Droplet Trapping in Oil Phase via Optically Driven Thermocapillary Convection

Herein, we propose a novel optical strategy for stable water droplet trapping in an immiscible oil phase by using a 1550 nm focused laser beam. Distinct from conventional optical strategies, this approach leverages a local photothermal effect-induced flow field surrounding the water droplet in the oil phase to achieve stable droplet trapping without prolonged interaction between the laser and the water droplet, allowing for low-temperature operation. This feature effectively minimizes potential thermal damage risks and thereby preserves the viability of samples within the droplet, which is particularly promising for biological applications. By analyzing the force balance in vertical and horizontal directions, the trapping sites are predicted, and the results agree well with experimental data. Remarkably, the local photothermal effect-induced flow field imparts rotational momentum to the water droplet, enabling self-rotation of the trapped droplet, which enhances the internal mixing efficiency. This unique feature is demonstrated by the synthesis of gold nanoparticles and E. coli cultivation. This optical strategy allows for highly precise droplet manipulation, eliminating physical contact with solid surfaces, thereby avoiding contamination risks and ensuring lossless transfer. This work presents an innovative optical strategy for droplet trapping in an immiscible phase, holding promise for various applications.