Atmospheric adsorbates break symmetry in oxide electrification

From sandstorms and volcanic plumes, electrical charging of small particles is of critical importance in many geophysical settings. How do the particles in these systems become charged in the first place? In this talk, I will discuss our experimental work on the transfer of electrical charge that occurs when two solid objects are contacted and separated. We focus on oxides (e.g., SiO₂) as they are the most abundant and relevant class of materials on the earth, which presents a number of challenges. First, they are extremely hard, which means their contact areas—and hence charge exchange—are extremely small. Second, direct handling introduces spurious charge that can overwhelm the signal we wish to measure. We overcome these challenges using acoustic levitation, which enables thousands of automated, hands-free contacts and charge measurements with few-hundred-electron resolution on macroscopic samples. Our experiments reveal that oxide contact electrification is not due to any bulk material property, but instead arises from surface adsorbates—specifically adventitious hydrocarbons—acquired by objects from the air that surrounds them. These findings, now in press at Nature, are the long sought source of particulate charging in settings ranging from desert sands to volcanoes and beyond.