Droplet spreading

Complete wetting if 5 > 0, P > 0. In this case, droplets spread to a uniform film or pancake. 

FIG. 36 Morphological changes of the sulfuric acid droplets as a function of time at about 94%. The three droplets (marked D) are the same as those shown in Fignre 35. After a period of time of abont 10 or more minntes, the droplets spread and coalesce (5). Spreading coincides with the corrosion reaction of the aluminnm snbstrate. Image size is 10 p,m X 10 p,m. (From Ref. 85.)... 

Figure 16. Schematic representation of binder droplet spreading and penetration into a porous sublayer.

However, the droplet method has its own drawbacks, such as the degradation of information about the analyte s localization at a spot where the matrix droplet spreads. In general, a dispensed matrix droplet makes a spot of more than 1 mm in diameter on a tissue surface because of the lower limit of a pipetting volume of 500 nL with an ordinary micropipette. For such a large spot, it is insufficient to perform a precise high-resolution IMS. Therefore, technical improvements are needed to dispense the smallest droplets possible. 

Considering a surface temperature which is higher than the Leidenfrost temperature of the liquid in this study, it is assumed that there exists a microscale vapor layer which prevents a direct contact of the droplet and the surface. Similar to Fujimoto and Hatta (1996), the no-slip boundary condition is adopted at the solid surface during the droplet-spreading process and the free-slip... 

Comparing the 3-D images simulated and the experimental photographs in Fig. 10, it can be seen that the droplet shapes are well reproduced by the present model. During the first 3.5 ms of the impact (frames 1-3), a liquid film with flattened disc shape is formed immediately after the impact. The inertial force drives the liquid to continue spreading on the solid surface, while the surface tension and the viscous forces resist the spreading of the liquid film. As a result, the droplet spreading speed decreases and the fluid mass starts to accumulate at... 

Fig. 14 shows the comparison of the photographs from Chandra and Avedisian (1991) with simulated images of this study for a subcooled 1.5 mm n-heptane droplet impact onto a stainless-steel surface of 200 °C. The impact velocity is 93 cm/s, which gives a Weber number of 43 and a Reynolds number of 2300. The initial temperature of the droplet is room temperature (20 °C). In Fig. 14, it can be seen that the evolution of droplet shapes are well simulated by the computation. In the first 2.5 ms of the impact (frames 1-2), the droplet spreads out right after the impact, and a disk-like shape liquid film is formed on the surface. After the droplet reaches the maximum diameter at about 2.1ms, the liquid film starts to retreat back to its center (frame 2 and 3) due to the surface-tension force induced from the periphery of the droplet. Beyond 6.0 ms, the droplet continues to recoil and forms an upward flow in the center of the... 

The impact process of a 3.8 mm water droplet under the conditions experimentally studied by Chen and Hsu (1995) is simulated and the simulation results are shown in Figs. 16 and 17. Their experiments involve water-droplet impact on a heated Inconel plate with Ni coating. The surface temperature in this simulation is set as 400 °C with the initial temperature of the droplet given as 20 °C. The impact velocity is lOOcm/s, which gives a Weber number of 54. Fig. 16 shows the calculated temperature distributions within the droplet and within the solid surface. The isotherm corresponding to 21 °C is plotted inside the droplet to represent the extent of the thermal boundary layer of the droplet that is affected by the heating of the solid surface. It can be seen that, in the droplet spreading process (0-7.0 ms), the bulk of the liquid droplet remains at its initial temperature and the thermal boundary layer is very thin. As the liquid film spreads on the solid surface, the heat-transfer rate on the liquid side of the droplet-vapor interface can be evaluated by... 

Figure 3.21. We-Oh map regimes of droplet spreading and splashing on a dry, solid surface.

F), in addition to the Reynolds and Weber numbers, to fully describe a droplet spreading and solidification process upon impact on a substrate. They introduced two new dimensionless numbers, defined as ... 

Hofmeister et al. 4l() employed two high-speed thermal imaging systems to record spatial and temporal temperature distributions at the splat-substrate interface, and to observe droplet spreading during impact and solidification on a quartz plate. They observed... 

Type R. This mode corresponds to low impinging velocities at any surface temperatures considered (200-400 °C). A droplet spreads as a radial film after impinging on a hot surface. Then, it shrinks and rebounds from the surface without breaking up. Hence, the mode is called R type. 

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