Laser-induced force on a microfluidic drop: origin and magnitude

Area of Science:

• Physics
• Fluid Dynamics
• Microfluidics

Background:

• Microfluidic drops are susceptible to surface tension gradients.
• Localized heating can alter fluid interfaces.

Purpose of the Study:

• Investigate the origin and magnitude of forces on microfluidic drops induced by localized laser heating.
• Characterize the resulting interfacial flows and drop dynamics.

Main Methods:

• Utilized fluorescent dye colocalization to visualize surfactant micelle distribution under laser heating.
• Employed time-resolved micro-Particle Image Velocimetry to measure interfacial flow patterns.
• Developed a microfluidic device to quantify the net force on the laser-heated drop.

Main Results:

• Demonstrated that laser heating alters surfactant distribution, creating out-of-equilibrium interfaces and reversed interfacial flows.
• Measured a force of 180 nN on a 300x100 micrometer drop with 100 mW laser power.
• Established a heating time scale of 4 ms, limiting force generation dynamics and maximum drop velocity (0.7 mm/s).

Conclusions:

• Localized heating induces a soluto-capillary effect that overcomes thermal surface tension dependence, generating significant forces on microfluidic drops.
• The magnitude of the force is directly related to laser power and absorption, while dynamics are governed by the heating time scale.
• A scaling model describes the blocking force in confined geometries due to viscous stresses.