Subcritical pitchfork bifurcation transition of a single nanoparticle in strong confinement

Confinement often influences the properties of fluids. Using molecular dynamics simulations with explicit solvents, we show that strong slit confinement induces a first-order transition between solvation states, where a nanoparticle either resides at the slit center or binds to the confining surfaces. We find that the stability transition of this bistable state follows a subcritical pitchfork bifurcation. Furthermore, along with the transition, the nanoparticle's lateral diffusion, which is Brownian in bulk, exhibits a discontinuous change. Although the transition depends on various parameters, such as interparticle interactions and confinement interface properties, the bifurcation framework consistently captures the observed stability transitions across all conditions studied. These results underscore the need to develop molecular-level hydrodynamic theories capable of accurately describing strongly confined systems.