Droplet spreading behavior

Figure 3.15. Spreading behavior of a single droplet impinging on flat (e/D0 = 0) and non-flat (e ID0 = 0.33, XID0 =2.8) surfaces at different Reynolds numbers (We = 2000) (Reprinted from Ref. 389, 1995, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X51GB, UK.)...

It should be noted that the dynamic conditions of droplet impact processes discussed above cover a large range of the actual conditions in many industrial processes, such as spray forming, thermal spray, spray combustion, spray cooling, and aircraft flight. Under these conditions, the spreading behavior of droplets on a flat surface is essentially governed by inertia and viscous effects (Fig. 

The spreading behavior of droplets on a non-flat surface is not only dependent on inertia and viscous effects, but also significantly influenced by an additional normal stress introduced by the curved surface. This stress leads to the acceleration-deceleration effect, or the hindering effect depending on the dimensionless roughness spacing, and causes the breakup and ejection of liquid. Increasing impact velocity, droplet diameter, liquid density, and/or... 

Figure 5.13. Comparison of spreading behavior of a single droplet of different...

FIGURE 6.11 Interfacial energy balances determine the wetting behavior of a liquid droplet in contact with a sohd surface, (a) The balance of the -direction components of the interfacial energy forces determines the contact angle 0. (b) If y,s y, 0 -> 0° and completely hydrophilic (spreading) behavior occurs, (c) If yis y v- nd completely hydropho-... 

The dried droplet diameters in the 1-2 mm range for the three concentrations of butyl acetate solutions deviate from each other, in the same way as for toluene. Again a difference in viscosity can explain the different spreading behaviors an increased viscosity limits the droplet spreading on the substrate and also affects the Z-number. Seaion 8.07.2.4 describes further research on the influence of viscosity on ink-jet-printed droplets and their spreading on a substrate. 

It was only recently that a study was presented wherein inks that contain a linear polymer with a systematically varied molar mass were investigated. The effect of the added polystyrene was related to the final dried droplet diameter on a substrate. Glass was foimd to be a suitable substrate for the investigations, since it is wetted very well by toluene, with a contact angle smaller than 5 , which provides a higher contrast between different spreading behaviors. Toluene was chosen, because of its good solubility properties with polystyrene. Polystyrene with a low polydispersity index (PDI< 1.07) has been used, in order to obtain a dependency on the molar mass and not on its (broad) molar mass distribution. 

The direct demonstration of a surfactant film in the airways is relatively recent (2-4), although a surface-active film had been inferred from physiological (5) and electron microscopic studies (6) many years before. The surface tension in large airways has been measured directly with a bronchoscope from the spreading behavior of oil droplets placed onto the tracheal walls or bronchi of anesthetized sheep and horses (7,8). A surface tension of approximately 32 mN/m has been recorded at the mucus-air interface in these animals. This relatively low surface tension suggests the presence of a surface film in large airways, because proteins, surface polymers of blood cells, polysaccharides, and other biopolymers, all have... 

Two extremes of droplet dispersion mode were investigated non-dispersed where the droplet stream remained tightly collimated as it traversed the reactor and dispersed, where the droplets were spread across the reactor but there was no side wall impingement. This approach allowed some control of droplet-droplet spacing and interaction from a highly collimated stream to a condition where individual droplet behavior was clearly evident. 

Several studies were recently carried out to better understand the mechanisms which dominate the behavior of an ink droplet on top of plain paper or inkjet paper and to enable better design of advanced inkjet substrates. A detailed study about the spreading on and penetration into thin, permeable print media and its application to inkjet printing was recently published. Adetailed review of the interaction... 

Have you ever broken a mercury thermometer If you have, you probably noticed that the mercury forms droplets on the surface on which it falls rather than spreading out and wetting it like water. Describe the difference between liquid mercury and liquid water that explains this different behavior. (Hint Consider the attractions between particles.)... 

Fig. III-29. The molecular dynamic simulation of the behavior of a droplet of liquid placed on a solid support a - spreading, b - gathering of liquid into a droplet in the case of nonwetting, c - fluctuations of a droplet under the condition of non-wetting, d - effect of a surfactant on wetting process [29]...

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