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Friction 7(1): 2–17 (2019) process [58]. However, due to the scope of our review, they will not be discussed in equal detail here. 4.1 Microstructure/pattern/geometry As stated in Section 2, an interesting phenomenon in the triboelectric effect is the tribocharging of chemically identical materials as the friction contact surfaces [61]. Different explanations and theories are provided to understand the electrification, among which pattern and geometry are believed to play a vital role. Wang et al. [53] proved that the differences in the microstructure of chemically identical materials trigger distinct tribo‐charging behavior. In this sense, as a strained surface will exhibit different microstructures due to voids and seams (which can be scaled from nano‐ to micrometers) and the different microstructures will trigger different surface potential energy minimum according to catastrophe theories [53, 54], as shown in Fig. 5, the strained surface may exhibit different triboelectric behavior [20, 53]. Except for the microstructure effects, the surface patterns can be tuned by the surface roughness, which can also tune the triboelectric behavior. Indeed, different area roughness will make the tribo‐charging more local and the mosaic pattern for charges (whether positive or negative) will exist [62]. Interestingly, the final charge distribution pattern, irrespective of the micro or macroscale [61–63], will also be affected, and in turn affect the triboelectric charging process. This indicates that the local charge can be much greater in magnitude than the net average charge on the surface, and that the net triboelectric effect represents an average of the possible con‐ tributions from both positively and negatively charged local regions [1]. 4.2 Particle size For the same materials in particle shape, when the particles are in different sizes, opposite polarities will form to help with the bipolar charging process [42, 64]. The size‐dependence of particle triboelectrification is observed in both natural phenomena [10, 65] and industrial processes [11, 66], which gives it priority in research fields and for which many modeling studies have been conducted [9, 67, 68]. Detailed studies on particle sizes have suggested 11 that the magnitude of the charge increases continuously with a decrease in particle size, as indicated in Table 4 [69]. A possible reason is thought to be the adhesion of fine particles (<40 μm) to coarse particles, which eventually enhance the surface roughness of the coarse particles, similar to the effects of the pattern/ microstructure shown in Table 4. Except for the pure charge (density) analyses, triboelectric series analyses have also been given equal importance [64, 70, 71]. These experiments and simulations confirm the general trend that large particles tend to charge positively, while small particles tend to charge negatively. The theory behind this trend can be summarized from the following several aspects [24, 70]. For a bimodal mixture particle system (as a simplest case) of masses in mL and ms , respectively, the mass fraction w is defined as mL B mm . With solid‐sphere Ls simplification for particles, the probability P(Di , Dj ) of a collision between two particles with sizes Di and Dj , respectively, will lead to the collision fraction occurring between a large particle and a small particle fLS and give it the form of [70] 1D /D 2 0.5 L s xsxL f2 Table 4 LS 1D /D 2 x2(D /D)2x20.5 L sLsLsL (18) where D indicates the diameters for either particle (in large or small size) and x indicates the number fractions for either large or small size in a charged state (positive or negative). Then, the charge segregation factor is calculated as x x LS s xx 2 x x LS (19) Effect of particle size on tribo-charging. Particle size (um) Charge (nC/g) 335−500 10 250−355 28 125−150 50 90−125 65 http://friction.tsinghuajournals.com∣www.Springer.com/journal/40544 | FrictionPDF Image | Fundamental theories basic principles of triboelectric effect
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