Optothermal control of microbubble clusters near the water–air interface via the dual Marangoni convection

The manipulation of microbubbles has significant applications in fields of drug delivery, optical assembly and sorting, and biomedicine. However, the generation, aggregation, and manipulation of multiple microbubbles using a single light source in a low-temperature environment remains challenging. Here, we demonstrate two-dimensional (2D) optothermal control of microbubble clusters near the water‑air interface using Marangoni flows generated by laser heating of a gold film in a low‑temperature‑rise environment. Nucleated microbubbles present in the solution grow into observable microbubbles within the fast water–air interface-induced Marangoni convection. Under the combined action of the Marangoni force induced by the water‑air interface and that induced by the bubble surfaces, the microbubbles aggregate near the interface to form clusters. Experiments confirm that both the position and morphology of the bubble clusters can be controlled by manipulating the laser power and the focus position, respectively. Furthermore, quantitative data are provided to demonstrate the effects of laser power and liquid layer thickness on the microbubble clusters. The forces acting on a single bubble in stable conditions were analyzed based on finite element flow field simulations, which suggest that the control of the microbubbles is governed by the dual Marangoni convection.