Dynamic Manipulation of Droplets on Liquid-Infused Surfaces Using Photoresponsive Surfactant.

Area of Science:

• Surface science
• Materials chemistry
• Fluid dynamics

Background:

• Programmable droplet transport is crucial for microfluidics, thermal management, and energy devices.
• Photoresponsive surfactants enable droplet manipulation via light-induced interfacial tension changes (photo-Marangoni effect).
• Previous studies focused on droplet migration in liquid media, leaving migration on solid and liquid-infused surfaces (LIS) as a challenge.

Purpose of the Study:

• To demonstrate and model photo-Marangoni droplet migration on diverse substrates, including LIS and solid capillary tubes.
• To synthesize novel photoswitches and characterize their performance in droplet manipulation.
• To provide insights for optimizing photoswitches and understanding migration mechanisms on LIS.

Main Methods:

• Synthesis of spiropyran and merocyanine-based photoswitches.
• Experimental demonstration of 2D droplet motion on liquid surfaces and LIS, and rectilinear motion in capillary tubes.
• Development of a scaling model for photo-Marangoni migration and numerical simulations for LIS.

Main Results:

• Achieved significant interfacial tension changes (up to 5.5 mN/m) with rapid photoswitching (as fast as 1.7 s).
• Demonstrated droplet migration speeds up to 5.5 mm/s on liquid and 0.25 mm/s on LIS.
• Identified an optimal droplet size for migration and validated a model predicting migration speed based on surfactant properties.

Conclusions:

• The study successfully extends photo-Marangoni droplet migration to liquid surfaces and LIS.
• A developed scaling model provides a framework for designing efficient photoswitches and predicting migration behavior.
• These findings pave the way for advanced droplet manipulation in microfluidic, thermal, and water harvesting technologies.