Development of an optical microactuator based on thermocavitation for the drop-on-demand deposition of viscous fluids

We present the design and experimental validation of an optical microactuator based on vapor microbubbles induced by a continuous wave laser in a highly absorbing solution. This optically driven system enables controlled drop-on-demand deposition of viscous fluids without the need for mechanical pumps or moving parts. It is a promising alternative to traditional techniques for handling high-viscosity fluids and deposition on unconventional substrates. The study details the design and fabrication of a dual-chamber microfluidic device, the experimental conditions, and a theoretical analysis of the behaviour of the deposited fluid using dimensionless numbers. To evaluate the performance of the optical device and the effects of viscosity and surface tension on the deposition quality, experiments were performed with liquids of different physical properties (deionised water, propylene glycol and SAE 40 oil). The results obtained show that the deposited microdroplet size mainly depends on the size of the vapor bubble, while the viscosity plays a crucial role in the deposition uniformity. The optical microactuator proved to be particularly effective with propylene glycol, where the depositions are reproducible within a working range, which is confirmed by the theoretical analysis. This work introduces a low-cost and versatile optical method for fluid actuation and microdroplet generation, offering promising applications in microfluidics, biofabrication, and laser-based material processing. The dual-chamber design thermally isolates the solution to be deposited from the laser interaction zone, allowing safe manipulation of thermally sensitive materials such as bio-inks or cell-laden suspensions. This opens up new opportunities for biomedical applications.