Drying-induced gravitational instability due to density inversion in colloidal films

Drying-induced instabilities in monodisperse colloidal films were investigated using lattice Boltzmann simulations. A two-step drying protocol was introduced in which evaporation was temporarily interrupted by an intermediate quiescent period during rapid drying. The initial rapid drying step produced stratified suspensions consisting of a dense upper layer above the dilute bulk region. Upon cessation of drying, a Rayleigh–Taylor instability is initiated, causing the internal interface between the dense and bulk regions to develop into fingerlike and mushroom-shaped structures that progressively relax the density inversion. The timing of the quiescent period strongly influenced the instability characteristics, with the maximum growth rate occurring when drying was interrupted at approximately half the initial film height. Predictions from the linear stability analysis quantitatively captured the observed behavior and revealed the competing effects of the density contrast and film thickness. Furthermore, a comparison with continuous drying demonstrated that the two-step drying suppressed the formation of a dense particle layer, leading to more homogeneous particle distributions. These results demonstrate that drying-induced instability is a controllable mechanism that can be exploited to design advanced drying protocols for colloidal film fabrication.