I am an intern at Nikhef and working on the ET-Pathfinder. This is an interferometer for gravitational wave detection designed by Nikhef in the Netherlands. I am using Molflow+ to simulate the high vacuum inside the towers where the mirrors and lasers are located. As you probably know water inside the system can cause serious problems. It will freeze in the mirrors at the temperatures of 8 to 20 kelvin .Usually the system is “baked” to 523 Kelvin for a long period of time to prevent stray water molecules. It won’t be possible to heat the ET-pathfinder higher than 338 kelvin. Because of thermal expansion of the mirrors and other electronics on the inside. The layers of water molecules on the inside surfaces causing the outgassing in the system will be the main priority and worry.
The water molecules will ‘’stick’’ to the surface with an x amount of surface tension. Now the water-water connection ( top layers/multilayers ) are a lot weaker than the lowest layer, water-Stainless steel connection ( mono layer). And thus the sticking factor and time spend on the surface will vary over time. I’ve made simulations with variating sticking factors ( higher sticking over time because of the mono layer ) using the Time --> Edit parameters window. But I would like the make a connection to the sticking factor and the amount of water molecule layers left on the surface.
Now my question:
I was wondering if water molecule layers is something that is implemented in Molflow+ and if it is possible to connect the sticking factor variable to the amount of layers left. Especially if the top layer molecules are gone and the monolayer molecules are exposed and going to desorb. Instead of using the time dependent parameters to change the value of the sticking factor in between a chosen time period.
Thank you for reaching out to us and the topic is very interesting. The problem with implementing such a feature is that you'd create a "catch 22": sticking factor would depend on the pressure (or its history), but pressure would depend on the sticking factor.
The solution is iterative simulation: one would have to run a series of (increasing) time steps, doing each step as a static simulation, then at the end update material properties (like sticking) based on the simulation history (such as total number of absorbed/desorbed molecules or layer thickness).
This is planned (see last point in our roadmap), and the good news is that a Phd thesis is in progress about the topic. However, it is not something that will be implemented this year (during your internship).
For your info, contamination control in satellites (mainly water) is an issue with which both a European commercial satellite manufacturer and a large American research institute has contacted us. We collaborate on the issue and a Phd is also in the work in T.U.Munich about contamination simulation using Molflow. If something comes out of it, we'll publish it ASAP as the topic is of interest to many.
So, despite our negative answer, good luck for your project and hope we can answer upcoming questions.
Thank you for the fast response. I will definitely try the iterative simulation method.
Fortunately, I can create a high-vacuum tank setup inside the institute and measure the pressure and precentage of water over time. So i will be able to know (estimate) the disorbtion of the multilayer water molecules. If the presentage of water is decreasing slower because of the higher sticking time to the surface, then i will be able to recreate the situation in Molflow+ and compare it to the iterative simulation (based on the amount of absorbed/desorbed molecules like you said). Very curious if I am able to get the same results.