Alejandro Lage Castellanos - Politecnico di Torino #
Upstream contamination by floating particles #
It has been known at least since the studies of Reynolds and Marangoni in the 
1880's that floating particulates strongly affect water surface behaviors, and 
research 
involving
particle-surface effects continues in modern applications ranging from microfluidics 
and
self-cleaning surfaces to colloidal dynamics and self-assembly. Here we analyze the
behavior of fine particles on a steady downstream flow. Surprisingly, we find that 
rapid
and robust vortices appear, transporting particulate material upstream at 
accelerations as
much as 20 times gravity. We confirm through experiments and simulations that this
upstream contamination is paradoxically driven by downstream flow of clean water 
which
establishes a surface tension gradient that sustains the particulate motion. We 
briefly
outline possible implications of this work.