Fluid displacement in porous media is a key process in many industrial and environmental applications. When a less viscous fluid displaces a more viscous one in porous media or Hele-Shaw cells, a finger-like interfacial pattern known as the Saffman-Taylor instability typically occurs. Recently, partially miscible systems have attracted attention because they exhibit multiple droplet formations instead of conventional fingering. While the effects of Hele-Shaw-cell gap width on hydrodynamic instability have been well established in fully miscible and immiscible systems, where narrower gaps produce thinner and more numerous fingers, the influence of gap confinement in partially miscible systems remains unclear. In this study, we experimentally investigated the effect of gap width on fluid displacement dynamics in a partially miscible $\mathrm{PEG}\text{−}{\mathrm{Na}}_2{\mathrm{SO}}_4$-water system. The droplet size decreased and the number of the droplets increased as the gap became smaller, indicating that confinement intensified droplet formation. The total interfacial length was successfully scaled using the flow rate, flow velocity, and gap width, collapsing onto a universal curve when expressed as a function of flow-rate-to-gap ratio. The derived empirical exponent was consistent with a force balance between inertial and capillary stresses, suggesting a transitional “inertia-modified capillary” regime. This scaling framework provides predictive insights into the interfacial morphology of partially miscible systems, bridging thermodynamic interfacial forces and hydrodynamic flow.

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• Fingering instability
• Flow instability
• Hele-shaw flows
• Saffman-Taylor instability