Periodic Reporting for period 3 - ADSNeSP (Active and Driven Systems: Nonequilibrium Statistical Physics)
Berichtszeitraum: 2020-10-01 bis 2022-03-31
Active and driven systems are all around us. Active systems include flocks of animals, the cytoskeleton inside each of our cells, swarms of bacteria and an increasing number of synthetic systems in which colloidal (micron-scale) particles suspended in a fluid are caused to self-propel. In each of these systems, local motion is sustained by the continuous use of an energy source. Driven systems differ from active ones, in that the motion of the constituent particles is not caused by local effect, but by sustained forcing at the boundaries; an example is pumping fluid down a pipe. In each case the system's behaviour, even when time-independent, can differ strongly from systems in thermal equilibrium, such as a suspension of colloidal particles that is neither self-propelled nor driven. The latter are never at rest, because Brownian motion causes them to move diffusively. However, Brownian motion, and thermal equilibrium in general, have a special character: the dynamics is reversible in the sense that a movie of the system looks the same running forwards and backwards. For driven systems this is hardly ever the case; for active systems the question is more subtle. If one views the microscopic motion of individual particles, irreversibility is easy to spot. But looking at the system at global scales, it can sometimes be very difficult to distinguish from thermal equilibrium (though sometimes very easy).
ADSNeSP’s agenda focuses on the following questions:
• When does local irreversibility of the dynamics have global consequences, and when does it not?
• When it doesn’t, can we find an effective equilibrium description at large scales?
• When it does, is the active system equivalent to some globally driven one, and in what sense?
• What can we learn by perturbatively connecting active to passive or globally driven models?
These objectives together aim to clarify the currently uncertain connections between active, driven, and equilibrium systems when viewed at scales much larger than their constituent particles.
This is important for society, because active systems underly the viability of almost all living objects. They also promise new ways of making functional materials where very precise microstructure is required (the same being true of driven systems). Driven systems such as sheared colloidal suspensions underpin large areas of technology such as the production of foodstuffs and homecare products. However, the science of active and driven systems lags approximately one century behind that of thermal equilibrium systems. This is because their reversibility makes the latter uniquely simple to understand (although still quite complicated enough!). It is therefore crucial to know which active and/or driven systems are amenable to the techniques developed for equilibrium systems and also which are equivalent to each other.
ADSNeSP’s agenda focuses on the following questions:
• When does local irreversibility of the dynamics have global consequences, and when does it not?
• When it doesn’t, can we find an effective equilibrium description at large scales?
• When it does, is the active system equivalent to some globally driven one, and in what sense?
• What can we learn by perturbatively connecting active to passive or globally driven models?
These objectives together aim to clarify the currently uncertain connections between active, driven, and equilibrium systems when viewed at scales much larger than their constituent particles.
This is important for society, because active systems underly the viability of almost all living objects. They also promise new ways of making functional materials where very precise microstructure is required (the same being true of driven systems). Driven systems such as sheared colloidal suspensions underpin large areas of technology such as the production of foodstuffs and homecare products. However, the science of active and driven systems lags approximately one century behind that of thermal equilibrium systems. This is because their reversibility makes the latter uniquely simple to understand (although still quite complicated enough!). It is therefore crucial to know which active and/or driven systems are amenable to the techniques developed for equilibrium systems and also which are equivalent to each other.
A major area of study has been the field theory of Motility-Induced Phase Separation (MIPS), a variant of the passive case where the final state is of bulk phase coexistence. When the particles are active, a variety of new behaviours are possible. We have studied these both for a purely diffusive model (Active Model B+) and a variant that also conserves fluid momentum (Active Model H). Active B+ shows a wide range of new behaviours, most spectacularly the cessation of phase separation kinetics in a state that is only partly demixed (microphase separation). We have understood the dynamics of Active Model B+ away from the so-called critical regime, and explained a novel reversal of the Ostwald process (which normally entails growth of large droplets and shrinkage of small) that underlies the microphase-separated regime. This work has also been extended to Active Model H where a second process is also possible, where droplets tear themselves apart due to active stresses which deform their surfaces. In the case of Active Model B+ we have performed the first renormalization group (RG) studies of the critical region and found new strong-coupling regimes for microphase separation. We have also studied the competition between chemical propulsion and momentum-conserving hydrodynamics in many-particle systems.
