**Fast Active Merging of Microdroplets in Microfluidic Chambers Driven by Photo-Isomerisation of Azobenzene Based Surfactants**

**Zain Hayat 1,†, Nizar Bchellaoui 1,‡, Claire Deo 2,§, Rémi Métivier 2, Nicolas Bogliotti 2, Juan Xie 2, Malcolm Buckle 3 and Abdel I. El Abed 1,\***


Received: 7 October 2019; Accepted: 28 October 2019; Published: 1 November 2019

**Abstract:** In this work, we report on the development of a newly synthesized photoactive reversible azobenzene derived surfactant polymer, which enables active and fast control of the merging of microdroplets in microfluidic chambers, driven by a pulsed UV laser optical stimulus and the well known *cis*-*trans* photo-isomerisation of azobenzene groups. We show for the first time that merging of microdroplets can be achieved optically based on a photo-isomerization process with a high spatio-temporal resolution. Our results show that the physical process lying behind the merging of microdroplets is not driven by a change in surface activity of the droplet stabilizing surfactant under UV illumination (as originally expected), and they sugges<sup>t</sup> an original mechanism for the merging of droplets based on the well-known opto-mechanical motion of azobenzene molecules triggered by light irradiation.

**Keywords:** microdroplets; photo-isomerisation; photokinetics; opto-mechanics; conformational states
