Problem converging a time dependent study

Hi Comsol community,

I started using Comsol few months ago. I aim to model the mixing by diffusion of two ionic solutions in a Y microfluidic junction (to eventually model precipitation of CaCO3). I am using the 'creeping flow' and 'Nernst-Planck' modules.

I started with a stationary study which converge. In the file attached:
-study 1(stationary): solve the flow (converge without problem);
-stydy 2 (sationary): step 1 solve the transport of ionic species (using the solution from study 1) while step 2 solve the equilibrium reactions (to model the speciation of carbonic speacies). The study converge.

However I have trouble when moving from a steady step problem to a time-dependent study. In the file attached:
Study 3 (time dependent): solve the flow, and converge without problem
Study 4 (time dependent): I attempt to solve the transport of ionic species (I disabled the equilibirum reaction for now) using either solution from study 1 (flow in stationary condition) or study 3 (time-dependent flow) but in each case the study fail to converge.

Any help would be greatly apreaciated.

Regards,
Thib