluminium stearate and form a structured coating on the surface of the acid-washed glass beads, where the hydrophobic tails are sticking out. On silanised glass beads, where the surfaces are hydrophobic, adherence of the stearic acid chains is favoured, resulting in a coating in which the hydrophilic heads are pointing outwards. Charge transfer happens at the contact interface and the different aluminium stearate coating structures could therefore alter the charge transfer process. Against PTFE, both acid-washed and silanised glass beads charge positively when coated with aluminium stearate. The only exception is the silanised glass beads coated with aluminium tri-stearate which charge negatively against PTFE, a highly surprising result. Against stainless steel, all of them charge negatively. However, a very effective charge reduction is observed in the acid-washed glass beads but not the silanised glass beads.

At 3 wt% (Figure 8), both the acid-washed and silanised glass beads are charged negatively against aluminium, contrasting to that of 0.5 wt%. In fact, this behaviour of acquiring the same charge polarity when they were charged against the same material is seen for all the four materials tested. When the concentration of aluminium stearate is high, the surface of the glass beads is heavily covered by it, and hence the charge transfer is potentially dominated by the interaction between the antistatic agent and the contacting surface. In addition, at high concentration, material transfer could also potentially play a vital role in the tribocharging behaviour of the coated glass beads. Nevertheless, whilst both acid-washed and silanised glass beads behave similarly when a high concentration of aluminium stearate is added to them, their behaviour is different from the observations made for pure aluminium stearate powders (Figure 3). Interestingly, when coated at 0.5 wt%, the behaviour of the coated acid-washed glass beads against the four contact materials is similar to the results obtained from dispersing pure aluminium stearate powders. However, at 3 wt%, the trend starts deviating, and the observations made in Figure 3 no longer apply. This is surprising, as one would expect the behaviour of the 3 wt% coated acid-washed glass beads to be more similar to that of the pure aluminium stearate powders. On the other hand, the trend of results of the coated silanised glass beads show no resemblance to the data collected for pure aluminium stearate powders (shown in Figure 3), regardless of the concentration. The experimental data presents a complex pattern of the effect of antistatic agent. Therefore, a first-principle molecular modelling approach based on density functional theory (DFT) could potentially provide valuable insights and help unravel the complex phenomenon observed here.