Highly reflective aluminum coatings may be formed on the surface of a substrate by impacting the surface with molten drops of aluminum. The droplets are discharged from an injector, proceed through an inert gas (helium), and must still be in a molten state at the time of impact. Consider conditions for which droplets with a diameter, velocity, and initial temperature of D 500 um, V 3 m/s, and T 1067 K, respectively, traverse a stagnant layer of atmospheric helium that is at a temperature of T- 300 K. What is the maximum allowable thickness of the helium layer needed to ensure that the temperature of droplets impacting the substrate is greater than or equal to the melting point of aluminum (T2 Tmp 933 K)? Properties of the molten aluminum may be approximated as rho-2550 kg/m3, cp = 1200 J/kg. K, and k-200 Win. K. (Hint: The surface temperature of the aluminum, 7s, is changing. Therefore, if you need to use T, to identify any properties, you need to sclect a proper 7, and please justify your selection.) Droplet injector -Molten alumnum, V, D Aluminum coating Cold substrate