Microjet Generator for Highly Viscous Fluids

Abstract

This paper describes a simple 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 interval [impact duration ≤ Oð10 −4 Þ s] and the focusing interval [focusing duration ≫ Oð10 −4 Þ s]. During the impact interval, the liquid accelerates suddenly due to the impact. During the focusing interval, the microjet emerges due to flow focusing. In order to explain the sudden acceleration inside the tube during the impact interval, 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 during the impact interval. In contrast, during the focusing interval, 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 the Reynolds number (the ratio between the inertia force and the viscous force). This 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.

Impulsive generation of jets by flow focusing

“…Impulsive generation of jets by flow focusing FIGURE 1. (a) Sketch of the two methods used here to generate the impulsive liquid jet: the left image illustrates the case in which the jet formation process is triggered by focusing a laser pulse on a liquid filled tube, whereas the right one corresponds to the case in which the jet is generated using the more complex geometry reported by Onuki et al (2018), when a metal rod is shot up by a coil gun upwards, impacting the tube. (b) Sequence showing the temporal evolution of the impulsively generated jet using the type of device described in Onuki et al (2018).…”

“…(c) Illustration of the different variables used to describe the jet ejection process. (d) Sketch of the device proposed in Onuki et al (2018) to generate the impulsive liquid jet.…”

“…Notice that the final drop velocity given in (2.24) is only slightly smaller than v jet because, in applications, We S 1. With the purpose of validating our theoretical approach, we have made use of the experimental data in Peters et al (2013) and Onuki et al (2018) (see the sketches in figure 1a). The way the relative liquid velocity with respect to the solid walls, V, is calculated, differs depending on the type of experimental set-up.…”

“…This type of unsteady liquid ligament, which can even reach supersonic velocities (Tagawa et al 2012), experience a strong stretching in the downstream direction until a drop, with a noticeably smaller diameter than the width of the cavity from which the jet emanates, is ejected from its tip (see figure 1b). These unique features of highly focused transient jets make them suitable to spread high viscosity liquids over a substrate (Onuki, Oi & Tagawa (2018), see figure 1a right) or to deliver, in a controlled manner, tiny amounts of drugs after the fast and thin jet penetrates the patient's skin (Tagawa et al 2013).…”

Microfluidics control the ballistic energy of thermocavitation liquid jets for needle-free injections

“…However, the control of these jets is difficult in practice because such additives change the better understood Newtonian liquids jetting conditions. The polymer type, concentration, liquid temperature and various non-linear phenomena, such as asymmetric jet formation and pinch-off, can in turn reduce the kinetic or ballistic energy of the resulting jets [37][38][39][40][41]. Besides manipulating the liquid properties, an elegant approach can be found in controlling the geometry of the microfluidic channel in which the vaporisation and jetting phenomena occur.…”