Sd Hoath

Inkjet printing - the physics of manipulating liquid jets and drops

Over the last 30 years inkjet printing technology has been developed for many applications including: product date codes, mailing shots, desktop printing, large-area graphics and, most recently, the direct writing of materials to form electronic, biological, polymeric and metallic devices. The new non-graphical applications require higher print rates, better resolution and higher reliability while printing more complex, non-Newtonian and heavily solids-loaded liquids. This makes the understanding of the physics involved in the precise manipulation of liquid jets and drops ever more important. The proper understanding and control of jet formation and subsequent motion of the jetted materials requires physical studies into material properties at very high shear rates, acoustic modes in print heads, instabilities of jets, drop formation, drop motion, stretching of fluid ligaments, the role of polymers in jet break up, electrical charging of drops and the aerodynamic and electrostatic interaction of jets and drops in flight. Techniques for observation, measurement and analysis are evolving to assist these studies. This paper presents some examples of the application of physics to understanding and implementing inkjet printing, including recent work at the Cambridge Inkjet Research Centre.

Evaluation of the inkjet fluid’s performance using the “Cambridge Trimaster” filament stretch and break-up device

This paper describes the design and initial results from the “Cambridge Trimaster,” a recently developed high speed filament stretch and break-up device that can be used for viscoelastic fluids with shear viscosities as low as 10 mPa s. Extensional viscosity and filament break-up behavior were studied optically using a high speed camera and extensional viscosity values determined for a series of mono-disperse polystyrene solutions up to a weight concentration of 5 wt % were measured as a function of the polymer loading. The transient stretching and break-up profiles recorded with the apparatus were observed and correlated with drop formation for drop-on-demand inkjet printing fluids. This allowed the filament break-up behavior to be ranked in terms of satellite drop and droplet filament behavior. Correlation with previous work on the jetting of similar low viscosity viscoelastic polymer solutions demonstrated the ability of this apparatus to characterize inkjet fluids.

Jetting behavior of polymer solutions in drop-on-demand inkjet printing

The jetting of dilute polymer solutions in drop-on-demand printing is investigated. A quantitative model is presented which predicts three different regimes of behavior depending upon the jet Weissenberg number Wi and extensibility of the polymer molecules. In regime I (Wi   L), the chains remain fully extended in the thinning ligament. The maximum polymer concentration at which a jet of a certain speed can be formed scales with molecular weight to the power of (1-3ν), (1-6ν), and −2ν in the three regimes, respectively, where ν is the solvent quality coefficient. Experimental data obtained with solutions of monodisperse polystyrene in diethyl phthalate with molecular weights between 24 and 488 kDa, previous numerical simulations of this system, and p...

A simple large-scale droplet generator for studies of inkjet printing

UNLABELLED A simple experimental device is presented, which can produce droplets on demand or in a continuous mode and provides a large-scale model for real inkjet printing systems. Experiments over different regimes of Reynolds and Weber number were carried out to test the system. The ranges of Reynolds and Weber numbers were adjusted by modifying the liquid properties or the jetting parameters. Reynolds numbers from 5.6 to 1000 and Weber numbers from 0.5 to 160 were obtained using water/glycerol mixtures in the drop-on-demand mode and Reynolds numbers from 30 to 5500 and Weber numbers from 20 to 550 for the continuous jet mode. The nozzle diameter can be varied from 0.15 to 3.00 mm and drop velocities were achieved in the range from 0.3 to 6.0 ms depending on the jetting parameters and the driving mode. KEYWORDS Droplet, printer nozzle, drop on demand and continuous jet.

Atomization patterns produced by the oblique collision of two Newtonian liquid jets

This paper reports a detailed experimental investigation of the formation, destabilization, and atomization of the liquid sheets created by the oblique impact of two laminar jets of a Newtonian liquid. Glycerol-water mixtures with viscosities between 4 and 30 mPa s were used to investigate the effects of viscosity and jet velocity. The jets were ejected from parallel cylindrical nozzles with an internal diameter of 0.85 mm. Collision of the jets resulted in various regimes of behavior which depend on the jet velocities and the liquid properties. We focus on the regime where the impinging jets form a liquid sheet which then breaks up into a regular succession of ligaments and droplets, a so-called “fishbone” pattern. We use short-duration, single-flash illumination combined with high-resolution digital photography to study the evolution of the sheet, its shape, and the form, size, and spacing of resulting ligaments and drops. Unexpectedly, we found fishbone regimes corresponding to lower Reynolds and Weber...

