The odd free surface flows of a colloidal chiral fluid

Simulations and measurements have established that water moves through carbon nanotubes with exceptionally high rates due to nearly frictionless interfaces1–4. These observations have stimulated interest in nanotube-based membranes for applications that range from desalination to nano-filtration and energy harvesting5–10, yet the exact water transport mechanisms inside the nanotubes and at the water-carbon interface continue to be controversially discussed11,12 because existing theories fail to provide a satisfying explanation for the limited number of experimental results available to date13. This is because even though controlled and systematic studies have explored transport through individual nanotubes8,9,14–7, none has met the considerable technical challenge of unambiguously measuring the permeability of a single nanotube11. Here we show that the pressure-driven flow rate across individual nanotubes can be determined with unprecedented sensitivity and without dyes from the hydrodynamics of water jets as they emerge from single nanotubes into a surrounding fluid. Our measurements reveal unexpectedly large and radius-dependent surface slippage in carbon nanotubes (CNT), and no slippage in boron-nitride nanotubes (BNNT) that are crystallographically similar to CNTs but differ electronically. This pronounced contrast between the two systems must originate from subtle differences in atomic-scale details of their solid-liquid interfaces, strikingly illustrating that nanofluidics is the frontier where the continuum picture of fluid mechanics confronts the atomic nature of matter.

Spontaneous pattern formation by self-assembly is of long-standing and continuing interest not only for its aesthetic appeal, but also for its fundamental and technological relevance. So far, the study of self-organization processes has mainly focused on static structures, but dynamic systems--those that develop order only when dissipating energy--are of particular interest for studying complex behaviour. Here we describe the formation of dynamic patterns of millimetre-sized magnetic disks at a liquid-air interface, subject to a magnetic field produced by a rotating permanent magnet. The disks spin around their axes with angular frequency equal to that of the magnet, and are attracted towards its axis of rotation while repelling each other. This repulsive hydrodynamic interaction is due to fluid motion associated with spinning; the interplay between attractive and repulsive interactions leads to the formation of patterns exhibiting various types of ordering, some of which are entirely new. This versatile system should lead to a better understanding of dynamic self-assembly, while providing a test-bed for stability theories of interacting point vortices and vortex patches.