Active liquid climbs walls defying gravity

Amazing stuff! What happens if you mix incompatible liquids? In this case, active and passive fluids.

"Researchers have created a fluid, that can climb walls, has turbulent interfaces and shows strange phase separation dynamics.

The fluid is a mixture of two immiscible liquids – polyethylene glycol (PEG) and dextran – microtubules and kinesin, a molecular motor protein that can walk along the microtubules. The microtubules sit in the dextran phase, where they are continuously moved by the molecular motors. This creates chaotic flows, which manifests in unusual behaviour at the liquid–liquid interface.

In a mixture without kinesin that has been shaken, PEG droplets coalesce slowly. At intermediate kinesin concentrations, they coalesce faster. At the highest kinesin concentrations, the droplets become active, incessantly merging and breaking apart again – an effect similar to a hot thermal bath. The molecular motor also creates an interface with undulations and waves large enough to be seen with the naked eye.
The mixture climbs up walls several hundred micrometres above the equilibrium capillary rise as the microtubule bundles align themselves with the wall. While this behaviour is similar to a superfluid like liquid helium, the underlying physical reasons are very different. ..."

"Incompatible liquids such as oil and water will phase separate with low interfacial tension. Adkins et al. investigated the dynamics of a one-dimensional interface separating an active nematic phase with a passive isotropic phase (see the Perspective by Palacci). They found a rich behavior of fluctuating interfaces in which the phase-separating fluids could form active emulsions that did not coarsen and in which droplets formed spontaneously. Macroscopic interfaces can also displayed propagating waves with a characteristic wave number and speed. Furthermore, the activity of one of the fluids, in which the addition of energy drove the ordering of that fluid, was able to modify the wetting transitions. The authors also observed active wetting of a solid surface whereby active extensile stresses parallel to the surface drove the fluid to climb a solid wall against gravity."

From the abstract:

"Controlling interfaces of phase-separating fluid mixtures is key to the creation of diverse functional soft materials. Traditionally, this is accomplished with surface-modifying chemical agents. Using experiment and theory, we studied how mechanical activity shapes soft interfaces that separate an active and a passive fluid. Chaotic flows in the active fluid give rise to giant interfacial fluctuations and noninertial propagating active waves. At high activities, stresses disrupt interface continuity and drive droplet generation, producing an emulsion-like active state composed of finite-sized droplets. When in contact with a solid boundary, active interfaces exhibit nonequilibrium wetting transitions, in which the fluid climbs the wall against gravity. These results demonstrate the promise of mechanically driven interfaces for creating a new class of soft active matter."

Active liquid climbs walls | Research | Chemistry World

Dynamics of active liquid interfaces (no public access)