Periodic Reporting for period 4 - ADSNeSP (Active and Driven Systems: Nonequilibrium Statistical Physics)
Berichtszeitraum: 2022-04-01 bis 2023-09-30
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. ADSNeSP’s agenda focused 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?
This work 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. Driven systems such as sheared colloidal suspensions underpin large areas of technology such as the production of foodstuffs and homecare products.
The project has led to a number of important conclusions. Firstly, it has led to a completely new understanding of how to quantify the irreversibility of active systems, by developing new tools to identify not only the physical origin of irreversibility but also the precise spatial locations of its causes. This new understanding has been deployed across a wide range of systems relevant to active matter. Second, the work has led to a new understanding of improbable processes in active matter and other irreversible systems. These improbably processes represent tipping points whereby a system which has been stable a long time suddenly makes a transition into a new state (such as nucleation of liquid from its vapour). Also, by understanding the unlikely process whereby a rare behaviour is achieved, it is possible to design a new process for which this behaviour is the normal one, creating an important route to material design. Along similar lines, we have developed new models for driven systems (especially dense suspensions of frictional particles) that allow precise connections to be made between flow behaviour and microscopic interactions. This has created a new tool for designing the flow properties of materials ranging from molten chocolate to ceramics.
• 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?
This work 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. Driven systems such as sheared colloidal suspensions underpin large areas of technology such as the production of foodstuffs and homecare products.
The project has led to a number of important conclusions. Firstly, it has led to a completely new understanding of how to quantify the irreversibility of active systems, by developing new tools to identify not only the physical origin of irreversibility but also the precise spatial locations of its causes. This new understanding has been deployed across a wide range of systems relevant to active matter. Second, the work has led to a new understanding of improbable processes in active matter and other irreversible systems. These improbably processes represent tipping points whereby a system which has been stable a long time suddenly makes a transition into a new state (such as nucleation of liquid from its vapour). Also, by understanding the unlikely process whereby a rare behaviour is achieved, it is possible to design a new process for which this behaviour is the normal one, creating an important route to material design. Along similar lines, we have developed new models for driven systems (especially dense suspensions of frictional particles) that allow precise connections to be made between flow behaviour and microscopic interactions. This has created a new tool for designing the flow properties of materials ranging from molten chocolate to ceramics.
We have studied several models for phase separation in active systems, discovering a wide range of new behaviours, such as the cessation of phase separation kinetics in a state that is only partly demixed (microphase separation). This is caused by a reversal of the 'Ostwald process' which normally entails growth of large droplets and shrinkage of small ones. We have performed the first renormalization group studies of active separation and found new strong-coupling regimes for microphase separation. We have also studied the competition between chemical propulsion and hydrodynamicflow, and developed new field-theoretic models for systems in which phase separation is coupled to birth-and-death population dynamics (such as bacterial colony formation).
A second major area has been the extension of equilibrium thermodynamics to address active matter. We have found principles for the optimization of work and new routes for using active matter to power mechanical devices,and extended to active systems the principles of entropy production and stochastic thermodynamics 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.
We have developed a number of new tools for studying stochastic trajectories and resolved longstanding difficulties in defining the probabilities of stochastic paths, and used these new methods to quantitatively confront experiments. Alongside this we have developed new numerical tools based on the Ritz method to identify the rare stochastic trajectories that carry a system from one metastable state to another.
We have examined the continuum (hydrodynamic) description of chiral active matter and also examined closely the relation between irreversibility and heat production, showing how irreversibility (entropy production) at large scales can be related to its microscopic counterpart.
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 found principles for the optimization of work and new routes for using active matter to power mechanical devices,and extended to active systems the principles of entropy production and stochastic thermodynamics 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.
We have developed a number of new tools for studying stochastic trajectories and resolved longstanding difficulties in defining the probabilities of stochastic paths, and used these new methods to quantitatively confront experiments. Alongside this we have developed new numerical tools based on the Ritz method to identify the rare stochastic trajectories that carry a system from one metastable state to another.
We have examined the continuum (hydrodynamic) description of chiral active matter and also examined closely the relation between irreversibility and heat production, showing how irreversibility (entropy production) at large scales can be related to its microscopic counterpart.
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 has vastly extended the state of the art in continuum field theories of active phase separation. The reverse Ostwald process is entirely novel, and our results for entropy production in phase-separated systems and near critical points has initiated a new field of study. Our work on active phase separation in systems with birth and death has elucidated issues relating to colony formation and also the formation of membraneless organelles within eukaryotic cells.
In our work on driven granular suspensions, we discovered a link to absorbing state physics via a model that can equally describe active systems, fulfilling our goal of establishing equivalences between active and driven classes of materials. We also created new design principles based on the first quantitative model of time-dependent flow in frictional suspensions.
Our work on atypical trajectories has led to a new conceptualization of design principles for active systems, based on the connections between active and driven forces, alongside new quantitative tools for finding rare event pathways (e.g.nuclation) in systems without microscopic reversibility.
Our work on active thermodynamics and active engines created tangible progress in transferring ideas from equilibrium to active settings where there was no guarantee of any such progress being possible. We discovered new channels for extracting work from active media, for example using mixtures of active and passive particles, or confining walls with temporally modulated properties, and also studied in detail the behaviour of active and passive particles and their mixtures in 1D channels with spatially asymmetric potentials.
Our work on stochastic thermodynamics has elucidated the rare event statistics and entropy production (or macroscopic irreversibility) of polar phases of active matter and in active lattice models.
In combination, and alongside a large number of other advances (resulting in over 60 publications in all), represent very substantial progress against all of the four main topics in ADSNeSP's agenda, as detailed in the overall objective section above.
In our work on driven granular suspensions, we discovered a link to absorbing state physics via a model that can equally describe active systems, fulfilling our goal of establishing equivalences between active and driven classes of materials. We also created new design principles based on the first quantitative model of time-dependent flow in frictional suspensions.
Our work on atypical trajectories has led to a new conceptualization of design principles for active systems, based on the connections between active and driven forces, alongside new quantitative tools for finding rare event pathways (e.g.nuclation) in systems without microscopic reversibility.
Our work on active thermodynamics and active engines created tangible progress in transferring ideas from equilibrium to active settings where there was no guarantee of any such progress being possible. We discovered new channels for extracting work from active media, for example using mixtures of active and passive particles, or confining walls with temporally modulated properties, and also studied in detail the behaviour of active and passive particles and their mixtures in 1D channels with spatially asymmetric potentials.
Our work on stochastic thermodynamics has elucidated the rare event statistics and entropy production (or macroscopic irreversibility) of polar phases of active matter and in active lattice models.
In combination, and alongside a large number of other advances (resulting in over 60 publications in all), represent very substantial progress against all of the four main topics in ADSNeSP's agenda, as detailed in the overall objective section above.