Electrically tunable partial coalescence of oppositely charged drops

Area of Science:

• Colloid and Interface Science
• Fluid Dynamics
• Electrokinetics

Background:

• Partial coalescence of charged droplets is crucial in various industrial processes.
• Controlling droplet size and charge transfer during coalescence remains a challenge.

Purpose of the Study:

• To investigate the critical ionic conductivity influencing droplet coalescence.
• To explore the role of electric fields in controlling droplet size and coalescence extent.
• To elucidate the mechanisms of charge transfer and daughter droplet formation.

Main Methods:

• Experimental study of dissimilarly sized water drops in oil under varying electric fields.
• Analysis of droplet coalescence extent and daughter droplet characteristics.
• Investigation of charge transfer mechanisms using fluid dynamics principles.

Main Results:

• A critical ionic conductivity was identified, below which partial coalescence occurs.
• Electric field strength precisely controls the extent of coalescence, from full to zero.
• Daughter droplet size and charge are independent of ionic conductivity.
• Charge transfer is dominated by vortex-driven convection during capillary-driven penetration.
• Daughter droplet size aligns with a scaling model balancing inertia and electrostatic repulsion.

Conclusions:

• External electric fields offer precise control over droplet coalescence and daughter droplet size.
• Charge transfer dynamics are governed by fluid convection, not solely ionic conductivity.
• The findings provide a fundamental understanding of electrocoalescence phenomena.