Laser-induced microdroplets: Generation, self-propelled motion and manipulation via the solutal Marangoni effect

Self-propelled microdroplets provide insight into the fundamental mechanisms underlying life-like behaviors [1] and hold promise for applications in micro-robotics and self-mixing. We introduce a non-contact, laser-actuated tracer-free platform for the on-demand generation and precise manipulation of sessile droplets on a light-absorbing surface [2]. We confine a binary mixture of volatile ethanol and nonvolatile PEG200 in a 5 µm gap between a gold-coated glass substrate and a cover slip. By focusing a 532 nm continuous-wave (CW) laser (approximately 1 mW, spot diameter ~ 4 μm) onto a bubble region enclosed by the reservoir, we selectively evaporate ethanol, creating a steep surface-tension gradient that nucleates droplets from the precursor film due to the solutal Marangoni effect. These droplets exhibit self-excited oscillatory and rotational motion around the irradiation spot in the closed bubble region, remaining stable for over 10 min. The droplets exhibit rapid rotation at frequencies exceeding 1 Hz, and both their rotation radius and speed can be modulated by varying the laser irradiation intensity. Upon laser shutoff, the droplets exhibit straight-line motion at speeds up to 30 µm s^-1 in the semi-open bubble. We perform image analysis of high-speed videography coupled with laminar-flow/species-transport FEM simulations (COMSOL Multiphysics) to quantify how the concentration gradient in the ~50 nm precursor film drives droplet motion. This versatile, contact-free approach provides a new active-matter platform for fundamental studies and paves the way for precise microdroplet handling in lab-on-a-chip and autonomous micro-robotic systems.