Violation of triboelectric charge conservation on colliding particles

In microgravity experiments, we quantified the net charge on systems of two identical, 434-$\mu \mathrm{m}$-diameter glass spheres before and after a collision. We find that charge conservation is significantly violated. Independent of the sign of the total charge, the systems regularly lose some of their net charges, that is, they slightly discharge. This implies that positive as well as negative charge carriers become entrained into the surrounding atmosphere during a collision.

Figure 1

Setup of the experiment taken from Steinpilz et al. [11].

Figure 2

Total charge on two particles after ($q_1^{\prime }+q_2^{\prime }$) and before ($q_1+q_2$) a collision. The bold black line marks the charge conservation (slope = 1). The dashed blue line is a linear trend to the data (slope = $0.80\pm 0.03$). Red square symbols mark cases of unambiguous violation of charge conservation. The inset in the top left is an example taken from the experiments to visualize the situation. Here, an overlay of 22 images during a collision is shown, covering a period of 120 ms. Other particles in the surroundings are removed.

Figure 3

Charge balance for different charge combinations in collisions between two particles. The radial distance of each data point to the center shows the sum of both charges ($q_1+q_2$). Each quadrant marks a certain polarity combination. Due to the arbitrary choice of the first and second particle, the top left and bottom right quadrant are equivalent. The direction of the position vector corresponds to the specific charge ratio $[\varphi =arctan(q_1/q_2)]$. Black and red dots mark the total charges before and after a collision, respectively. Inward directed arrows mark a loss in total charge, outward directed arrows mark a gain in total charge. Top: All measured values; Bottom: Zoom into the inner rings up to total charges of $5\times {10}^5\mathrm{e}$.