DLS and "flow mode" don't go well together. Are you thinking of a stop-flow system?

DLS works by detecting the relative motion of particles in a liquid. In a truly stationary liquid, the only motion should be diffusion and that's what the technique estimates usually by autocorrelation.

In the presence of bulk flow, you will almost certainly have a spread of particle velocities (e.g., turbulence etc) that presents additional relative motion. This often manifests as large, slow oscillations in the correlation function. DLS instruments are supposed to be designed to keep all motion other than diffusion to a minimum.

Hello Francesco Giorgi , in practice it is possible to do DLS in flow mode. It turns out that the flow contribution to the correlation function can be minimized with the optical setup and measurement conditions. We found that in backscattering this can work for a flow rate up to ~2mL/min . Some of the reasons why it can work:

• q and v almost perpendicular
• actual measurement volume only a small cross section of the cell / flow front
• monodisperse sample during peak in chromatography

You can find a few examples in https://www.americanlaboratory.com/913-Technical-Articles/18720-Absolute-Size-Exclusion-Chromatography/

and there are a few app notes available for example https://www.malvernpanalytical.com/en/learn/knowledge-center/application-notes/AN101104SEC-DLS

You will probably find the technical note on practical setup details quite interesting: https://www.malvernpanalytical.com/en/learn/knowledge-center/technical-notes/TN101104ZetasizerNanoSECDetector.html

Good luck!