Hello, I am setting up my model using transport of diluted species. I am simulating in a way that after certain time (24hr) my boundary concentration changes from 100 nM to zero. I used a step function to set this condition. My geometry is a circle and a concentration is supposed to come from outside boundary to the center. After 24hrs, when concentration changes to 0, there is more concentration in the center compared to the edge. Considering Fick's law of diffusion, the concentration is supposed to move following its gradient. However, in my case, the concentration just keeps going towards the center regardless of the concentration gradient. I was wondering if there is a way to control that in the transport of diluted species physics to make the transport just follow concentration gradient instead of being a unidirectional movement. Any help will be higly appreciated.
Thank you