Demixing in Binary Mixtures with Differential Diffusivity at High Density

Channel instability in binary mixtures with differential diffusivity

Rupture dynamics and pinch-off phenomena are fundamental for understanding instabilities in fluid dynamics and biological systems. In this study, we investigate the rupture of two-dimensional, channel-like configurations in a binary mixture of particles with differential diffusivities. Through computational simulations, we analyze the evolution of this instability under various conditions, identifying key parameters such as aspect ratio, particle density, and drift strength that influence the system's stability. While its behavior resembles the Plateau-Rayleigh instability (PRI), the underlying mechanism differs fundamentally, as PRI is restricted to three-dimensional systems. Interestingly, similar instabilities have been observed in chiral fluids, further supporting the existence of a novel instability mechanism unique to two-dimensional non-equilibrium systems. Our results suggest that this phenomenon is not a finite-size effect, but rather an intrinsic property of systems with differential diffusivities, offering new insights into pattern formation and instability dynamics in active matter.

Algebraic Depletion Interactions in Two-Temperature Mixtures

The phase separation that occurs in two-temperature mixtures, which are driven out of equilibrium at the local scale, has been thoroughly characterized, but much less is known about the depletion interactions that drive it. Using numerical simulations in dimension 2, we show that the depletion interactions extend beyond two particle diameters in dilute systems, as expected at equilibrium, and decay algebraically with an exponent -4. Solving for the N-particle distribution function in the stationary state, perturbatively in the interaction potential, we show that algebraic correlations with an exponent -2d arise from triplets of particles at different temperatures in spatial dimension d. Finally, simulations allow us to extend our results beyond the perturbative limit.

Cell Sorting in an Active Nematic Vertex Model

We study a mixture of extensile and contractile cells using a vertex model extended to include active nematic stresses. The two cell populations phase separate over time. While phase separation strengthens monotonically with an increasing magnitude of contractile activity, the dependence on extensile activity is nonmonotonic, so that sufficiently high values reduce the extent of sorting. We interpret this by showing that extensile activity renders the system motile, enabling cells to undergo neighbor exchanges. Contractile cells that come into contact as a result are then more likely to stay connected due to an effective attraction arising from contractile activity.

Diverse morphology and motility induced emergent order in bacterial collectives

Microbial communities exhibit complex behaviors driven by species interactions and individual characteristics. In this study, we delve into the dynamics of a mixed bacterial population comprising two distinct species with different morphology and motility aspects. Employing agent-based modeling and computer simulations, we analyze the impacts of size ratios and packing fractions on dispersal patterns, aggregate formation, clustering, and spatial ordering. Notably, we find that motility and anisotropy of elongated bacteria significantly influence the distribution and spatial organization of nonmotile spherical species. Passive spherical cells display a superdiffusive behavior, particularly at larger size ratios in the ballistic regime. As the size ratio increases, clustering of passive cells is observed, accompanied by enhanced alignment and closer packing of active cells in the presence of higher passive cell area fractions. In addition, we identify the pivotal role of passive cell area fraction in influencing the response of active cells toward nematicity, with its dependence on size ratio. These findings shed light on the significance of morphology and motility in shaping the collective behavior of microbial communities, providing valuable insights into complex microbial behaviors with implications for ecology, biotechnology, and bioengineering.

Cell sorting by active forces in a phase-field model of cell monolayers

Cell sorting, the segregation of cells with different properties into distinct domains, is a key phenomenon in biological processes such as embryogenesis. We use a phase-field model of a confluent cell layer to study the role of activity in cell sorting. We find that a mixture of cells with extensile or contractile dipolar activity, and which are identical apart from their activity, quickly sort into small, elongated patches which then grow slowly in time. We interpret the sorting as driven by the different diffusivity of the extensile and contractile cells, mirroring the ordering of Brownian particles connected to different hot and cold thermostats. We check that the free energy is not changed by either partial or complete sorting, thus confirming that activity can be responsible for the ordering even in the absence of thermodynamic mechanisms.