Active matter under deformable confinement
Creators: name: Uplap, Sarvesh
Publisher: name: Brandeis University
Year: 2023
Subjects: Active Matter
Molecular dynamics simulations
Self-propelled particles
Confinement
Theoretical Physics

description: Active matter is a class of systems where the constituent particles consume energy at the microscopic scale to generate 'active' forces and motion. These systems are inherently driven out of equilibrium and show novel behaviors that are not observed in traditional equilibrium systems. Collective organisation of active forces leads to fascinating emergent behaviors at length and time scales that are much larger that of the microscopic dynamics. Understanding the physics of such systems could open avenues to create new materials with life-like properties. Many biological systems, across a wide spectrum of size scales, harness active forces to perform vital functions for example, cellular functions such as cell migration, morphogenesis and cytoplasmic streaming. Simplified models of active matter could provide a suitable framework to understand the underlying physical principles of these functions.
description: Active matter systems are highly influenced by their boundary conditions, which bias the collective emergent behavior. Biological processes such as cell migration are driven by organisation of active forces that emerge from active components confined within a cell membrane. The presence of the cell membrane and its interaction with the active components play a key roles in the process. Building off this idea, we use a minimal model of active particles (rods and semi-flexible filaments) and study the emergent behavior of systems that are comprised of these active particles in deformable confinements.
description: First, we use coarse-grained molecular dynamics simulations to study the transport properties of ring-like 2D vesicles containing polar active rods as a function of the vesicle deformability and the properties of the enclosed rods. Properties such as the length of rods, the number density and the active force. Above a threshold value of the rod length, distinct dynamical regimes emerge, including a dramatic enhancement of vesicle motility characterized by a highly persistent random walk. These regimes are determined by organisation of the rods within the vesicle; the maximum motility state arises when the rods organise into one long-lived polar cluster. We develop a scaling theory that predicts the dynamical regimes as a function of control parameters, and shows that feedback between activity and passive membrane forces govern the rod organization. These findings yield design principles for building self-propelled superstructures using independent active agents under deformable confinement.
description: Second, we use coarse-grained molecular dynamics simulations to study the fluctuations of a spherical membrane driven by semi-flexible active filaments (having a nematic active force) in 3D. Experiments show that giant unilamellar vesicles containing an active minimal cytoskeleton composed of microtubules and molecular motors exibit large shape fluctuations. These fluctuations are non-equilibrium in nature and are enhanced by the active forces. The coupling of the deformable membrane with the underlying active cytoskeleton breaks the equilibrium relationship between the spatial scale and temporal response of the membrane fluctuations. The temporal correlations are now determined solely by fluctuations in active stress arising from the cytoskeletal dynamics, rather than elastic relaxation of the membrane. We model the minimal cytoskeleton as semiflexible active filaments and a triangular mesh model for the membrane. Molecular dynamics simulations using this simplified model capture the phenomenology observed in experiments and give key insights into the nature of active force fluctuations. We also explore the role of membrane deformability using our simulation model and find that, when the membrane has a high bending modulus it supresses short wavelength fluctuations. These results from experiments and simulations are a first step towards realizing of realistic model systems to study the interplay between membrane and enclosed active matter. Understanding this dynamics is a fundamental step in the program of reconstituting a synthetic cell or in the effort to construct biologically-inspired soft robots.