Influence of Wettability and Geometry on Contact Electrification between Nonionic Insulators

Abstract

Contact electrification is an interfacial process in which two surfaces exchange electrical charges when they are in contact with one another. Consequently, the surfaces may gain opposite polarity, inducing an electrostatic attraction. Therefore, this principle can be exploited to generate electricity, which has been precisely done in triboelectric nanogenerators (TENGs) over the last decades. The details of the underlying mechanisms are still ill-understood, especially the influence of relative humidity (RH). Using the colloidal probe technique, we convincingly show that water plays an important role in the charge exchange process when two distinct insulators with different wettability are contacted and separated in <1 s at ambient conditions. The charging process is faster, and more charge is acquired with increasing relative humidity, also beyond RH = 40% (at which TENGs have their maximum power generation), due to the geometrical asymmetry (curved colloid surface vs planar substrate) introduced in the system. In addition, the charging time constant is determined, which is found to decrease with increasing relative humidity. Altogether, the current study adds to our understanding of how humidity levels affect the charging process between two solid surfaces, which is even enhanced up to RH = 90% as long as the curved surface is hydrophilic, paving the way for designing novel and more efficient TENGs, eco-energy harvesting devices which utilize water and solid charge interaction mechanism, self-powered sensors, and tribotronics.

Toward the Assembly of 2D Tunable Crystal Patterns of Spherical Colloids on a Wafer-Scale

“…A similar observation was previously made, where it was found that the cohesive silica powder required more energy than the polystyrene powder in order to be fluidized. 39 To eradicate the water content from the silica powder and thus reduce F p−p among the silica particles such that the aggregate can be easily crushed into single particles, 40,41 we repeated the experiments in a zero-humidity glovebox. As a consequence, Figure 1d shows that we can assemble the silica powder into pronounced HCP structures, albeit with some defects, on the fluorocarbon-coated substrates inside of the glovebox.…”

“…The fact that the tribocharging-induced electrostatic attraction on the fluorocarbon-coated substrate is stronger in comparison to the SiO 2 substrates is corroborated by our recent colloidal probe findings. 40 Of course, intuitively, one expects that for both of these hydrophilic SiO 2 substrates and silica powder, the capillary force may also dominate the adhesion force under ambient conditions. Therefore, we also performed experiments on a hydrophilic silicon substrate carrying a 2 nm native oxide layer, which is stiffer than that of the 500 nm SiO 2 substrate.…”

“…Note that on the fluorocarbon-coated substrate, tribocharging is also the strongest, which adds to the adhesion. 40 On the other hand, the monolayers on the pristine silicon substrates contain less pronounced HCP crystal structures of PMMA microspheres, albeit significantly more than the silica microspheres. This result can be ascribed to the lower Young's modulus of the PMMA microspheres than the silica ones, which promotes the adhesion between PMMA particles and the pristine silicon substrates.…”

“…This is supported by our colloidal probe study in which contact electrification between a polymer sphere and a silicon substrate was measured within a second. 40 In addition, the charging of PMMA particles due to the PDMS stamp, i.e., charging between two polymers by means of material transfer, 46,47 cannot be excluded.…”

“…Furthermore, the data in Figure 1e,f, particularly those obtained using the silica particles, shows that the strongest tribocharging-induced electrostatic attraction emerges on the fluorocarbon-coated substrate. 34,40 4.2. Different Stamps.…”