We find a supersonic microjet with highly-focused shape (Fig.1). The maximum speed of the jet of 10 µm diameter is 850 m/s. This ultra-high speed makes the microjet unique in the field of micro-fluids since the small jet have large inertia compared to the viscosity (high Reynolds number). Possible applications of the microjet are needle-free injection systems and micro-cleaning devices.
Fig.1 Snapshot of a supersonic microjet (Tagawa et al., Physical Review X, 2012)
The velocity of the microjet is more than 340 m/s, with the diameter of ~ 5 µm.
VISCOUS LIQUID MICROJET
We invent a device for generating highly-viscous liquid jets up to 10,000 cSt (10,000 times viscous than water). See Fig. 2. Compared to existing ink-jet printers that is able to eject a liquid of low viscosity (< 20 cSt), the ability of ejecting an ultra-viscous liquid is extraordinary. Our device has a simple structure, leading to its easy installation in various devices, such as needle-free injection devices, ink-jet printers, and printed-electro devices.
Fig. 2 Left: A generator of highly-viscous microjets. Right: An example of printing high-viscous jets (manicure ~ 1,000 cSt) using our device (Thanks to NAC img. tech. co.)
A drop is able to levitate over a moving wall without heating or vibrating the wall (Movie 1, 2). This simple and beautiful fluid dynamics are seen in processes of spray coating and ink-jet printings. There is an air film of a few micro-meters between the drop and the wall, which enables the drop levitate. We measure the shape with sub-micron precision and investigate its mechanics.
Movie 1 Steady drop levitation（Saito, et al., APS/DFD 2014, Gallery of Fluid Motion）
Movie 2 made by Sawaguchi, Hama, et al., Droplets2015 Best Video Award
PARTICLES IN TURBULENCE
Inertia particles (or bubbles) cluster in turbulence. In order to analyze the cluster in Lagrangian point of view, we develop a method employing Voronoi tessellation (Fig. 3). Using this method, clustering lifetimes are defined quantitatively. This Voronoi analysis is expected to open a new door for Lagrangian analysis of clusters in turbulence.
Fig. 3 Voronoi tessellation
(Tagawa et al., Journal of Fluid Mechanics, 2012)
SINGLE BUBBLE IN WATER
A bubble of 2 mm diameter in a quiescent liquid shows 3D motion, i.e. zigzagging or spiraling. This phenomenon often occurs in purification systems and chemical aeration devices. Interestingly, the addition of small amount of surfactant drastically change the motion of bubbles. We measure the 3D motion by synchronized high-speed cameras and reveal forces acting on the bubble.
Fig. 4 (a) 3D measurement system (b) Motion of a bubble of 2 mm in diameter.
(Tagawa et al., Journal of Fluid Mechanics, 2014)