An inclusion of particles in a Newtonian liquid can fundamentally change the interfacial dynamics and even cause interfacial instabilities. For instance, viscous fingering can arise even in the absence of the destabilizing viscosity ratio between invading and defending phases, when particles are added to the viscous invading fluid inside a Hele-Shaw cell. In the same flow configuration, the formation and breakup of a dense particle band are observed on the interface, only when the particle diameter d becomes comparable to the channel gap thickness h. We experimentally characterize the evolution of the fluid-fluid interface in this new physical regime and propose a simple model for the particle band that successfully captures the fingering onset as a function of the particle concentration and h/d.
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