By Kun-Ta Wu, Ph.D.
Pumping water through a pipe solves the need to provide water in every house. By turning on faucets, we retrieve water at home without needing to carry it from a reservoir with buckets. However, driving water through a pipe requires external pressure; such pressure increases linearly with pipe length. Longer pipes need to be more rigid for sustaining proportionally-increased pressure, preventing pipes from exploding. Hence, transporting fluids through pipes has been a challenging problem for physics as well as engineering communities.
To overcome such a problem, Postdoctoral Associate Kun-Ta Wu and colleagues from the Dogic and Fraden labs, and Brandeis MRSEC doped water with 0.1% v/v active matter. The active matter mainly consisted of kinesin-driven microtubules. These microtubules were extracted from cow brain tissues. In cells, microtubules play an important role in cell activity, such as cell division and nutrient transport. The activity originates from kinesin molecular motors walking along microtubules. In cargo transport, microtubules are like rail tracks; kinesin motors are like trains. When these tracks and trains are doped in water, their motion drives surrounding fluids, generating vortices. The vortices only circulate locally; there is no global net flow.
Figure: Increasing the height of the annulus induces a transition from locally turbulent to globally coherent flows of a confined active isotropic fluid. The left and right half-plane of each annulus illustrate the instantaneous and time-averaged flow and vorticity map of the self-organized flows. The transition to coherent flows is an intrinsically 3D phenomenon that is controlled by the aspect ratio of the channel cross section and vanishes for channels that are either too shallow or too thin. Adapted from Wu et al. Science 355, eaal1979 (2017).
To create a net flow like rivers, Wu et al. discovered that confining such an active fluid in a toroid triggers a transition from turbulent to coherent flowing states (Figure, above left). Unlike rivers, which gain kinetic energy from their gravitational potential, such a fluid is driven by kinesin motors consuming internal chemical energy (adenosine triphosphate), self-pumping without external pressure. However, not all toroidal geometries sustain coherent flows. Wu et al. pointed out that sustaining coherent flows requires the toroidal cross section to have a shape similar to a square or circle. In a thin and wide toroid, the coherent flow is suppressed (Figure, above right). Wu et al. demonstrated that with a proper (circular) cross section, the coherent flow can persist for meters, proving the feasibility of transporting fluids at a macroscopic scale without external pressure.
In conclusion, Wu et al. invented self-pumping fluids. The fluids flow in a pipe without external pressure. However, the knowledge of self-pumping fluids remains limited. For example, why does the transition from turbulent to coherent flow depend on the shape of the pipe cross section, rather than its absolute size? How does the coherent flow respond to external pressure? Answering these questions are the key to understanding underlying principles of self-organization of active fluids; however, Wu et al.’s invention already opens the door to pumping fluids without a pump.
Kun-Ta Wu, Jean Bernard Hishamunda, Daniel T.N. Chen, Stephen J. DeCamp, Ya-Wen Chang, Alberto Fernández-Nieves, Seth Fraden, and Zvonimir Dogic. Transition from turbulent to coherent flows in confined three-dimensional active fluids. Science 355, eaal1979 (2017). doi:10.1126/science.aal1979.
