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|You are here: Home > Technique > Processes > Scientific report of the LGP2 > Printing processes > Influence of rheology on drop formation and their impact: application to ink-jet printing||Update: July 21, 2011|
|Scientific report of the LGP2 (2006-2009)|
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|Researchers of the LGP2
The objective of this project is to determine how the rheology of the fluid, in particular the yield stress, governs on one hand by the drop formation mechanisms and on the other hand by the mechanisms of the impact of this drop on PMMA substrates whose surface is perfectly controlled. The aim is ultimately to better control the final shape of the deposited drop.
In this study, carbopol gels were chosen. These gels are made of dispersed polymer in water and neutralized with sodium hydroxide. Their rheological behavior was characterized using a cone-plate rheometer. It is modeled by an Herschel-Bulkley law, whose parameters depend on the polymer concentration. Drops of these gels were ejected under their own weight using an experimental device equipped with an high-speed camera and an image processing software. Once the events recorded at a speed of 2000 fps, the contact angle, diameter and height of the drop during the impact and the spreading process can be determined every 0.5ms. Thus, the dynamic behavior of the impacting millimeter drops can be characterized.
With the increasing yield stress, the drops are more elongated and their shape becomes more elliptical. On the other hand, they show a change in volume. The variation of droplet size and the time necessary for their formation appears to be related to high sensitivity between the surface tension and the yield stress.
To highlight the effect of yield stress at different inertial regimes, the drops have been ejected on a range of impact velocity from 0.3 to 3 m/s. Phenomena visualized at each impact velocity were analyzed and compared with the behavior of a Newtonian fluid (water). At relatively low impact velocity, where the drops are almost deposited on the substrate, a subsidence of the drops is observed. At the contact moment with the support, the drop deforms. The drop crashes slightly and then regains the height. By increasing the impact velocity (1.4 to 3 m/s), the drops spread to a maximum diameter and then retract to their final steady state. In a yield stress fluid, the drop loses its velocity when its spreads; stresses also decrease until they eventually fall below their yield value. When the limit is reached, the spreading stops suddenly. The use of yield stress fluid may therefore contribute to the control of the profile and especially the deposition of drops of fluid. The next stages of the project are to study the effect of the consistency of the fluid at constant yield stress and the surface of the substrate.
|Figure 1 - Évolution de la forme des gouttes en fonction du seuil.|
|Figure 2 - Impact du Carbopol gel d’un seuil de 34 Pa sur le film de PMMA à 3 m/s|
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