Focused Liquid Pinching in Coaxial Drop Capsule Generation

Area of Science:

• Fluid Dynamics
• Rheology
• Microfluidics

Background:

• Investigating droplet dynamics is crucial for understanding various industrial processes.
• Pinching instability in fluid interfaces is a fundamental phenomenon with significant implications.
• Coaxial pendant drop systems offer a unique platform for studying complex interfacial flows.

Purpose of the Study:

• To experimentally and numerically investigate the pinching dynamics of an inviscid inner drop within a coaxial pendant drop structure.
• To determine the factors influencing the thinning rate of the inner drop during pinching.
• To identify the conditions leading to the formation of satellite droplets.

Main Methods:

• Experimental investigation of inviscid inner drop pinching in a coaxial pendant drop.
• Numerical simulations to model the fluid flow and interface dynamics.
• Analysis of thinning rates and neck radii differences (Δh_min) under varying conditions.

Main Results:

• Inner drop pinching is accelerated in a coaxial configuration compared to a single drop, attributed to flow focusing.
• The flow focusing effect intensifies significantly when the inner to outer nozzle radii ratio (R̃) exceeds 0.67.
• The difference in minimum neck radii (Δh_min) reliably predicts inner drop thinning rates and satellite droplet formation, observed when Δh_min is 0.3%-1% of capillary length.

Conclusions:

• The coaxial pendant drop geometry significantly alters inner drop pinching dynamics through flow focusing.
• The ratio of nozzle radii and the difference in minimum neck radii are key parameters governing pinching behavior and satellite droplet generation.
• Understanding these dynamics is vital for controlling droplet breakup and formation in microfluidic devices and other applications.