Determination of dynamic surface tension and viscosity of non-Newtonian fluids from drop oscillations

The oscillations of free-falling drops with size range from pl to μl have been used to measure the transient shear viscosity and the dynamic surface tension of shear-thinning fluids on the timescale of 10−5–10−2 s. The method is first validated with Newtonian fluids. For a given surface tension, the lower and upper limits for accurate measurement of the viscosity are determined as a function of drop size. The dynamic properties of two types of shear-thinning fluids with varying viscoelasticity are reported: aqueous suspensions of the antifungal drug griseofulvin and of the organic light-emitting diode material poly(3,4-ethylenedioxythiophene): polystyrene-sulphonate. In both cases, the free-falling drop retains the high-shear viscosity.

Fundamentals of inkjet printing : the science of inkjet and droplets

From droplet formation to final applications, this practical book presents the subject in a comprehensive and clear form, using only content derived from the latest published results. Starting at the very beginning, the topic of fluid mechanics is explained, allowing for a suitable regime for printing inks to subsequently be selected. There then follows a discussion on different print-head types and how to form droplets, covering the behavior of droplets in flight and upon impact with the substrate, as well as the droplets wetting and drying behavior at the substrate. Commonly observed effects, such as the coffee ring effect, are included as well as printing in the third dimension. The book concludes with a look at what the future holds. As a unique feature, worked examples both at the practical and simulation level, as well as case studies are included. As a result, students and engineers in R&D will come to fully understand the complete process of inkjet printing.

A simple criterion for filament break-up in drop-on-demand inkjet printing

Jets from drop-on-demand inkjet print-heads consist of a main drop with a trailing filament, which either condenses into the main drop, or breaks up into satellite drops. Filament behaviour is quantitatively similar to that of larger, free symmetrical filaments, and can be predicted from the aspect ratio and Ohnesorge number. Symmetrical filaments generated from inkjet print-heads show the same behaviour. A simple model, based on competition between the processes of axial shortening and radial necking, predicts the critical aspect ratio below which the jet condenses into a single drop. The success of this simple criterion supports the underlying physical model.

Velocity Profiles in a Cylindrical Liquid Jet by Reconstructed Velocimetry

An experimental setup and a simple reconstruction method are presented to measure velocity fields inside slightly tapering cylindrical liquid jets traveling through still air. Particle image velocimetry algorithms are used to calculate velocity fields from high speed images of jets of transparent liquid containing seed particles. An inner central plane is illuminated by a laser sheet pointed at the center of the jet and visualized through the jet by a high speed camera. Optical distortions produced by the shape of the jet and the difference between the refractive index of the fluid and the surrounding air are corrected by using a ray tracing method. The effect of the jet speed on the velocity fields is investigated at four jet speeds. The relaxation rate for the velocity profile downstream of the nozzle exit is reasonably consistent with theoretical expectations for the low Reynolds numbers and the fluid used, although the velocity profiles are considerably flatter than expected.

Jetted mixtures of particle suspensions and resins

Drop-on-demand (DoD) ink-jetting of hard particle suspensions with volume fraction Φ ∼ 0.25 has been surveyed using 1000 ultra-high speed videos as a function of particle size (d90 = 0.8—3.6 μm), with added 2 wt. % acrylic (250 kDa) or 0.5 wt. % cellulose (370 kDa) resin, and also compared with Newtonian analogues. Jet break-off times from 80 μm diameter nozzles were insensitive (120 ± 10 μs) to particle size, and resin jet break-off times were not significantly altered by >30 wt. % added particles. Different particle size grades can be jetted equally well in practice, while resin content effectively controls DoD break-off times.