Discussion Forum

Transport of diluted species is not suitable for tracking individual molecules, it works on the scale where one can speak of concentrations. I do not know if COMSOL is suitable for simulations of molecular interactions at all.

Actually I want to track the flux of the molecules on the absorbing boundary and not individual molecules. Also I would like to define some boundaries that reflect the flux back into the reservoir.

Thanks

By reflecting you mean that the boundary is closed (i.e. no flux)?
What kind of boundary condition do you have in mind for an 'absorbing' boundary? You mean that every molecule of your diffusing species is immediately removed from the solution if it reaches that boundary? Then you could simply put a zero concentration boundary.
The flux can be extracted from your calculations. Do you only want to access it after the calculations are done or do you need this flux as the input for a different part of you model during the calculations?

Yes. By reflecting I mean that boundary is closed but the molecules are not going to be accumulating in the closed boundaries and instead they are going to be reflected back.
And again Yes. By absorbing boundary I mean that once the molecules reach the boundaries the are going to be removed from solution. At the end i want to monitor their flux rate on the absorbing boundaries respect to time. I don't need them as an input . At least for now.

Thanks much

Yes, then indeed you can put the boundary condition as I said above.

The flux through a boundary is called chds.bndFlux_c (if chds is your transport of diluted species physics and c is your diffusing species). You can get total flux through the absorbing boundaries by putting in the Results -> derived values node an integral over the absorbing boundaries (select the relevant boundaries when you have this node active), and put as the expression to integrate chds.bndFlux_c (or a similar name if your physics and/or species are called differently). If you evaluate this a table will be created (in my gui it opens below the graphics, and you can re-open it by selecting it under Results -> Tables). Above this table there will be a button 'Table graph', which will create a plot of the data in the table.

Thank you so much,
First of all do you mean line integral in Derived Values? If so I did as you advised. However as I said I want the Flux vs Time (ie. the flux at time=.2, the flux at t=.3,...) when I use integral it gives the whole flux through the absorbing boundaries over the passed time. so the graph is not like it should be...
Thanks

If you have specified the times at which you want the flux also as times in the time dependent study, and then if you choose for the line integral (you have a 2D model I guess?) to evaluate it at all time steps it should evaluate to a table that has one column with the specified times and another with the corresponding fluxes at those times. If you make a plot of this table it should result in a graph of flux versus time.