rspa.royalsocietypublishing.org Research Cite this article: Bianchini S, Lage A, Siu T, Shinbrot T, Altshuler E. 2013 Upstream contamination by foating particles. Proc R Soc A 469: 20130067. http://dx.doi.org/10.1098/rspa.2013.0067 Received: 1 February 2013 Accepted: 4 June 2013 Subject Areas: fuid mechanics, chemical engineering, biomechanics Keywords: surface tension, Marangoni forces, particulate transport Authors for correspondence: Troy Shinbrot e-mail: shinbrot@rutgers.edu Ernesto Altshuler e-mail: ealtshuler@fsica.uh.cu † These authors contributed equally to this study. Electronic supplementary material is available at http://dx.doi.org/10.1098/rspa.2013.0067 or via http://rspa.royalsocietypublishing.org. Upstream contamination by foating particles Sebastian Bianchini 1,† , Alejandro Lage 1,† , Theo Siu 2,† , Troy Shinbrot 2 and Ernesto Altshuler 1 1 Facultad de Fisica, Universidad de la Habana, La Habana, Cuba 2 Department of Physics, Rutgers University, Piscataway, NJ 08854, USA 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. When water is poured into a teacup, it seems self-evident that material in the cup will not make its way upstream into the teapot. Similarly, when chemicals are pipetted onto a culture plate, it is taken for granted that cells from the plate will not contaminate the chemical source. In this work, we demonstrate that contrary to expectation, floating particles can contaminate upstream reservoirs by travelling in rapid jets at accelerations substantially larger than the gravitational downstream acceleration. This counterintuitive phenomenon was first observed during the preparation of mate tea, when hot water was poured from a pot into a cup containing tea leaves: it was found that when the spout was within 1 cm above the leaves, floating leaves would find their way from the cup into the pot (see the electronic supplementary material, figure S1). 2013 The Author(s) Published by the Royal Society. All rights reserved.