A microfluidic device and sensing method that utilize a resonating tube configured to have sufficient sensitivity to be capable of sensing the volume of a gas present as bubbles in a liquid or the flow rate and/or density of a gas or gas mixture flowing through the tube. The tube has a freestanding tube portion supported above a surface of a substrate so as to be capable of vibrating in a plane normal to the surface of the substrate. As a gas-containing fluid flows through an internal passage of the tube, a drive signal vibrates the freestanding tube portion at a resonant frequency thereof. Coriolis-induced deflections of the freestanding tube portion are sensed relative to the substrate to produce an output corresponding to the sensed deflections, and the drive signal and/or the output are assessed to determine the volume, density and/or flow rate of the gas of the gas-containing fluid.