6.2 Gas-Surface Modeling: Adsorption and Desorption Rates
The trends in friction coefficient with water vapor pressure and surface temperature
have been established experimentally. To compare that data to modeling of surface
adsorption and desorption of water vapor, friction coefficient has to be related back to
surface coverage. This is done in the same manner as the modeling in Chapter 2, using a
linear rule of mixtures. The difference in this chapter the friction coefficient data is
transformed into coverage instead transforming the model equations to friction
coefficient. This is just a simple rearrangement of eqn 2.22 to the form in eqn 6.1.
(= _-0) eqn 6.1
Using the same values for nascent and fully covered friction coefficient as in previous
modeling (to = 0.006 and pI = 0.12), the friction coefficient data in Figure 6-2 is
correlated to fraction surface coverage (0) in Figure 6-4.
0.12- 1.0 o 123 Pa (01% RH)
a 1233Pa(10%RH)
0.10- 0.8 2837 Pa (23% RH)
W m 4933 Pa (40% RH)
S0.08
E 0.6
u 0.06 =
"4 0.4
'" 0.04 -
0.2
0.02 -
0.0 8 o B B = B S r'
0.00 0.0
300 320 340 360 380
temperature (K)
Figure 6-4 Experimental results of friction coefficient (g) and surface coverage (0) as a
function of counterface surface temperature and water vapor pressure.
Modeling of the trends in surface coverage with changes in water vapor pressure
and counterface temperature is going to be performed using the Langmuir model for