Capillary-assisted printing of droplets at a solid-like liquid-liquid interface

Area of Science:

• Soft Matter Physics
• Colloid Science
• Materials Science

Background:

• Nanoparticle surfactants (NPS) form solid-like films at oil-water interfaces.
• These films are expected to stabilize aqueous droplets against coalescence.
• Droplet deformation at the interface can induce capillary forces.

Purpose of the Study:

• To investigate the stability and assembly of aqueous droplets on nanoparticle surfactant films.
• To explore the directed assembly of droplets into larger structures.
• To demonstrate control over droplet behavior using light-responsive interfaces.

Main Methods:

• Placing aqueous droplets on cellulose nanocrystal surfactant (CNCS) films at oil-water interfaces.
• Quantifying droplet dynamics using single-particle tracking.
• Utilizing a custom droplet printer for controlled assembly.
• Modifying droplet and surfactant compositions for heterogeneity and light responsiveness.

Main Results:

• Droplets remained stable at the interface for extended periods.
• Microlitre droplets exhibited attractive forces over millimetric distances.
• Observed dynamics deviated from theoretical predictions for pristine interfaces.
• Inter-droplet capillary attraction enabled self-building droplet structures.
• Plasmon-assisted optofluidics with gold nanoparticles allowed laser-induced manipulation.

Conclusions:

• Nanoparticle surfactant films provide a stable platform for aqueous droplets.
• Inter-droplet capillary forces drive the formation of large, ordered droplet structures.
• This system offers a novel approach for printing and manipulating complex fluidic architectures.