Spinning Drop Dynamics in Miscible and Immiscible Environments.

Area of Science:

• Fluid dynamics
• Rheology
• Interfacial phenomena

Background:

• The behavior of drops in rotating fluids is crucial for understanding various industrial processes.
• Spinning drop tensiometry is a common technique for measuring interfacial tension, but its applicability in miscible systems is debated.

Purpose of the Study:

• To investigate the extensional dynamics of spinning drops in both miscible and immiscible fluids after a sudden change in rotation speed.
• To compare experimental observations with existing theoretical models for drop deformation and relaxation.

Main Methods:

• Experimental observation of spinning drops subjected to a rotational speed jump.
• Analysis of drop shape evolution over time in different fluid environments.
• Quantitative comparison of experimental data with theoretical predictions.

Main Results:

• Drops in immiscible fluids exhibit exponential relaxation to equilibrium shape, independent of centrifugal force, matching Stone and Bush's model.
• Drops in miscible fluids elongate indefinitely, following a power-law consistent with inviscid drop dynamics (Lister and Stone).
• Experimental data suggests low compositional gradients in miscible fluids do not yield measurable interfacial tension via spinning drop tensiometry.

Conclusions:

• The extensional dynamics of spinning drops differ significantly between miscible and immiscible fluids.
• Existing models accurately predict drop behavior in immiscible fluids but require adaptation for miscible systems.
• Spinning drop tensiometry may not be suitable for quantifying interfacial tension in systems with low compositional gradients in miscible fluids.