Spreading dynamics of drops on a solid surface submerged in different outer fluids

Hypothesis: Surrounding fluids affect critically drop wetting dynamics in many applications involving viscous environments. Although macroscopic effects of outer fluid viscosity on contact line motion have been documented, the extent to which the outer fluid modulates internal flow pattern is still not well understood, largely due to experimental challenges. It is hypothesized that the external fluid exerts a dominant effect on the internal flow fields and energy dissipation, thereby altering dynamic contact angle evolution and overall wetting behavior. Elucidating this coupling mechanism is essential for advancing our understanding of multiphase spreading in complex fluid systems. Experiments: We investigate the spreading of Newtonian and non-Newtonian shear-thinning aqueous drops in air versus in oil, using high-speed imaging and custom-built micro-PIV. Internal velocity and viscosity fields are measured to quantitatively relate internal flow evolution to contact line motion. Dynamic contact angle was measured and analyzed using composite model incorporating hysteresis and pinning. Scaling laws were derived to compare spreading dynamics under different outer fluid viscosities and substrate wettabilities. Findings: In air, capillary waves trigger Laplace pressure gradients that drive rapid, outward internal flow as well as fast contact line motion. In contrast, viscous oils suppress wave formation and generate recirculating vortices, resulting in a significantly slower spreading process dominated by viscous drag. Despite power-law spreading in both cases, the governing timescales reflect fundamentally different mechanisms: inertial forces within the drop dominate in air, whereas external fluid viscosity controls the spreading dynamics in oil. A unified scaling incorporating outer-fluid viscosity and equilibrium contact angle gathers diverse data onto a master curve. These results underscore the central role played by outer-fluid induced internal flow in governing wetting dynamics.

💡 Research Summary

This paper presents a comprehensive experimental investigation into how the surrounding fluid fundamentally alters the spreading dynamics of a liquid drop on a solid surface. The study challenges classical wetting theories, which often assume an inviscid outer fluid like air, by systematically comparing spreading in air versus in viscous oils (mineral and silicone oil).

The core hypothesis was that the outer fluid exerts a dominant influence not just macroscopically on contact line speed, but also microscopically by modulating the internal flow patterns and energy dissipation mechanisms within the drop. To test this, the authors employed a dual-method approach: high-speed imaging to capture macroscopic shape evolution (contact diameter, dynamic contact angle) and a custom-built high-speed micro-PIV system to quantitatively map the internal velocity fields for the first time during such rapid spreading events. They used both Newtonian (water) and shear-thinning non-Newtonian (polyacrylamide solution) drops on substrates with different wettabilities (PMMA and stainless steel).

The findings revealed a stark contrast in mechanisms. In air, the initial contact excites strong capillary waves on the drop interface. These waves create Laplace pressure gradients that drive a rapid, outward-directed internal flow, leading to fast, inertia-dominated spreading following an approximate r ~ t^(1/2) scaling. Conversely, in viscous oils, the outer fluid’s viscosity heavily damps any interfacial waves. The spreading is significantly slower and is characterized by the development of internal recirculating vortex pairs. The process is dominated by viscous drag from the outer fluid, shifting the dynamics towards a viscosity-controlled regime.

A major technical achievement was the first experimental estimation of the spatiotemporal viscosity distribution inside the spreading non-Newtonian drop. The PIV-derived shear rate maps showed pronounced shear-thinning, especially near the moving contact line, where local viscosity could drop dramatically, influencing the contact line motion.

The most significant outcome was the development of a unified scaling law. By incorporating the outer fluid viscosity (η_out) and the equilibrium contact angle (θ_e) into a modified capillary number, the authors successfully collapsed all their spreading data—across different inner/outer fluids and substrates—onto a single master curve. This scaling elegantly bridges the inertial regime (dominant in air) and the viscous regime (dominant in oil).

In conclusion, this work provides unprecedented insight into the coupled dynamics of inner and outer fluids during wetting. It demonstrates that the outer fluid is not a passive bystander but an active agent that reshapes the internal flow architecture, thereby governing the overall spreading kinetics. The proposed unified scaling offers a powerful predictive tool for designing and optimizing processes involving multiphase spreading in complex fluid environments, such as immersion coating, bioprinting, and lubricated surfaces.