We numerically investigated liquid droplet impact behavior onto a dry and flat surface. The numerical method consists of a coupled level set and volume-of-fluid framework, volume/surface integrated average based multimoment method, and a continuum surface force model. The numerical simulation reproduces the experimentally observed droplet behavior quantitatively, in both the spreading and receding phases, only when we use a dynamic contact angle model based on experimental observations. If we use a sensible simplified dynamic contact angle model, the predicted time dependence of droplet behavior is poorly reproduced. The result shows that precise dynamic contact angle modeling plays an important role in the modeling of droplet impact behavior.

This paper reports experimental investigations of drop impacts onto chemically treated surfaces with wettability from 5° to 160°. To follow in time the drop spreading, a high speed video camera was used, and it allows us to determine precisely the expansion of the drop and the profile of the free surface at the contact line. By changing the impact velocity, between less than 0.5 and 5 m/s, and the viscosity, from 1 to 100 mPa s, at constant surface tension, a broad range of Reynolds and Weber numbers is explored. This paper is divided into two parts. In the first part, the experimental drop evolution during spreading is directly reported and compared with previous works. Secondly, the emphasis is on the importance of the apparent dynamic contact angle for the prediction of the maximum spreading diameter. This achievement is manifested at low Reynolds numbers at which the matching between the experiment and the model is improved greatly.

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

The addition of a starch or gum-based thickener to patient fluids with dysphagia is commonly carried out, but the mechanism behind the efficacy of this treatment is not fully understood. This paper describes the rheological behavior of two commercially available thickening powders and an additional xanthan gum solution with a view to explaining the efficacy of thickened fluids in terms of their rheology. Both linear viscoelastic and steady shear data were obtained for the fluids together with filament extensional stretch, decay, and breakup data. In order to follow the behavior of the fluids in a processing situation, a mechanical "Cambridge Throat" was designed and tested. The action of the tongue was modeled using a constant torque cam that forced fluid contained within a flexible membrane through a model throat. Movie photography captured images of the fluid behavior and showed that for a constant tongue torque, the transit time within the model throat increased with increasing fluid viscosity, with implications for the time available for the successful function of the larynx, throat muscles, and epiglottis. V

In this paper we introduce an experimental protocol to reliably determine extensional relaxation times from capillary thinning experiments of weakly-elastic dilute polymer solutions. Relaxation times for polystyrene in diethyl phthalate solutions as low as 80 µs are reported: the lowest relaxation times in uniaxial extensional flows that have been assessed so far. These data are compared to the linear viscoelastic relaxation times that are obtained from fitting the Zimm spectrum to high frequency oscillatory squeeze flow data measured with a piezo-axial vibrator (PAV). This comparison demonstrates that the extensional relaxation time reduced by the Zimm time, ext / z , is not solely a function of the reduced concentration c/c* , as is commonly stated in the literature: an additional dependence on the molecular weight is observed.

This article links measurements of ink jetting performance in drop-on-demand printing with the high-frequency rheological properties of model viscoelastic fluids containing linear polymers with various molecular weights. Jet formation and evolution were studied for solutions of polystyrene in diethyl phthalate. Ligament length, initial jet ejection speeds, and ligament extension and retraction rates were determined by high-resolution imaging with high time resolution. For these fluids, the viscosity measured under low shearrate conditions showed no correlation with their jetting performance. The jetting behavior was, however, well correlated with high frequency rheological properties measured at 5 kHz using a piezoelectric axial vibrator rheometer. This study shows that high frequency rheometry can provide useful predictive data about the jettability of fluids, and differentiate between inks that have similar low shearrate viscosity yet show different jetting behavior. A phenomenological model has been proposed and fitted to the evolution of the average ligament length from emergence, through break-up and into the final state of unmerged drops and associated satellites in order to help discuss the influence of viscoelastic behavior on the fixed speed drop-on-demand jetting and printability of fluids. The values of the parameters of this model obtained from the fitting are shown to have a consistent correlation with the rheological properties of the jetted fluids.

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.

This paper is concerned with the rheology of algae suspensions relevant to algae biofuel processing for a range of concentrations up to 15 vol. % using mostly a piezoaxial vibrator (PAV) rheometer as a method of measuring rheological properties. Linear viscoelastic (LVE) measurements of a Scenedesmus obliquus [culture collection of algae and protozoa (CCAP) 276=7] living algae strain were obtained and a curve for complex viscosity (g*) as a function of concentration=volume fraction derived. The PAV complex viscosity data increased exponentially with cell concentration and elasticity (G 0) developed in a similar way with increasing concentration. The results indicated the presence of interaction between algae cells at all measured concentrations. For concentrations above $5 vol. %, steady shear data obtained using a Couette geometry showed non-Newtonian "shear-thinning" behavior and at higher concentrations there was a divergence from the Cox-Merz rule. A difference in the LVE rheological measurements was found for cells that were either alive or dead indicating that cell motility and significant interparticle contact and interactions influenced levels of viscoelasticity. The results are of potential scientific relevance and also useful in relation to the design of algae bioprocessing for the production of biofuels. V