Swimming droplets in 1D geometries: an active Bretherton problem

Area of Science:

• Colloid and Interface Science
• Microfluidics
• Soft Matter Physics

Background:

• Self-propelled droplets generate motion via interfacial activity, specifically swollen micelles.
• Droplet behavior in confinement is crucial for microfluidic applications.
• Previous models often simplify interfacial dynamics and confinement effects.

Purpose of the Study:

• To experimentally investigate the behavior of active droplets in confined geometries.
• To explore the impact of confinement on droplet velocity and shape.
• To develop an analytical model incorporating interfacial activity and micelle transport.

Main Methods:

• Experimental confinement of water-in-oil droplets in square and circular capillaries.
• Utilizing stretched circular capillaries for higher confinement studies.
• Developing an analytical model based on the Bretherton approach, including interface activity and micelle transport.

Main Results:

• Droplet velocity decreases with confinement but converges to a non-zero value.
• High confinement leads to non-uniform lubrication layer thickness and neck formation.
• Spontaneous droplet splitting occurs at large enough confinement.
• The analytical model successfully predicts velocity convergence and non-classical lubrication layer shapes.

Conclusions:

• Droplet interface activity critically influences behavior under confinement.
• The developed model accurately captures key phenomena like velocity saturation and droplet splitting.
• Micelle concentration saturation explains lubrication layer divergence and droplet division.