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

SynopsisThis paper is concerned with the experimental ability to measure viscoelasticity of low viscosity ink jet fluids and demonstrates the capability of both a piezo axial vibrator and torsion resonator rheometer to capture high frequency rheological data for both model and commercial ink jet fluids. Results are presented for polymer and particle laden suspensions together with a commercial ink. The data demonstrate that high frequency linear viscoelastic rheology can be captured using both rheometers and that both the presence of polymer and particles can induce viscoelasticity within the fluid. It is believed that the physical origin of viscoelastic effects produced by the presence of polymer or particles is different, and this results in a different high frequency limiting slope for the GЈ data.

Drop-on-demand (DoD) ink-jetting of hard particle suspensions with volume fraction ∼ 0.25 has been surveyed using 1000 ultra-high speed videos as a function of particle size (d 90 = 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 breakoff 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. C 2014 AIP Publishing LLC.Many materials applications are being implemented using additive manufacturing, in particular, using digital processes such as drop-on-demand (DoD) inkjet printing. Reliable inkjet printing of colloids, and hard particle suspensions such as inorganic pigments, is desirable for modern manufacturing processes. There has been much prior effort to understand ceramic inkjet printing, 1 but perhaps surprisingly, there are still many unresolved aspects of complex fluid behaviour under jetting conditions although recent progress has been reported 2 for the understanding of polymeric fluid jetting.The deposition of hard particles is of increasing interest for larger scale decorative applications, but is currently restricted to relatively small particle (d 90 ∼1 μm) sizes. Close packed spheres have a maximum occupied volume of ∼64 vol. %. The rheology for model suspensions of spherical polystyrene particles is well-described 3 by the Zarraga model 4 η(ϕ) = η 0 exp(-2.34 ϕ)/(1 -ϕ/ϕ m ) 3 where η 0 is the base fluid viscosity (200 mPa s) and the maximum volume fraction is ϕ m = 0.62. However, in DoD inkjet applications the base fluid viscosity is typically 5 mPa s, very much lower than that used in liquid bridge and dripping experiments with particles, and jetted particle loadings are limited to 35 wt. %, for which the corresponding volume fraction is typically 0.20-0.30 (but still well beyond the Einstein regime). Recent numerical modelling of liquid bridges 5 and experimental work on the dripping of liquids containing particles 3, 6 suggest that DoD break-off times could be significantly lowered (by typically >10%) for particle loadings with > 0.20.We report here the DoD jetting of complex liquids containing particles and/or resins observed using ultra-high speed imaging techniques. A Shimadzu HPV-1 ultra-high speed camera and 500 W flash lamp recorded single jets in a 312 × 260 pixel frame at a rate of 500 000 frames per second for 102 frames of 0.5 μs exposure time in conventional shadowgraph mode at typically 1.7 μm/pixel image. Off-line analyses determined the jet and drop speeds, while jet break-off times (±3 μs) were deduced from the video image counter.Jets and drops in DoD inkjet printing have speeds and volumes that appear to vary almost linearly with print head drive voltage above a threshold value of drive voltage that depends on viscosity. The DoD jetting thresh...

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