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 behaviour 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 mono-disperse polystyrene in diethyl phthalate with molecular weights between 24 - 488 kDa, previous numerical simulations of this system, and previously published data for this and another linear polymer in a variety of “good” solvents, all show good agreement with the scaling predictions of the model

Giovanni Canestrini’s models of Leonardo da Vinci’s friction experiments

Among the exhibits at the 2016 Leonardo da Vinci exhibition at the Science Museum, London, was one that purported to illustrate Leonardo’s experiments on friction. The models involved were the work of Giovanni Canestrini (1893-1975) who contributed to both the 1939 and 1953 Leonardo exhibitions in Milan. This article discusses the original sources and history of these models, in the light of recent research into Leonardo’s work on friction. It concludes that, while being relevant to Leonardo’s study of mechanics, these models seriously misrepresent his experimental investigations of friction

Holographic measurement of drop-on-demand drops in flight

The analysis of images of ink drops in flight can provide information about jet straightness, drop velocity and volume. However trade-offs between field of view, optical and digital resolution and other factors such as depth of field and optical distortion, limit the accuracy and amount of information available from a single image. In-line, digital holograms of drops in flight can capture information from fields of view at least as large as the area of the digital sensor. Using mathematical reconstruction techniques particularly suited to sparse, small objects of regular geometry the accuracy of measurement can potentially be submicrometer on drop position and diameter. This paper describes our experimental apparatus, hologram reconstruction techniques and the results of experiments on imaging drops. We also discuss techniques to improve the accuracy of the technique in the direction of the optical axis

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.This work was sponsored by EPSRC grant number RG5560

Aerodynamic effects in ink-jet printing on a moving web

This work was sponsored by EPSRC grant number RG5560

Jetting, in-nozzle meniscus motion and nozzle-plate flooding in an industrial drop-on-demand print head

The state of the ink film at and near the nozzles of a drop-ondemand(DoD) print head during jetting has a direct impact on printing performance and reliability. We have developed highspeed imaging apparatus and analytical techniques to investigate the ink film dynamics on an industrial print head nozzle-plate in real-time. In addition to a direct correlation between the jet emergence velocity and drive voltage, drive-dependent variations in the oscillation of the ink meniscus in adjacent nozzles were also observed. Using a ray-tracing model to analyze the meniscus shape, the meniscus oscillations for both printing and nonprinting nozzles were found to be complex and involve elements such as pre-oscillation and high-order surface waves. The flooding of non-firing nozzles, deliberately caused by the application of maximum drive voltage to a neighboring nozzle, has been recorded and analyzed dynamically. The build-up of fluid in an annulus around the nozzle (flooding rate) has been characterized and compared with models for the net ink flow through the nozzle

High speed shadowgraphy for the study of liquid drops

The book contains invited lectures and selected contributions presented at the Enzo Levi and XVII Annual Meeting of the Fluid Dynamic Division of the Mexican Physical Society in 2011.This was work was sponsored by EPSRC grant number RG5560

Breakup of liquid filaments.

Whether a thin filament of liquid separates into two or more droplets or eventually condenses lengthwise to form a single larger drop depends on the liquid's density, viscosity, and surface tension and on the initial dimensions of the filament. Surface tension drives two competing processes, pinching-off and shortening, and the relative time scales of these, controlled by the balance between capillary and viscous forces, determine the final outcome. Here we provide experimental evidence for the conditions under which a liquid filament will break up into drops, in terms of a wide range of two dimensionless quantities: the aspect ratio of the filament and the Ohnesorge number. Filaments which do not break up into multiple droplets demand a high liquid viscosity or a small aspect ratio.This work was supported by EPSRC (RG53364 and RG55605

Some innovative surface texturing techniques for tribological purposes

This paper reviews methods for texturing surfaces for tribological applications and presents some innovative methods that could make surface texturing more cost-effective. Possible texturing methods were identified and classified according to their physical principles. This involved identifying existing texturing methods and also led to proposals for new possible methods. Three innovative texturing methods with low cost and high texturing speed are then presented: (i) a simpler and cheaper version of photochemical texturing, (ii) maskless electrochemical texturing, and (iii) masking surfaces by ink-jet printing followed by etching. From these, maskless electrochemical texturing was the cheapest and fastest, but the minimum size of the texture features was the largest. Ink-jet printing followed by etching is an alternative that may potentially provide a good combination of cost and resolution, but the texturing time depends on the surface area. Then, an attempt was made to delimit tribological applications where the use of such processes could be beneficial, based on analysis of experimental results of their tribological evaluation. These showed that the methods proposed could be particularly suited for components with contact areas larger than the width of the texture features under either hydrodynamic lubrication or starved lubrication. This work was supported by Fapemig/Brazil, Capes/Brazil, and the Royal Society (UK)This version is the author accepted manuscript. The final published article can be found here: http://pij.sagepub.com/content/early/2014/06/18/1350650114539936.full.pdf+htm