Conventional fluids flow only in response to external forces, but scientists are increasingly interested in “active matter” that consumes energy and moves itself. Previously, this has been seen only on millimetre or centimetre scales, but now researchers in the US have observed tiny molecular motors moving around metre-long tubes.
Flocks of birds, swarms of bees and other collective animal movements can be explained by cognitive decisions of the organisms involved. A bird, for example, sits in the wake of the bird in front to minimize drag. However, collective behaviour can also be found at the cellular and sub-cellular level. “Non-equilibrium behaviour is more general than what living or sentient creatures exhibit,” explains Seth Fraden of Brandeis University in Massachusetts.
The mechanisms through which this kind of self-organization occurs are not well understood. To gain more insight, Fraden, together with colleagues at Brandeis and Georgia Institute of Technology, studied the motor protein kinesin – which Brandeis’ Zvonomir Dogic describes as “the simplest biological engines capable of transducing chemical energy into mechanical motion”. They dispersed the kinesin in water with filaments called microtubules extracted from the brain tissue of cows. When fed with ATP – a chemical fuel used in the human body – the kinesin moves microtubules against each other, pulling the water along by viscous drag.
Experiments on concentrated bacterial suspensions reported in 2004 by the group of Raymond Goldstein at the University of Cambridge revealed a hallmark behaviour of active fluids. They observed that in an unconfined bulk, different parts of fluids move in different directions, creating turbulent vortices with a characteristic size that depends on the fluid. However, they later showed that when confined, these fluids can produce self-organized bulk motion on macroscopic scales.
These experiments, as well as theoretical simulations, have suggested that this requires the smallest confinement dimension to be smaller than the characteristic vortex size of the fluid. In the case of the Brandeis experiments, this would mean about 100 μm. To the Brandeis researchers’ astonishment, however, they observed coherent flows persisting in channels of various shapes that were over a millimetre wide and over a metre in length. “We found that you could keep going larger and larger and larger,” says Fraden. “No one knows what the upper bound is or whether there is an upper bound. We were limited by the number of cows that had to be slaughtered [to provide sufficient sample material].”
The researchers found that – regardless of the size – the aspect ratio of the space into which the fluid is confined appears crucial. If the cross-section of the channel is more than about three times as wide as it is tall, or vice versa, then the flow becomes turbulent. The reason, however, remains puzzling. “No one knows the answer – no one even knew it was a question until this experimental discovery,” says Fraden. “Here we have channels that are 50 vortices wide and we still have coherent flow, so there’s something more going on, and this is something that no theorist has even considered yet.” The researchers suspect this has not been seen previously because aspect ratio is an inherently 3D feature. Dogic says: “The vast majority of investigations so far – both experimentally and theoretically – have been confined to 2D systems, which are much simpler to investigate.” “This is a new direction that people have not really thought about.”
Not seen in nature
The large-scale, self-organized flow does not appear to be used in nature, and applications in engineering are probably “many, many years away”, says Dogic – partly because synthetic molecular motors are much less efficient than the biological ones used here. Fraden, however, points out that there were no applications to the classical equations of hydrodynamics when Claude-Louis Navier and George Gabriel Stokes formulated them 150 years ago – whereas today the equations are used to design aircraft. “In 150 years we’re going to have buildings and all sorts of things we haven’t imagined based on this kind of continuum mechanics where the boundary between animate and inanimate has become so blurred as to be completely unrecognisable,” he predicts.
Andreas Bausch of the Technical University of Munich is excited by the researchers’ ability to control whether flow is turbulent or coherent by tailoring the width and height of the channels: “This is a real scaling up of properties,” he says, “The flow is driven in long-range flow patterns by nanometre motors.”
The research is described in Science.