Polymer-monovalent salt-induced single-molecule DNA compaction/condensation in a microfluidic stagnation point flow was studied by analyzing both DNA compaction images and time trajectories. For the whole DNA compaction process we observed three successive steps: Step I, a relaxation process of the stretched DNA that occurs slowly along the whole DNA chain, Step II, nucleus formation and growth, and Step III, corresponding to a rapid compaction of the chain. A memory effect was observed between Steps I and III, and a new (intruder-induced) nucleation mode was observed for the first time. This study extends the use of the microfluidic stagnation point flow, which we have previously used for sequence detection and to measure enzyme kinetics site-specifically.