Double Diffusive Instability

We analyse numerically experimentally and through linear stability theory a reaction diffusion system in which either the reactant Y is radially injected into an autocatalytic medium of X in a circular geometry in the presence of radial advection or X radially invades Y through a cubic autocatalytic reaction in the absence of advection. Since X and Y have different diffusivities, we observe in the first case the formation of a previously unreported sunray-like instability pattern whose amplitude increases with the flow rate and the diffusivity ratio. In the second case, where only the autocatalyst propagates via reaction-diffusion without advection, we show that the radial geometry is more unstable than the rectilinear configuration for the same diffusivity ratio. This result contrasts with earlier findings that curvature has no effect on the critical diffusivity ratio. See the work here:

https://doi.org/10.1103/s8rs-2bq9

Reactive Density Fingering in Porous Media

This video showcases MATLAB Livelink & COMSOL simulations of Rayleigh–Taylor instability in incompressible reactive fluids within porous media. The study focuses on the reaction A+B→C, where the product alters density and permeability, driving convection–diffusion–reaction dynamics. The system’s well-posedness is established analytically (existence, uniqueness, continuous dependence), and numerical validation is performed using finite-element simulations.

See the work here:

https://arxiv.org/abs/2505.17943

Shear instability interaction with traveling autocatalytic fronts

We investigate how vertical viscosity stratification in pressure-driven channel flows can trigger shear instabilities that roll up into convective patterns. Unlike miscible non-reactive systems, the mixing zone here evolves around a travelling autocatalytic reaction–diffusion front that self-organizes viscosity changes. Numerical simulations and linear stability analysis reveal how the front’s position and speed control the onset and growth of instabilities, offering new insight into reactive shear-driven dynamics.

See the published work here:

https://doi.org/10.1098/rspa.2024.0891

Shear Instability in layered channel flow

When a less viscous fluid is sheared over a more viscous one, in a channel, roll-up or ligament type pattern formation occurs by the shear induced by viscosity stratification. This instability only originates at an intermediate viscosity ratio, and the flow become stable if the viscosity ratio is increased further. By the virtue of this fact, maximum mixing occurs for intermediate viscosity ratios

See the published work here:

https://doi.org/10.1063/5.0164830

which has been selected as an featured article of PoF.

Kelvin-Helmholtz roll-ups in a displacement channel flow

In a 2D channel, reactant A displaces another reactant B having the same viscosity. A simple reaction A + B -> C modifies the viscosity at the interface. The local gradients in viscosity produce a favorable shear; as such, Kelvin-Helmholtz roll-ups develop at the C - B reaction front. The concentration of fluid B is shown in the movie. The flow equation is Navier-Stokes equation here.

See the related published works here: https://doi.org/10.1017/jfm.2021.630

https://aip.scitation.org/doi/10.1063/5.0078776

Viscous Fingering

A new numerical sheme is designed to simulate Viscous Fingering (VF) instability in a highly convection-dominated regime.

VF occurs when a less viscous fluid tries to push a more viscous one in a Hele-Shaw cell or porous medium. The flow equation is Darcy's equation here. Stay tuned for more on this...