LAUSR

C electrification is the phenomenon in which static charge is generated when two materials come into contact and are then separated. After contact, one material charges positively and the other charges negatively (Figure 1). This phenomenon is ubiquitous: almost all types of materials (e.g., metal, semiconductor, inorganic materials, and polymers) charge upon contact. Hence, contact electrification has a vast range of influences in our lives. In many of our daily activities, we frequently encounter slight annoyances as a result of static charge (e.g., the adherence of dust particles on charged surfaces and the sticking of clothes after drying). The phenomenon is also problematic for many types of industries (e.g., petrochemical, chemical, pharmaceutical, semiconductor, and packaging). For example, particles charged by contact electrification may adhere onto the walls of vessels and hinder effective heat transfer. In addition, excessive accumulation of static charge can cause electrostatic discharge (e.g., sparks). Electrostatic discharge can damage equipment and is known to cost the electronic industry billions of dollars per year. Importantly, it can also cause explosion of flammable gas, liquid, and solid. On the other hand, many applications rely on contact electrification; a few representative examples include electrophotography, electrostatic separation, electrostatic coating, microfluidics (e.g., manipulation of droplets), and electrostatic self-assembly. In particular, there has been a lot