  This paper describes a simple yet novel system for generating a highly
viscous microjet. The jet is produced inside a wettable thin tube partially
submerged in a liquid. The gas-liquid interface inside the tube, which is
initially concave, is kept much deeper than that outside the tube. An impulsive
force applied at the bottom of a liquid container leads to significant
acceleration of the liquid inside the tube followed by flow-focusing due to the
concave interface. The jet generation process can be divided into two parts
that occur in different time scales, i.e. the Impact time (impact duration $\le
O(10^{-4})$ s) and Focusing time (focusing duration $\gg O(10^{-4})$ s). In
Impact time, the liquid accelerates suddenly due to the impact. In Focusing
time, the microjet emerges due to flow-focusing. In order to explain the sudden
acceleration inside the tube in Impact time, we develop a physical model based
on a pressure impulse approach. Numerical simulations confirm the proposed
model, indicating that the basic mechanism of the acceleration of the liquid
due to the impulsive force is elucidated. Remarkably, the viscous effect is
negligible in Impact time. In contrast, in Focusing time, the viscosity plays
an important role in the microjet generation. We experimentally and numerically
investigate the velocity of microjets with various viscosities. We find that
higher viscosities lead to reduction of the jet velocity, which can be
described by using Reynolds number (the ratio between the inertia force and the
viscous force). This novel device may be a starting point for next-generation
technologies, such as high-viscosity inkjet printers including bioprinters and
needle-free injection devices for minimally invasive medical treatments.