A second major area has been the extension of equilibrium thermodynamics to address active matter. We have not only found principles for the optimization of work by a single active colloid but also discovered new ways to pull work out of many-particle systems by inclusion of asymmetrically shaped objects that move spontaneously in an active fluid and which can be used to power mechanical devices. This body of work also addresses general principles of entropy production and stochastic thermodynamics, as exemplified in work on thermodynamic uncertainty relations which we have now generalized to periodically driven systems. We have also addressed the statistics of outlying rare states in active particle systems characterized by unusually high or low dissipation, showing the appearance of new types of order and generating new design principles.
In most swimming systems such orientational order is polar but in some it is also chiral. We have examined the continuum (hydrodynamic) description of chiral active matter and shown, surprisingly, that the microscopic concept of a 'torque dipole' is not a complete enough specification of local chirality to give unambiguous large scale equations of motion.
A major achievement involves granular suspensions, in which suspended particles are large enough to have negligible Brownian motion, under external drive such as shear flow. We have found a connection to the physics of the so-called absorbing state transition which explains some anomalous features of oscillatory flow, and come up with a new constitutive model for time-dependent flows in the shear-thickening regime in which particles undergo a jamming transition.
A second major area has been the extension of equilibrium thermodynamics to address active matter. We have not only found principles for the optimization of work by a single active colloid but also discovered new ways to pull work out of many-particle systems by inclusion of asymmetrically shaped objects that move spontaneously in an active fluid and which can be used to power mechanical devices. This body of work also addresses general principles of entropy production and stochastic thermodynamics, as exemplified in work on thermodynamic uncertainty relations which we have now generalized to periodically driven systems. We have also addressed the statistics of outlying rare states in active particle systems characterized by unusually high or low dissipation, showing the appearance of new types of order and generating new design principles.
In most swimming systems such orientational order is polar but in some it is also chiral. We have examined the continuum (hydrodynamic) description of chiral active matter and shown, surprisingly, that the microscopic concept of a 'torque dipole' is not a complete enough specification of local chirality to give unambiguous large scale equations of motion.
A major achievement involves granular suspensions, in which suspended particles are large enough to have negligible Brownian motion, under external drive such as shear flow. We have found a connection to the physics of the so-called absorbing state transition which explains some anomalous features of oscillatory flow, and come up with a new constitutive model for time-dependent flows in the shear-thickening regime in which particles undergo a jamming transition.
Our work on Active Models B+ and H vastly extends the state of the art in continuum field theories of active phase separation; our RG work is the first of its kind, and the newly discovered Reverse Ostwald process is entirely novel.
In driven granular suspensions the link to absorbing state physics is via a model that can equally describe active systems -- this is the first success in our agenda to establish equivalences between active and driven classes of materials.
Our work on atypical trajectories has led to a new conceptualization of design principles for active systems.
Our work on active thermodynamics and active engines creates tangible process in transferring ideas from equilibrium to active settings where there was no guarantee of any such progress being possible.
Between now and the end of the project we expect substantial further progress on all these problems, including:
Entropy production and irreversibility at large scales near phase-separation critical points.
Active phase separation in systems with birth and death of particles e.g. bacteria.
Stochastic thermodynamics and rare event statistics in polar phases of active matter and in active lattice models.
Further progress with granular suspensions and the links between driven and active materials.
Quantification of rare event pathways (e.g. nucleation) in systems without microscopic reversibility.
These areas of progress, alongside what is already achieved, will deliver major advances against all of the four main topics in ADSNeSP's agenda as detailed in the overall objective section above.
In driven granular suspensions the link to absorbing state physics is via a model that can equally describe active systems -- this is the first success in our agenda to establish equivalences between active and driven classes of materials.
Our work on atypical trajectories has led to a new conceptualization of design principles for active systems.
Our work on active thermodynamics and active engines creates tangible process in transferring ideas from equilibrium to active settings where there was no guarantee of any such progress being possible.
Between now and the end of the project we expect substantial further progress on all these problems, including:
Entropy production and irreversibility at large scales near phase-separation critical points.
Active phase separation in systems with birth and death of particles e.g. bacteria.
Stochastic thermodynamics and rare event statistics in polar phases of active matter and in active lattice models.
Further progress with granular suspensions and the links between driven and active materials.
Quantification of rare event pathways (e.g. nucleation) in systems without microscopic reversibility.
These areas of progress, alongside what is already achieved, will deliver major advances against all of the four main topics in ADSNeSP's agenda as detailed in the overall objective section above.