Upstream contamination by floating particles

It has been known at least since the work of Reynolds and Marangoni in the 1880s that floating particulates strongly affect water surface behaviour, and research involving particle-fluid interactions continues in modern applications ranging from microfluidics and cellular morphogenesis to colloidal dynamics and self-assembly. Here, we report and analyse an unexpected result from a simple experiment: clean water is discharged along an inclined channel into a lower container contaminated with floating particles. Surprisingly, the floating particles are transported both up a waterfall as long as 1 cm, and upstream in channels to lengths of at least several metres. We confirm through experiments and simulations that this upstream contamination is paradoxically driven by the downstream flow of clean water, which establishes a surface tension gradient that sustains the particulate motion. We also show that contamination may occur in practical applications, such as the discharge of a standard pipette or simulated release of waste into larger scale channels.