Fig. 1.

(A) Scanning electron image of a channel of inner height $2.5\mu$m. Narrow openings at the two ends are marked by red asterisks. Inset illustrates the channel’s geometry: the laser-scanning contour, $f(x)$, the wall inner boundary, $h(x)$, and the effective boundary of the space accessible to the particle center, $g(x)$; $d_t\simeq \mathrm{\hspace{0.17em}0.7}\mu$m and $r=\mathrm{\hspace{0.17em}0.5}\mu$m are the wall thickness and the particle radius, respectively. (B) Spatial distribution of particle counts in a typical 20-h-long experiment. The effective channel boundary is marked by black lines and is denoted by $\pm g(x)$ (Eq. 1); here, $g_n=\mathrm{\hspace{0.17em}0.5}\mu$m, and $g_w=\mathrm{\hspace{0.17em}4.5}\mu$m.