Droplet-substrate interactions

The fundamental issues to be addressed in the process modeling include spray enthalpy, gas consumption, spray mass distribution, microstructure of solidified droplets, and droplet-substrate interactions. The effects of atomization gas chemistry, alloy composition and operation conditions on the resultant droplet properties are also to be investigated in the process modeling. 

The modeling results of the spray stage provide input data and initial conditions for the modeling of droplet-substrate interactions in the ensuing deposition stage of the spray forming process. 

To analyze the dependences of chemical potentials, we employ the expressions resulting from the Flory-Huggins theory [29] modified to take into account the interactions of amphiphilic molecules with the environment and the underlying molecules of a substrate, namely, interactions of their hydrophilic head groups with water and their hydrophobic tails with the substrate molecules (under the droplet), and interactions of hydrophilic head groups with the air and hydro-phobic tails with underlying substrate molecules (outside the droplet). This leads to the following expression ... 

Roll-up. The principal means by which oily soil is removed is probably roU-up. The appHcable theory is simply the theory of wetting. In briefest outline, a droplet of oily soil attached to the substrate forms at equiUbrium a definite contact angle at the oil-sohd-air boundary line. This contact angle (Fig. 4) is the result of the interaction of interfacial forces in the three phase boundaries of the system. These interfacial forces, expressed in mN/m(= dyn/cm), or interfacial free energy values expressed in mj/m (erg/cm s) are conveniently designated 1SA iSlj subscripts relate to the Hquid-air,... 

The interaction of a simple fluid with a single chemically heterogeneous substrate has also been studied. Koch et al. consider a semiinfinite planar substrate with a sharp junction between weakly and strongly attractive portions and investigate the influence of this junction on the density profile of the fluid in front of the substrate [172-174]. Lenz and Lipowsky, on the other hand, are concerned with formation and morphology of micrometer droplets [175]. 

In cases when the two surfaces are non-equivalent (e.g., an attractive substrate on one side, an air on the other side), similar to the problem of a semi-infinite system in contact with a wall, wetting can also occur (the term dewetting appHes if the homogeneous film breaks up upon cooHng into droplets). We consider adsorption of chains only in the case where all monomers experience the same interaction energy with the surface. An important alternative case occurs for chains that are end-grafted at the walls polymer brushes which may also undergo collapse transition when the solvent quality deteriorates. Simulation of polymer brushes has been reviewed recently [9,29] and will not be considered here. 

A solid is not wetted if k < -1, partly wetted for -1< k< 1 and fully wetted for k > 1. Wetting is favoured when the difference (yss - ysl) approaches and becomes larger than o s. In this case the interaction between the droplet and the substrate increases and the contact angle decreases. It follows that materials with high surface energy are better substrates for deposition of another phase than substrates with low surface energy. One consequence is that metal surfaces are often readily wetted while polymeric surfaces often are not. 

Figure 1.79 Interaction of a liquid droplet with a solid substrate (a) Partially wetting (0° < 6> < 180°), (b) completely wetting (6> 0°), and (c) completely nonwetting 0 = 180°).

Wall-coated flow tube reactors have been used to study the uptake coefficients onto liquid and solid surfaces. This method is sensitive over a wide range of y (10" to 10 1). For liquids this method has the advantage that the liquid surface is constantly renewed, however if the uptake rate is fast, the liquid phase becomes saturated with the species and the process is limited by diffusion within the liquid, so that corrections must be applied [70,72,74]. Many experiments were designed to investigate the interaction of atmospheric species on solid surfaces. In this case the walls of the flow tube were cooled and thin films of substrate material were frozen on the wall. Most of the reaction probabilities were obtained from studies on flow tubes coated with water-ice, NAT or frozen sulfate. Droplet train flow tube reactors have used where liquid droplets are generated by means of a vibrating orifice [75]. The uptake of gaseous species in contact with these droplets has been measured by tunable diode laser spectroscopy [41]. 

Powell, G. L., Beecroft, M., Dombrowski, M., Mattison, P. and Szczesniak, M. M. (2003) FTIR imaging of the interaction of oil droplets with various substrates. PITTCON Paper 1703—4. 

The mercury film electrode (MFE), used for stripping analysis or flow amperometry, consists of a very thin (10-100-pm) layer of mercury covering a conducting and inert support. Because of the adherent oxide films on metal surfaces, and the interaction of metals with mercury, glassy carbon is most often used as a substrate for the MFE. The mercury film formed on a glassy carbon support is actually composed of many droplets. Because they do not... 

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... 

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