With the rapid development of studies involving droplet microfluidics, drug delivery, cell detection, and microparticle synthesis, several models have been proposed to study the flow of fluid-filled bodies.

Motivated by results for the propagation of active–passive interfaces of bacterial Serratia marcescens swarms [Nat. Commun., 2018, 9, 5373], we used a hydrodynamic multiphase model to investigate the propagation of interfaces of active nematics on substrates. We characterized the active nematic phase of the model through the calculation of the spatial and temporal auto correlation functions and the energy spectrum and discussed its description of the statistical dynamics of the swarms reported in the experiment. We then studied the propagation of circular and flat active–passive interfaces. We found that the closing time of the circular passive domain decays quadratically with the activity and that the structure factor of the flat interface is similar to that reported for the swarms, with an activity dependent exponent. Finally, the effect of the substrate friction was investigated. We found an activity dependent threshold, above which the turbulent active nematic forms isolated islands that shrink until the system becomes isotropic and below which the active nematic expands, with a well defined propagating interface. We also found that the interface becomes static in the presence of a friction gradient.

We provide numerical evidence that a Kelvin-Helmholtz instability occurs in the Dirac fluid of electrons in graphene and can be detected in current experiments. This instability appears for electrons in the viscous regime passing though a micrometer-scale obstacle and affects measurements on the time scale of nanoseconds. A possible realization with a needle-shaped obstacle is proposed to produce and detect this instability by measuring the electric potential difference between contact points located before and after the obstacle. We also show that, for our setup, the Kelvin-Helmholtz instability leads to the formation of whirlpools similar to the ones reported in Bandurin et al. [ Science 351, 1055 (2016)]. To perform the simulations, we develop a lattice Boltzmann method able to recover the full dissipation in a fluid of massless particles.

Immiscible fluid displacement in porous media occurs in several natural and industrial processes. For example, during petroleum extraction from porous rock reservoirs, water is used to displace oil. In this paper, we investigate the primary drainage and imbibition in a heterogeneous porous medium using an improved numerical model based on the multicomponent pseudopotential lattice Boltzmann method. We apply recent developments from the literature and develop new pressure boundary conditions. We show that the proposed model is able to simulate realistic viscosity ratios, and it allows independent tuning of surface tension from viscosity. Moreover, the model suppresses a nonphysical behavior of previous schemes, in which trapped fluid volumes significantly change with time. Furthermore, we show that the developed model correctly captures the underlying physical phenomena of fluid displacements. We simulate oil–water flows and verify that the measured values of irreducible water and residual oil saturations are realistic. Finally, we vary the wetting conditions of the porous medium to represent different wettability states. For the different scenarios, we show that the simulations are in good agreement with the experimental results.

The hydrodynamic behavior of the interface between nematic (N) and isotropic (I) phases of a confined active liquid crystal is investigated using numerical simulations.