Currently, optical or mechanical resonances are commonly used in microfluidic research. However, optomechanical oscillations by light pressure were not shown with liquids. This is because replacing the surrounding air with water inherently increases the acoustical impedance and hence, the associated acoustical radiation losses. Here, we bridge between microfluidics and optomechanics by fabricating a hollow-bubble resonator with liquid inside and optically exciting vibrations with 100 MHz rates using only mW optical-input power. This constitutes the first time that any microfluidic system is optomechanically actuated. We further prove the feasibility of microfluidic optomechanics on liquids by demonstrating vibrations on organic fluids with viscous dissipation higher than blood viscosity while measuring density changes in the liquid via the vibration frequency shift. Our device will enable using cavity optomechanics for studying non-solid phases of matter, while light is easily coupled from the outer dry side of the capillary and fluid is provided using a standard syringe pump. Keywords: nonlinear optics; optical materials and devices; optomechanics INTRODUCTION A major application of optical resonators is in the field of sensing where microresonators were used to sense biomarkers in serum 1 and to detect viruses and nanoparticles 2-4 in an aqueous environment. With similar motivation and in a parallel effort, mechanical resonators were used to weigh biomolecules and cells. 5,6 Just like we use more than one sense (e.g., eyes and ears) to detect hazards, optomechanics might suggest a bridge between the seemingly parallel optical and mechanical detection fields. The recent availability of liquid containing bubble shaped resonators, 7,8 in combination with optomechanical vibrations at .GHz rate, 9-12 might pave the way for ultrasound investigations on analytes in liquids. Nevertheless, cavity optomechanics on non-solid phases of material was never before demonstrated.One of the major 'show stoppers' on the way to microfluidic optomechanics originates from the fact that water has acoustical impedance that is more than 4000 times larger than air. Hence, naively immersing optomechanical devices in water will accordingly increase the acoustical radiation losses. Liquid submerged optomechanical oscillators are therefore challenging, as sound will tend to escape from the cavity by radiating out rather than being confined to the resonator. Here, we confine the high-impedance water inside 6,13 a silica-bubble resonator so that its acoustical quality factor is minimally affected. In this manner, when both mechanical and optical quality factors, Q m and Q O , of microdevices are sufficiently leveraged, their optical and mechanical modes can be parametrically coupled, allowing the optical excitation of vibrations. 14 In spherical shapes 9 such as our bubble,