Theoretical limits of energy extraction in active fluids

Active materials form a class of far-from-equilibrium systems that are driven internally and exhibit self-organization, which can be harnessed to perform mechanical work. Inspired by experiments on synthetic active polymer networks, in this paper we examine limits of work extraction from an active viscoelastic medium by analyzing the transport of a particle. The active viscoelastic material possesses an equilibrium density where the active and passive forces are balanced out. In a one-dimensional system, a gliding activation front (AF), which converts a passive to an active medium, provides active energy at a constant rate, which is injected into the system at one end and propagates to the other. We demonstrate that there exists a maximum velocity of the AF, above which the activated region fails to deliver the transport power. We hypothesize and intuitively argue based on the limit cases that the feasibility and the velocity of transport of the particle can be interpreted in terms of the velocity of an equilibration domain wall of the field, which is set by two parameters: (i) a measure of activity, and (ii) the viscoelastic timescale. The phase diagram is divided into “transport” and “no-transport” sectors, namely, for any pair of the two parameters, there exists a threshold velocity of the AF above which the particle transport becomes impossible. Constructing the phase diagram we find that (1) there are regions of the phase diagram for which the threshold velocity of the AF diverges, and (2) larger viscoelastic timescale makes the transport region more accessible and also increases the transport velocity therein. Furthermore, we find that increasing the velocity of AF, results in larger extracted power but smaller transport coefficient; defined as the ratio of the transport velocity and that of the AF. Our model provides a framework for understanding the energetics of transport phenomena in biology and designing efficient mechanisms of transport in synthetic active materials. Published by the American Physical Society 2024