Enhancement of interfacial instabilities by solid particles during fast stretching of a liquid suspension bridge

In this experimental study, the rapid stretching dynamics and interfacial instabilities of a suspension liquid bridge are investigated using a high-speed video system. The bridge is formed between two parallel plates with one plate remaining fixed, while the other propagates with a constant acceleration reaching up to 160 m s^-2. In these experiments, the initial gap width ranges from 30 to 60 μm. Sizes of the solid particles within the suspensions vary from 6 to 30 μm. Rapid stretching of the viscous liquid bridge can induce interfacial instability in the bridge meniscus, leading to the formation of finger-like structures. The presence of particles notably increases the number of fingers during the initial stretching phase, in comparison to the pure Newtonian liquids, which cannot be explained only by the changes of the effective viscosity of the liquid. Moreover, particles facilitate the emergence of numerous filaments (secondary bridges) surrounding the primary central bridge (central filament). In this study a correlation is established between the number of secondary bridges and both the particle volume fraction and size. Notably, intermediate particle sizes (15 μm) result in the highest number of secondary bridges, whereas the smallest particles (6 μm) produce only a few. The significant influence of solid particles in the suspension on the stability of the stretching bridge is attributed to particle accumulations that hinder the movement of the meniscus, leading to interface destabilization resulting in the fingering instability in the early stages of the stretching process. In the later stages, the dynamics of the stretching process are further affected by different regimes of particle interaction with the residual liquid film, including particle deposition within the film, partial or complete deposition of particles by the receding meniscus. Another mechanism involves the formation of periodic particulate structures when the liquid bridge is compressed between the approaching plates during experiment preparation. This is associated with a local increase in particle volume fraction of up to 12% compared to the overall volume fraction. The presence of these high-concentration structures facilitates a more uniform distribution of secondary bridges and predetermines the spatial locations of both the fingers and the resulting secondary bridges. Finally, particle image velocimetry (PIV) measurements reveal that flow separation caused by jamming of particles in regions of high particle concentration is the key mechanism driving filament formation.