**4. Conclusions**

Active merging of microdroplets is one of the most important manipulations enabled by droplet-microfluidics, as it enables for on demand droplet targeting and sequencing of complex chemical and biological reactions. Nevertheless, the standard voltage-based methods are generally limited to spatially isolated pairs of droplets, as the long-range effect of the applied electric field does not have the potential to differentiate between flowing or confined droplets. We developed an optofluidic approach, which overcomes such limitations and allows for a rapid controlled coalescence of targeted microdroplets thanks to the application of a ps UV pulsed laser. Our approach is based on the photo-isomerization process of an azobenzene-derived surfactant. The success of such controlled coalescence is far from trivial. In fact, the dynamics of the change of surface tension at the microscale driven by photo-isomerisation processes; the diffusion of the new surfactant molecules to the interface; as well as the adsorption on the droplet surface, are complex and not well understood. By incorporating light activated switching in the surfactant, release or mixing of droplets contents could be programmed. In this study, we present the fast and dynamic switching of azobenzene derived surfactant for controlled coalescence of spatially selectable microdroplets as proof of concept.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6374/9/4/129/s1, Figure S1: Chemical synthesis procedure of KryAz600 surfactant molecules.

**Author Contributions:** Conceptualization, M.B. and A.I.E.A.; Data curation, Z.H., N.B. (Nizar Bchellaoui), R.M. and A.I.E.A.; Formal analysis, Z.H. and A.I.E.A.; Funding acquisition, M.B. and A.I.E.A.; Investigation, Z.H., R.M. and A.I.E.A.; Methodology, C.D., Juan Xie, M.B. and A.I.E.A.; Project administration, M.B. and A.I.E.A.; Resources, C.D., Remi Metivier, N.B. (Nicolas Bogliotti), J.X., M.B. and A.I.E.A.; Software, Z.H. and A.I.E.A.; Supervision, N.B. (Nicolas Bogliotti), M.B. and A.I.E.A.; Validation, Z.H. and A.I.E.A.; Visualization, Z.H. and A.I.E.A.; Writing–original draft, Z.H. and A.I.E.A.; Writing–review editing, A.I.E.A.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank Emile Auria for his contribution to the synthesis of the azobenzene-derived surfactant. **Conflicts of Interest:** The authors declare no conflict of interests.
