Han Seo Ko

Drop-on-demand printing of conductive ink by electrostatic field induced inkjet head

Recently, inkjet printing technology has become crucial in many industrial fabrication fields mainly due to its advantages of noncontact and fast pattern generation. In this paper, we investigate an electrostatic field induced inkjet printing system, which is based on an electrohydrodynamic process, for drop-on-demand jetting. In order to locate the optimal jetting conditions, we tested jetting performance for various bias voltages and pulse signals. To investigate the characteristics of drop-on-demand operation and micropatterning, we used conductive silver ink and examined the drops and lines patterned on a substrate.

Direct measurement of slip flows in superhydrophobic microchannels with transverse grooves

Slippage effects in microchannels that depend on the surface characteristics are investigated, taking into account hydrophilic, hydrophobic, and superhydrophobic wettabilities. Microscale grooves are fabricated along the vertical walls to form superhydrophobic surfaces, which enable both the visualization of the flow field near the walls and the direct measurement of the slip length. Velocity profiles are measured using microparticle image velocimetry and those in hydrophilic glass, hydrophobic polydimethylsiloxane (PDMS), and superhydrophobic PDMS microchannels are compared. For the hydrophilic glass surface, the velocity near the wall smoothly decreases to zero, which is consistent with the well-known, no-slip boundary condition. On the other hand, for the flow in the hydrophobic PDMS microchannel, the velocity profile approaches some finite value at the wall, showing a slip length of approximately 2μm. In addition, to directly measure the velocity in the superhydrophobic microchannel, transverse groove...

Electrostatic droplet formation and ejection of colloid

This paper presents a novel mechanism of electrostatic micro droplet formation and ejection using charged colloid as fluid. Unlike conventional electrostatic jetting which requires a high operating voltage, we offer a commercially viable, low operating voltage for droplet-on-demand operation based on a layered electrode structure of nozzle. The layered electrode structure of nozzle allows the formation, acceleration, and ejection of a droplet to be controlled by the voltage signals applied to individual electrodes. A simulation study indicates that the proposed mechanism allows less than 100 volt of operating voltage for the formation and ejection of a few pico-liter volume of droplet. Investigation includes the effects of the following factors on the droplet formation and ejection: 1) The shape of meniscus inside the nozzle. 2) The electric field distribution. 3) The density of charged nano particles in colloid.

Comparative Study on Ejection Phenomena of Droplets from Electro-Hydrodynamic Jet by Hydrophobic and Hydrophilic Coatings of Nozzles

An electro-hydrodynamic (EHD) jet from an electrostatic inkjet head shows advantages in printing microsize patterns because it can generate submicron droplets and can use highly viscous inks. Since the basic principle of the EHD jet is to form a droplet from the apex of a meniscus at the end of a nozzle, the stable ejection of the droplet greatly depends on the shape of the meniscus, which is affected by surface characteristics of the nozzle, electric potential, and ink properties. Hence, experiments have been performed using nozzles with hydrophobic and hydrophilic coatings to investigate the droplet ejection.

Comparative study on basis functions for projection matrix of three-dimensional tomographic reconstruction for analysis of dropletbehavior from electrohydrodynamic jet

Three-dimensional optical tomography techniques were developed to reconstruct three-dimensional objects using a set of two-dimensional projection images. Five basis functions, such as cubic B-spline, o-Moms, keys, and cosine functions and Gaussian basis functions, were used to calculate the weighting coefficients for a projection matrix. Two different forms of a multiplicative algebraic reconstruction technique were also used to solve inverse problems. The reconstruction algorithm was examined by using several phantoms, which included droplet behaviors and random distributions of particles in a volume. The three-dimensional volume comprised of particles was reconstructed from four projection angles, which were positioned at an offset angle of 45° between each other. Then, three-dimensional velocity fields were obtained from the reconstructed particle volume by three-dimensional cross correlation. The velocity field of the synthetic vortex flow was reconstructed to analyze the three-dimensional tomography algorithm.

