Real-time Sub-nanogram Resolution Measurement of Inertial Mass and Density Using Magnetic-field Guided Bubble Microthruster

Artificial micro/nanomotors utilizing active particles exhibit enormous potential in applications such as drug delivery and microfabrication. However, their substantial upgrade to micro/nanorobots capable of executing precise tasks with sophisticated functions remains a significant challenge. In this paper, we develop a bubble microthruster (BMT) – a new variation on the bubble driven microrobot – that focuses energy from a collapsed microbubble to create a hydrodynamic jet to exert inertial impact on nearby micro-objects. Benefiting from the easy magnetic-field guided control via gamepad, the BMT can perform real-time measurements on designated micro-objects with the assistance of high-speed imaging. By measuring the transient response of the micro-object and fitting the relaxation time, the inertial mass and density of micro-objects can be determined. Our study validates our approach by measuring the mass of polystyrene microparticles and the effective density of hollow glass microspheres, and unravel a sub-nanogram resolution that could be suitable to detect variation in mass or density of cells or embryos. The BMT technique illuminates the development of a chip-free method for biological microparticles, integrating with manipulation using microrobot and detection of physical properties. This technique may provide versatility and simplicity in complex environments compared to current methods using cantilever/resonator in microfluidic chips.