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44 PRODUCTION MACHINING :: DECEMBER 2016 PARTS CLEANING M any current industrial surface metal or composite material treatment operations, such as cleaning, passivation and etching, are limited by the mass transfer process of chemicals from bulk fluid to the part's surface. At the surface of parts there exists a distinct layer of fluid called the mass transfer boundary layer, where chemical concentrations and velocities are significantly lower than in the bulk solution away from the surface. In order to enhance chemical transferability through the mass transfer boundary layer, bulk fluid agitation (to decrease the boundary layer as much as possible) and/or increased chemical concentrations are needed. By Donald Gray, Ph.D., Contributor Understanding the Vacuum Cycling Nucleation Process VCN can offer enhanced surface treatment performance in both aqueous and solvent applications. But serious limitations still exist because of the boundary layer, where most of the resistance to mass transfer takes place; the fluid velocity used to convectively transfer the treating chemicals to or away from the surface is dampened and decreases rapidly near and at the solid surface. It is within this boundary layer that the rate of mass transfer slows because of a dependence upon molecular transfer mecha- nisms as opposed to the more rapid eddy transfer mechanism encountered in the bulk fluid region. Since the mass transfer rate slows near a surface, a rapid change in the active ingredient chemical concentration occurs across this boundary layer. Figure 1 shows a typical concentra- tion profile developed for a chemical reacting at the surface, thus showing a zero concentration at the surface. Although the boundary layer in this example is only 1 mil, 80 percent of the change in concentration occurs across the 1-mil thick boundary layer. Increasing bulk concentration linearly increases the concentration within the boundary layer. Decreasing the boundary layer thickness shortens the length of the resistance path to the surface. Reducing Total Pressure e vacuum cycling nucleation (VCN) process is accomplished by reducing the total pressure in a controlled-environment chamber that contains (submerged in :: Vacuum cycle nucleation enhances the transfer of material to or from the surface of the solid by reducing the total pressure in a controlled environmental chamber containing a part submerged in a liquid to below the vapor pressure of the liquid.