Study on Concurrent Measurements of Velocity and Density Distributions for High-Speed CO2 Flow

Velocity and density distributions of a high-speed and initial CO2 jet flow have been analyzed concurrently by a developed three-dimensional digital speckle tomography and a particle image velocimetry (PIV). Two high-speed cameras have been used for the tomography and one for the PIV since a shape of a nozzle for the jet flow is bilaterally symmetric and the starting flow is fast and unsteady. The speckle movements between no flow and CO2 jet flow have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The three-dimensional density fields for the high-speed CO2 jet flow have been reconstructed from the deflection angles by the real-time tomography method, and the two-dimensional velocity fields have been calculated by the PIV method concurrently and instantaneously.

Investigations of the Mechanisms of the Electrostatic Droplet Ejections

This paper presents an experimental investigation of a micro dripping mode of electrostatic droplet ejection for the liquid solvents, such as DI water, acetone, dioxane, dioxane+ DI water (10% ,25% ,50% ,66.7%), of different physical properties. Emphasis is given to how the electrical conductivities of liquid solvents affect the stability of a micro dripping mode of drop-on-demand droplet ejection. The experimentation is based on a pair of electroplate with a hole at the center and a micro capillary tube with a pole inside, and on capturing the images of droplet ejection by a high speed camera. The results clearly demonstrate the influence of the applied voltage, the liquid conductivity and the in-flow rate on the formation of stable droplets for a micro dripping mode of droplet ejection. Especially, it is discovered that, given the applied voltage and flow rate, there exists an optimal range of conductivity, from 10-6 to 10-6 S/cm, required for stable droplet ejection in a micro dripping mode. This may be compared with the earlier report on the range of liquid conductivity, from 10-17 S/cm to 10-9 S/cm or larger, necessary for successful electrostatic spraying of liquid paraffin. This could be accounted for in terms of the electric force high enough for pulling away a droplet from the meniscus, but low enough for keeping the droplet from atomizing, which is something to do with charge relaxation time

Analysis of electrohydrodynamic jetting behaviors using three-dimensional shadowgraphic tomography

Electrohydrodynamic jetting behaviors of liquid menisci were analyzed experimentally by three-dimensional optical shadowgraphic tomography. The tomographic algorithm was developed after a series of multiplicative algebraic reconstruction techniques updated the objects intensities by using a cubic cosine basis function to determine the weighting coefficients of the projection matrix. The algorithm was evaluated initially by using a synthesized three-dimensional droplet phantom. Three-dimensional reconstructions of several jetting modes were built based on three images of projection data captured by three high-speed cameras, which were positioned at an offset angle of 45° relative to one another.

Electrostatic Ejection of Single-Walled Carbon Nanotubes Suspended in Solution

We propose to use electrostatic drop-on-demand devices, with precise metering and accurate delivery capabilities, to manipulate single-walled carbon nanotubes suspended in solution. The conductivity of the solution increases significantly by the addition of anionic surfactants and nanotubes, generating favorable conditions for electrostatic ejection. This technique could find applications, combined with chemical functionalization, in the fabrication of carbon nanotube field emission devices as well as electronic circuits. hanseoko@yurim.skku.ac.kr

Analysis of Simultaneous Velocity and Density Distributions for High-Speed CO2 Flow Using Particle Image Velocimetry and Digital Speckle Tomography

Velocity and density distributions of a high-speed and initial CO2 jet flow have been analyzed simultaneously by a developed three-dimensional digital speckle tomography and a particle image velocimetry (PIV). Three high-speed cameras have been used for the tomography and the PIV since a shape of a nozzle for the jet flow is asymmetric and the initial flow is fast and unsteady. The speckle movements between no flow and CO2 jet flow have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The three-dimensional density fields for the high-speed CO2 jet flow have been reconstructed from the deflection angles by the real-time tomography method, and the two-dimensional velocity fields have been calculated by the PIV method simultaneously.